2.3.3.    Nesting Habitat assessment

Site visit

  1. During the period 13th-23rd June 2022 a site visit was undertaken by NBC/WMIL to estimate potential nest site availability on Inchcolm for each the key species. Estimates of nesting space were obtained by both carrying out a walkover a walkover survey of accessible areas and a circuiting the island by boat to take counts and photograph birds nesting on cliffs. Care was taken not to disturb sensitive nesting areas. The results of this work are presented in full within the feasibility study (Cain et al., 2022).
  2. Since the survey was conducted during peak breeding period it was not possible to carry out precise study of the physical attributes of the nesting areas. This would need to be carried out outside of the breeding period and would involve gathering additional information on the dimensions of individual rock crevices for puffin and razorbill and the depth and type of superficial soils for burrow nesting species (i.e. Puffin).
  3. Photographs of habitat of interest were taken and observations of seabird activity were recorded. Unoccupied areas of cliffs and slopes for nesting were noted and these areas were crudely measured with the aid of a laser rangefinder and reference to Ordnance Survey maps and Google Earth imagery. The high tide mark plus a 2m ‘splash zone’ was subtracted from the measured height of the ‘unoccupied’ island feature to provide an estimate of the total area of habitat available for additional nesting. Habitats were photographed and matched to the known nesting preferences of the key species to assess potential habitat availability.

Habitat preferences and nesting densities of the key species

  1. Guillemot and Razorbill nest in broadly similar habitat types and share colony space (Harris & Wanless, 1987), although Razorbill show a preference for nesting in cavities and crevices as well as nesting on ledges (Hipfner & Dussureault, 2001). The density of nests in Guillemot breeding areas varies between site. Densities as high as 46 pairs m-2 have been recorded (Harris & Wanless, 1987), although a lower density of 20 pairs m-2 is used in these calculations (Harris & Birkhead, 1985). Based on the species composition of other seabird colonies within the Forth Islands (Table 2.4), coupled with regular sightings of Guillemot around Inchcolm, it seems feasible to suggest that Inchcolm has the potential to support a mixture of Guillemots and Razorbills, especially given the proximity of other nearby colonies, such as Inchkeith, which could be a source of potential colonists.
  2. When assessing habitat availability for Guillemot and Razorbill on Inchcolm, only horizontal rocky ledges were selected. This is a conservative estimate, as short ledges and small rocky crevices and other rocky features can also be used, and inclined ledges can be used if flat areas for egg placement or nest building are present. A crude estimate of the total length of the marked areas (total ledge length) was made. Based on the topography of the cliffs and the width/depth of the ledges as observed during the preliminary site visits, a conservative estimate of an average of 0.3m ledge depth was assumed. This width also aligns well with the published literature, with Birkhead (1977) recording a 0.29 m mean width for ledges occupied by Guillemot. Where photographs showed large, flat rock areas (here referred to as platforms), depth was estimated as 0.6 m. Potential nesting space available for Guillemot and Razorbill (number of pairs) was calculated by measuring the ledge length (in meters), multiplying by the ledge depth (in meters) and finally multiplying by an average nesting density of 20 pairs m-2, as generated from the scientific literature (Harris & Birkhead 1985).
  3. Kittiwakes favour steep cliffs with horizontal ledges for nesting, often sharing these with other seabirds, particularly Guillemot and Razorbill. Nests are built up on grassy knolls in crevices and on ledges using a mix of grasses and seaweeds. A crude estimate suggests a Kittiwake nesting density of 3 nests per linear metre of suitable ledge is achievable for a healthy colony (Massaro et al. 2001). For kittiwake, only horizontal ledges, recesses and grassy knolls with an estimated depth equal to or greater than 0.3 m visible on the photographs were selected. Estimation of available habitat was calculated by measuring the total length of the marked areas, multiplying by the ledge depth (0.3m) and then multiplying by an average nesting density of 3 pairs m-2.
  4. Puffins nest colonially on cliff tops and grassy slopes, digging a burrow in which a single egg is laid. Puffin burrow density has been shown to be negatively correlated with distance from the cliff edge and positively correlated with angle of slope. These correlations are biologically significant in that close to the cliff edge, where the angle of slope was steep, breeding success was significantly higher than on adjacent level habitat (Nettleship 1972). On St Kilda burrow densities averaging approximately 0.5 burrows m-2 have been reported (Harris and Rothery, 1988). To reinforce the suitability of the Forth islands habitat for Puffin, on the Isle of May, located 45 km to the east of Inchcolm, only five pairs of puffins were breeding in 1958, while 20 years later, 10,000 pairs were present (Boag et al. 1986).
  5. For Puffins, estimates are based on the areas of steep grassy banks at the top of cliffs or rocky outcrops only. This is a conservative estimate as boulder fields and shallow inclines can also be used if vegetation is managed to improve access, and on Inchcolm it is known that the boulder field on the south-west of the island was historically used as a nesting area. Potential nesting space for puffin was calculated by multiplying the area of grassy bank on top of cliffs or rocky outcrop (in m2) by an average nesting density of 0.5 pairs m-2 (based on Harris and Rothery 1988).
  6. In all instances estimates are based on the best available measurements from photographs and on conservative assumptions of ledge length, width, and available nesting space from photographs taken from a vessel. Smaller niches and short sections of ledge, not clearly distinguishable on whole area images, are likely to be available and could provide additional nest habitat.

Estimation of available habitat

  1. The results of the habitat assessment are summarised in Figure 2.3, which shows the locations of potential areas for colony expansion and Table 2.3 which shows the size and potential capacity of different nesting areas for each of the key species. Photographs showing each of the areas shown on Figure 2.3 and delineating the available ledge species for each of the key species are included in Appendix 1 of this document as Figures A1 to A7.

Figure 2.3:
Areas of Inchcolm assessed as most suitable for supporting the expansion of target seabird species.

Figure 2.3: Areas of Inchcolm assessed as most suitable for supporting the expansion of target seabird species.

  1. The habitat assessments do not take into account predation pressure from large gulls, and competition with other species for breeding space, or other factors such as human disturbance.
  2. Since there is overlap between habitat that could be used by Kittiwake and by Guillemots and/or Razorbills, the results in Table 2.3 are presented in three ways: i) the total estimated number of pairs (of each species) that could be supported by the available good quality habitat, ii) the estimated number of pairs of each species assuming that Kittiwake outcompete Guillemot/Razorbills for suitable shared good quality habitat, and iii) the estimated number of pairs of each species assuming that Guillemot and/or Razorbill outcompete Kittiwake for suitable shared good quality habitat.
  3. The habitat assessment indicates that there is capacity on Inchcolm to support an additional 250 pairs of Puffins, giving a total of 255 pairs[10]. Since there is some degree of overlap in habitat between the other three species, assumptions have to be made to produce a broad estimate of how many pairs of each species could occupy the available habitat. Table 2.3 shows the number of pairs of each species that could be accommodated under two scenarios: i) whereby Kittiwake outcompete Guillemot/Razorbill, and ii) whereby Guillemot/Razorbill outcompete Kittiwake. Both scenarios assume that Puffin will not compete with Guillemot/Razorbill or Kittiwake. The feasibility study (Cain et al., 2022)states that the reality is likely to be somewhere between these two scenarios. Therefore, an interim value has been calculated based equal colonisation of the available habitat by both Guillemot/Razorbill and Kittiwake (i.e. a value half way between scenario 1 and scenario 2).
  4. Based on these assumptions, there is capacity on Inchcolm to support 176 additional pairs of Kittiwake, giving a total of 239 pairs, and to support an additional 210 pairs of Guillemot/Razorbill, giving a total of 222 pairs. Since species specific compensation targets are needed to present the compensation in a metric that is comparable with the predicted impacts of Berwick Bank, then it was necessary to establish what proportion of these birds were likely to be Guillemots and what proportion were likely to be Razorbills.
  5. The mean ratio of Guillemots to Razorbills within the other Forth Islands (listed in Table 2.4) was examined. Ratios varied between 1.6:1 (Guillemots:Razorbills) on Inchkeith to 21:1 on Lamb. Since Inchkeith is the nearest island to Inchcolm, the ratio from this site was used. This was viewed to be reasonable on the basis that a lower number of Guillemots would be expected as they are not currently established on Inchcolm. Using the Guillemot to Razorbill ratio of 1.6:1 from Inchkeith, the additional 210 pairs were assumed to constitute 129 pairs of Guillemots and 81 pairs of Razorbills. Adding on the number of Razorbills currently present (12 pairs), then the habitat assessment suggests that Inchcolm has the potential to support 93 pairs of Razorbills and 129 pairs of Guillemots.
  6. However, these numbers are to a certain extent arbitrary and are intended only as an approximate guide so that the compensation benefits can be evaluated in a similar metric to the potential impacts of Berwick Bank.
  7. It is acknowledged that the benefits of rat eradication are likely to be influenced by a range of site-specific factors, which include (but are not limited to) the following:
  • The number of rats present;
  • How accessible nesting areas are to rats;
  • Current population trends, e.g. if a species is declining rapidly then achieving any measurable improvement is unlikely although it may be possible to slow the rate of decline;
  • Large scale climatic events and other unpredictable forces majeure;
  • Other factors that may impact negatively on breeding success, e.g. poor weather, floods, storms; and
  • How easy it is to remove rats and keep the site rat free (e.g. are rope access routes around cliffs required? Is it possible to remove all the rats or is there a significant risk that eradication will be unsuccessful? Is the risk of reinvasion high.)
Table 2.2:
Size and potential capacity of different nesting areas on Inchcolm Island for each of the key species.

Table 2.2: Size and potential capacity of different nesting areas on Inchcolm Island for each of the key species.

2.3.4.    Project description

  1. The project to eradicate Black Rat from Inchcolm would be developed in accordance with the UK Rodent Eradication Best Practice Toolkit (Thomas et al. 2017a). A brief outline of the work is included here, and further detail is provided within the Implementation and Monitoring Plan. Further information on the approach to developing a Biosecurity Plan, incorporating information on how incursion of invasive mammalian predators will be prevented, surveillance to monitor for incursion, and incursion response plans should an incursion occur are all included within the Implementation and Monitoring Plan.
  2. Stakeholder engagement with local groups including HES, the Forth Seabird Group, and the Forth Islands Heritage Group, has been positive and details of a proposed Communication and Engagement Strategy going forward are included in the Implementation and Monitoring Plan. The outputs of the feasibility study also indicate that the project is both technically feasible and would improve seabird breeding success. Although there is a high incursion risk, this can be minimised through positive stakeholder engagement, most notably with the local ferry operator and HES, as well as implementation of a Biosecurity Plan. All funding for this work would be supplied by the Applicant, as outlined in the Implementation and Monitoring Plan.
  3. As is standard practice, rodent eradication would be carried out during the winter months. The details of rodent removal methods and indicative programme are included within the Implementation and Monitoring Plan.
  4. A monitoring programme to undertake seabird counts and to monitor productivity would be required to study how key species respond following rodent eradication. Details are included within the Implementation and Monitoring Plan.
  5. Since Inchcolm is not an SPA and is not covered by any nature conservation designations, there is currently no resource to deal with any work on seabirds. There is clear potential for Applicant funded staff involved in the surveillance and monitoring phase of the project to also tackle any other minor issues that would be part of routine colony management at designated sites, and which are likely to improve seabird breeding success.
  6. During the stakeholder consultation process undertaken to develop the colony measures it was identified that Inchcolm currently supports Tree Mallow in places around the Abbey gardens. It is recommended that this could be removed to prevent the issues experienced on the other Forth Islands where this invasive species has colonised and grown to such an extent that Puffins are obstructed from entering and leaving their burrows. Additional vegetation management in Puffin nesting areas could also be beneficial. The accumulation of plastic litter on the beaches was also raised. Although HES maintain the Abbey grounds there is currently no mechanism to remove plastic from the rest of the island. It was suggested that an annual plastic pick-up could be enough to maintain Inchcolm in a better condition for both seabirds and also Grey Seal, which use the beach as a breeding site and are vulnerable to entanglement in plastic litter and old fishing nets. Any additional measures to improve seabird nesting habitat and the wider habitats on Inchcolm is detailed in in the Implementation and Monitoring Plan.
  7. Putting measures in place to reduce human disturbance, particularly during the first half of each calendar year, would also be of benefit to nesting birds, and it would be beneficial if guidelines to minimise disturbance were developed with HES as a component of this project

2.3.5.    Conservation targets

  1. Establishing conservation targets for Inchcolm is problematic as the site has always supported Black Rat for as long as seabird counts have been conducted. Therefore, any predictions of how the bird populations would respond to rat eradication are to a certain extent speculative.
  2. Maximum counts represent the maximum number of birds that are known to have nested on Inchcolm. For Puffin this is thought to be around 200 birds during the period 1992-1995 (R. Morris pers. comm.). For Kittiwake it is 190 AON (1995), and for Razorbill 21 AOS (2021). However, since Black Rat has always been present, and numbers of birds in general are very low relative to other islands in the Firth of Forth (see Table 2.4), they are not considered representative of what the site may support without Black Rat.

 

Table 2.3:
Numbers of the key species nesting on different Forth Islands.

Table 2.3: Numbers of the key species nesting on different Forth Islands.

Data sources: FIHG 2021, Forth Seabird Group 2017, 2018, 2019, 2020 & 2021. Note The size of colonies nesting on Inchcolm are notably low.

 

  1. The results of rat eradication projects at other islands can provide helpful information with regards to establishing potential benefits. Examination of data from 12 islands from which rodents have been removed shows increases in the numbers of nesting birds at 10 of these islands (Thomas et al. 2017a, Table 2.5). The two sites where increases were not recorded were Looe Island in Cornwall and Inchgarvie in the Firth of Forth. Both of these sites have had rat re-invasions. The rats have been removed a second time from Looe Island but the seabird population trends are uncertain. Anecdotal rat sightings have been reported from Inchgarvie, and at present there appears to be no monitoring and no resource to carry out further rat removals (Thomas et al. 2017a). Following the production of this review, rats have recently recolonised Handa.
Table 2.4:
UK & Crown Dependency rodent-eradication attempts. (Reproduced in full from Thomas et al. 2017a

Table 2.4: UK & Crown Dependency rodent-eradication attempts. (Reproduced in full from Thomas et al. 2017a

 

  1. At five of the 12 islands, previously lost seabird species recolonised after rodent eradication including Puffin at one site, European Storm-petrel at three sites and Manx Shearwater at one site. At three islands there have been very substantial increases in numbers of Puffins and Manx Shearwaters: on Handa Puffins increased rapidly in the five years following rat eradication, at Ramsey Manx Shearwaters have increased more than five-fold, and at Lundy Manx Shearwater has increased 10-fold and Puffin numbers have increased from five birds to more than 200 pairs (Thomas et al. 2017a).
  2. Other ecosystem benefits include increases in passerines and ducks as well as other species such as Pygmy Shrew Sorex minutus, Slow Worm Anguis fragilis, Common Lizard Zootoca vivipara, Lesser White-toothed Shrew Crocidura suaveolens, Bank Vole Myodes glareolus and even moths (Thomas et al. 2017a). On this basis, the eradication of rodents from islands is clearly of very significant benefit to seabirds, other avian species, and the whole ecosystem. However, there is an element of uncertainty in predicting recolonisation, and which species will benefit and how rapidly.
  3. The restoration potential of Inchcolm is very clear both when viewed in the context of the other Forth Islands (Table 2.4) and when the benefits of rat eradication achieved by other eradication programmes are taken into account. On this basis, and on the grounds that data gathered in the absence of rats is lacking, the number of birds that might occupy Inchcolm based on the habitat assessment appears to be the most suitable measure available to use as a basis for conservation targets. However, the results of the habitat assessment are discussed below by species in relation to historical counts, the size of colonies present on the other Forth Islands, and the results of rat eradication programmes conducted elsewhere to ensure that the conservation targets are sense checked and appear reasonable and achievable.

Puffin

  1. Currently there are five pairs of Puffin nesting on Inchcolm, where they are restricted to the steepest cliffs on the north west of the island. During the period 1992-1995 around 200 Puffin are known to have nested on Inchcolm (R. Morris pers. comm.), utilising a boulder field on the south of the island. Many of the Forth Islands support substantial Puffin colonies, with the nearest on Inchkeith numbering 2178 AOB. Even accounting for Inchkeith’s larger size, it is clear that Puffin numbers on Inchcolm are severely depressed.
  2. The benefits of rodent eradication for Puffin have been observed at Lundy, Ailsa Craig, Canna, and the Shiant Isles. At Lundy, Puffin have increased by 369% (Brooker et al. 2018), rising from 13 birds in 2000 to 848 in 2021[11]. At Ailsa Craig they have recolonised since rat eradication in 1994 and numbered 130 nests in 2015[12]. On the Shiant Isles productivity increased from 0.66 to 0.79 over the two years following rat eradication and numbers have increased significantly[13]. At Handa, Puffin numbers increased from 472 Puffin in 1996 to 735 Puffins in 1997. This equates to a 56% increase and a gain of ~44 Puffin per year (see Section 3.4.5).
  3. At Canna, Puffin numbers were low before rat eradication, and they were confined to two virtually inaccessible stacks, making counting them impossible. However, a count of rafting Puffin in 1999 gave 1190 individuals, whilst in 2016, 2050 were counted suggesting a percentage increase in the region of a 70% (although the imprecise nature of counting rafting birds is acknowledged). After rat eradication in 2005/06 Puffin recolonised the main island and have spread to several sites along the north coast of Canna itself.
  4. The only site where Puffin have been present and not increased significantly following rodent eradication has been Puffin Island, Anglesey. However, there are some indications that even here a slow recovery is underway with an increase from 8 pairs in 2010 to 29 birds in 2018 (still a 263% increase).
  5. Nationally, Puffin numbers have increased in some locations and declined at others. However, on the grounds that many of the increases described above have been substantial (well in excess of national trends) then it is clear that rat eradication is a powerful measure for Puffin.
  6. The habitat assessment indicates that Inchcolm has the potential to support an estimated 255 pairs of Puffin. This seems very feasible when compared with the large numbers of Puffins that currently nest on other similar sized islands within the Firth of Forth (e.g. Craigleith, Lamb, Fidra, Inchkeith - see Table 2.4). Inchcolm supported 200 Puffin historically even when rats were present. On this basis the conservation target of 255 pairs of Puffin seems reasonable and achievable based on the proven success of other rat eradication projects for this species.
  7. It is acknowledged that predation from large gulls and Peregrine Falcon may inhibit recovery and it is suggested that artificial ground cover could be used to reduce avian predation. Previous studies show that Guillemots nesting in areas with artificial cover installed over the cliff tops produced twice as many eggs (Parish and Paine 1996). Diversionary feeding of the Peregrine Falcon pair could also provide an acceptable means of temporarily reducing predation pressure to facilitate the recovery of auk (and also Kittiwake) populations if recovery is slow, although this would require specific investigation (see Section 6).
  8. Vegetation management, comprising reduction in height and density of grasses and shrubs and loosening of soils on tops of steep slopes, could be adopted prior to the start of the nesting season to optimise conditions and improve access for Puffin. Other practical measures to assist population recovery could include controlling human disturbance around areas of suitable habitat (including the boulder field).

Razorbill

  1. Currently there are 12 pairs of Razorbills nesting on Inchcolm, where they are restricted to the steepest cliffs on the north west of the island. Razorbill numbers have been very gradually increasing on Inchcolm with maximum counts of 21 AOS recorded in 2020. However, in comparison with the other rat-free Forth Islands, numbers at Inchcolm are extremely low (Table 3.2). Looking at number of AOS per hectare for each island, Inchcolm scores lowest at only one Razorbill per hectare. This figure is highest for The Isle of May at 108 AOS per hectare. Fidra has the lowest number of AOS per hectare of the rat-free Forth islands at 13 AOS per hectare. Since current numbers for Inchcolm are so low, and Razorbills often nest in locations that are accessible to rodents, it seems reasonable to assume that they are significantly impacted by the rats.
  2. There is less available information on the benefits of rodent eradication and biosecurity for Razorbill, although this will change as more data emerges. However, since Razorbills nest either on lower cliff ledges or among boulders at the bottom of cliffs, then significant improvements to breeding success following rodent eradication are anticipated. Indeed, there is evidence of increases in Razorbill numbers at five islands where they have been monitored following rodent eradiation, namely Canna, Lundy, Ramsey, the Shiants and Ailsa Craig (Table 2.5, Brooker et al. 2018[14], RSPB News from the Rock 2021). However, Razorbill has been increasing nationally and so caution is required in interpreting this information.
  3. At Lundy, Razorbills increased from 950 in 2000 to 3,533 in 2021, an increase of 272%[15]. When rats were eradicated from Canna over the winter of 2005/06 Razorbill numbers initially increased sharply but then levelled off. However, eggs appeared in areas that had been clear of nesting for several years. Since Razorbill numbers had been declining, rat eradication had the effect of halting the decline (Luxmoore et al 2019). More recently (as the prey supply offshore has ameliorated) there have been big increases in Razorbills nesting on Canna, with numbers doubling between 2016 and 2019. They are now at their highest level since 1995 (The Seabird Group 2019). Comparison of all island Razorbill counts on Handa from 1997 (the year of rat eradication) and 2001 show that Razorbills increased from 15,573 to 17,042, an increase of 1,469 individuals (or 9%) (see Sections 2.4.2 and 2.4.5).
  4. The habitat assessment indicates that Inchcolm has the nesting space to support an additional 81 pairs of Razorbills (a total of 93 pairs), assuming a mixed Guillemot and Razorbill colony. This is only an approximation and there may be greater numbers of Razorbill if Guillemot are either slow to recolonise or do not recolonise. Habitat assessment indicates that capacity for Guillemot/Razorbill combined is 420 pairs, although some of this habitat would also be suitable for Kittiwake. On this basis achieving a conservation target of 93 pairs seems very feasible in the context of the Razorbill colonies on the other Forth Islands, and acknowledging the benefits achieved elsewhere e.g. Lundy and Canna.
  5. The recommendations described above to aid Puffin recovery would also help Razorbill populations to increase.

Guillemot

  1. Although there are no Guillemots on Inchcolm, it is a species that is observed (single birds in 2007 and 2008, and 14 individuals in 2014 – data from annual Forth Seabird Group reports). The habitat assessment (Cain et al., 2022) concludes that Guillemot could reasonably be expected to colonise Inchcolm following rat removal, based on habitat available and the proximity of other islands and sites supporting large breeding colonies. Since other rat free seabird islands within the Firth of Forth support both Guillemots and Razorbill (Table 2.4), it is considered likely, based on the habitat, that Inchcolm could similarly support colonies of both species if nesting conditions were more favourable. Habitat assessment indicated that Inchcolm has the capacity to support 129 pairs of Guillemots. This is considered feasible in view of the numbers of Guillemots supported elsewhere, including on the nearby island of Inchkeith, which could act as a source of potential colonists.
  2. Rat eradication programmes at other sites have proven successful for Guillemot. For example, following eradication of rats from Lundy, Guillemot numbers increased by 51% (Brooker et al. 2018). At Canna rat eradication had the effect of slowing the rate of decline (Luxmoore et al. 2019), although more recently Guillemots have increased reaching their highest number since 2004 (The Seabird Group 2019).
  3. Colonisation prospects could be further improved by using decoys and playbacks. Other studies have shown these techniques to be highly effective Parker et al. (2007). Breeding Guillemots were lost from a colony in California following an oil spill in 1986 and did not naturally recolonise over the following eight years. During this period Guillemots were not observed on site. In January 1996 decoys, playbacks and mirrors were installed to attract Guillemots. Following installation Guillemots were observed on site, with 6 pairs breeding in June 1996. By 2004 numbers had increased to 190 pairs (Parker et al. 2007).
  4. In some seabird species, white paint has been used to simulate guano at potential breeding sites (Gummer, 2003; Sawyer and Fogle, 2013). This could be used to encourage colonisation by all of the auk species, potentially alongside the use of decoys and playbacks, with the aim of increasing colonisation rates following rat eradication.

Kittiwake

  1. Currently there are 63 pairs of Kittiwake nesting on Inchcolm (FIHG 2021), where they are restricted to the steepest cliffs on the north-west of the island. Although Kittiwakes generally select steep cliff faces for nesting to avoid ground predator, the cliffs on Inchcolm are not that steep, so it is likely that nesting areas will be accessible to rats. Maximum Kittiwake counts on Inchcolm date from 1995 when 190 AON were present (despite the presence of Black rat).
  2. Although rat eradication projects have not focussed on this species previously many Kittiwakes nest at the top of cliffs and need relatively substantial ledges on which to build nests. The nests themselves also add extra material and structure to the cliff face which may make it easier for rats to climb. Data from Canna shows that Kittiwake numbers have risen significantly since rat eradication in 2005, at a time when Kittiwake was experiencing significant decline elsewhere in Scotland (see Figure 3.1 and also Figure 2.4).
  3. The habitat assessment shows that Inchcolm has the habitat to support 176 additional pairs of Kittiwake, totalling 239 pairs when adding the 63 pairs currently present (FIHG 2021). This number is relatively small in comparison to some of the other Forth islands (Table 2.4) but is considered reasonable and achievable as a conservation target: much of Inchcolm is relatively low lying and it does not support the same quantity of steep cliff faces as many of the other Forth Islands. However, higher historic counts confirm that nonetheless Inchcolm is capable of supporting a larger Kittiwake colony.

Figure 2.4:
Number of Kittiwake (AON) nesting on Canna by year. Rats were eradicated in 2005. Reproduced in full from The Seabird Group 2019.

Figure 2.4: Number of Kittiwake (AON) nesting on Canna by year. Rats were eradicated in 2005. Reproduced in full from The Seabird Group 2019.

Summary

  1. Conservation targets for Inchcolm are expressed in Table 2.6 both as the total number of birds that would be generated throughout the 35 year operational lifespan of the Proposed Development, assuming that conservation targets are met, and also as a number of birds that would be provided per year. The numbers are based on the assumptions that have been described above in sections 2.3.3 and 2.3.5. In reality, it is anticipated that it would take considerably less than 35 years to achieve conservation targets – seabird populations have expanded rapidly over 5-10 years on many islands where rats have been eradicated. However, the metric is useful in enabling comparison with predicted annual mortality from the wind farm. 
  2. Although the habitat assessment provides a measure of how many nesting birds Inchcolm could accommodate if conditions were more favourable, it is acknowledged that many factors (including predation pressure from large gulls, and competition with other species for breeding space, or other factors such as human disturbance )may impact on recovery, and as such there is uncertainty associated with whether the conservation targets can be achieved. Conversely, recovery may be enhanced by the measures described in the previous section, such as the use of decoys and playback for Guillemot and artificial ground cover to reduce avian predation.
  3. Whilst the conservation targets indicate what could potentially be achieved at Inchcolm, they do not measure how improved growth and colony productivity could positively contribute towards seabird populations within the Forth Islands. If the conservation targets are achieved within the next 10 years then it is conceivable that the expanded colonies would produce reasonable numbers of fledged chicks per year, and that this would impact positively on the Forth Islands colonies.
  4. For example, assuming a productivity value of 0.69 chicks per pair (JNCC 2021) an additional 250 pairs of Puffins could produce an additional 146 fledged chicks per year. Using the mean annual survival rates presented in Horwill & Robinson (2015) and assuming a first breeding age of seven years (Harris & Wanless 2011) this would result in the addition of 72 adults per year being added to the Forth Islands population.
  5. Similarly, assuming a productivity value of 0.57 chicks per pair (JNCC 2021) an additional 81 pairs of Razorbill could produce an additional 46 fledged chicks per year. Using the mean annual survival rates presented in Horwill & Robinson (2015) and assuming a first breeding age of 4 years (Lavers et al. 2008) this would ultimately result in the addition of 21 adults per year to the Forth Islands population.
  6. Assuming that colonisation of Inchcolm by Guillemot is achieved and based on a potential 129 pairs of Guillemot occupying the available habitat on Inchcolm and a productivity value of 0.70 chicks per pair (JNCC 2021), then 90 fledged chicks would be produced per year. Using the mean annual survival rates presented in Horwill & Robinson (2015) and assuming a first breeding age of 6.6 years (Harris et al. 2016) this would ultimately result in the addition of 30 adults per year to the Forth Islands population.
  7. Assuming an additional 176 pairs of Kittiwakes nest on Inchcolm, and assuming a productivity of 0.83 chicks per pair (JNCC 2021), then an additional 146 fledged chicks would be produced per year. Using the mean annual survival rates presented in Horwill & Robinson (2015) and assuming a first breeding age of 4 years (Coulson 2011) this would ultimately result in the addition of 72 adults per year to the Forth Islands population.
  8. Although these potential benefits are acknowledged, they have not been incorporated within the main compensation benefits tables as they indicate what may be achievable over the longer term following a recovery phase. Currently bird numbers on Inchcolm are very low, and it is acknowledged that recovery may take time. Although it is possible that rats may be the principal reason why bird numbers are so low, and their removal may facilitate a rapid recovery, the uncertainty in this situation is acknowledged and a precautionary approach adopted in the calculation of the potential compensation benefits.
Table 2.5:
Preliminary conservation targets and associated increases for each key species on Inchcolm island. All numbers are expressed as single birds.

Table 2.5: Preliminary conservation targets and associated increases for each key species on Inchcolm island. All numbers are expressed as single birds.

  1. Whilst habitat availability provides a theoretical estimate of the numbers of birds that could physically occupy Inchcolm, it does not capture the influence of the factors listed above. Rat activity on Inchcolm is classified as ‘moderate to high’, the nesting areas are accessible to rats, and on this basis significant benefits of rat eradication are anticipated.
  2. Although Inchcolm supports large colonies of Herring Gull and Lesser Black-backed Gull, which may suppress numbers of Kittiwakes and auks, it is notable that the Isle of May also supports extensive large gull colonies, as does Fidra and Craigleith. However, Inchcolm is located some distance down the Forth Estuary, so it is possible that it may always be less desirable habitat for species that consistently forage offshore, such as Puffin.

2.3.6.    Benefits to other species

  1. Removing rats from Inchcolm could also benefit other birds most notably Fulmar (currently 174 AOS), Shag (27 AON) and Eider (122 AON) (Forth Islands Heritage Group 2021). Other ground-nesting birds known to occur on Inchcolm that would significantly benefit from rat eradication include Mallard Anas platyrhynchos, Shelduck Tadorna tadorna, Oystercatcher Haematopus ostralegus and Wood Pigeon Columba palumbus (Morris 2003). Other species that may benefit to a lesser extent include Rock Dove Columbia livia, and Jackdaw Corvus monedula. A study of islands with and without rats showed that islands with invasive rats had reduced species richness of passerines (Tabak et al. 2015). Therefore, rat eradiation may also offer benefits to the passerine species that are routinely observed on Inchcolm, which include Pied Wagtail Motacilla alba, Dunnock Prunella modularis, European Robin Erithacus rubecula, Blackbird Turdus merula, Chaffinch Fringilla coelebs and Starling Sturnus vulgaris (Morris 2003).
  2. Previously terns are known to have nested in the local area, either on Inchcolm itself or more commonly on the nearby islet of Carr Craig. Common and Arctic terns are also recorded as nesting on Swallow Craig, a small rocky islet in Inchcolm’s harbour. Previously Common Tern Sterna hirundo, Roseate Tern Sterna dougallii and Sandwich Tern Thalasseus sandvicensis have nested on Inchcolm. It is possible that Common or Arctic Terns could return to Inchcolm, potentially recolonising from the Isle of May, if conditions were more favourable. However, the chances of this would be much improved if there was a possibility of maintaining some areas as gull free.
  3. Inchcolm is known to support a number of plants typical of coastal grassland and sand dune habitats. These include Sea Rocket Cakile maritima and various Atriplex species (Morris 2003). Both of these plant species increased significantly following the 1997 rat eradication from Handa (see Section 2.4.2), and it is possible that rodent eradication from Inchcolm could benefit both these, and other plant species.

2.3.7.    Timescale and mechanism for delivery

  1. Full details of how the project could be delivered are included within the Implementation and Monitoring Plan, and only a brief summary is included here.
  2. It is anticipated that the initial rat eradication phase would be undertaken by an eradication specialist during the winter months, with surveillance and seabird monitoring conducted by a member of staff funded by the Applicant, who would also be responsible for implementing incursion response plans should an incursion occur.
  3. Further stakeholder consultation would be required before this specific measure could be implemented, the intention is not to take this measure forward as compensation at this stage. Instead, this measure is being presented as a secondary measure that could be implemented as an adaptive management measure if required post consent.

2.3.8.    Additionality & uncertainty

  1. Inchcolm does not have any type of nature conservation designations. Currently annual seabird counts are conducted by FIHG, a local volunteer group. There are no plans from any conservation bodies to carry out rat eradication from non-SPA islands. Therefore, all the work proposed is additional; there is no other known mechanism whereby any sort of seabird work at Inchcolm would otherwise be undertaken.
  2. The factors that may affect the success of rat eradication are listed in above. It is also acknowledged that the positive response of seabird populations following rodent eradication varies significantly between sites and is difficult to predict, especially with regards to recolonisation of species.
  3. In some locations, birds have not responded as predicted following rat eradication. For example, it was thought that Manx Shearwaters would increase on Canna and Sanday following rodent eradication. These islands are only 4 km from Rum, which holds a third of the global population of Manx Shearwater. However, Manx Shearwater numbers have remained low, despite the increases amongst other species. Similarly, the timescale in which existing species increase is also very variable between sites.
  4. Although Inchcolm has significant restoration potential, it is possible that Brown Rat may colonise from the mainland, as it is within swimming distance. However, the risks can be reduced by adopting a robust surveillance system and incursion response plan.
  5. Inchcolm is one of the few remaining locations in the UK that supports Black Rat. On this basis, there may be some opposition to removing them, as was the case in the Shiants, that was also known to support Black Rat (Mcdonald & Hutchings 1997). However, responses to stakeholder consultation have so far been nearly all positive. Black Rat is not native to the UK, and is widespread throughout its native range in Asia, where populations are stable. It is also commonly encountered across the globe, where it has been introduced and has subsequently successfully colonised. Furthermore, the success of rat eradication from the Shiants provides further support in favour of their eradication from other seabird islands.

2.4. HANDA: RAT ERADICATION & BIOSECURITY

2.4.1.    Site description

Site details and designations

  1. Handa Island is 367 hectares in area and is located ~350 m from mainland Scotland. A map of Handa, showing place names, is included as Figure A10 in the Appendix.
  2. Handa has high Torridonian sandstone sea-cliffs that provide tiered ledges used by a range of nesting seabird species. It is designated as an SPA and at the time of designation in 1990 supported populations of European importance for Guillemot (98,686 individuals – 9.3% of the British population and 2.9% of the North Atlantic biogeographic population) and Razorbill (16,394 individuals – 11% of the British population and 1.9% of the Alca torda islandica population). It also supports nationally important colonies of Kittiwake (10,732 pairs, 2.2% of the British population), as well as several hundred Puffins (735 AOB). The most recent counts show there are an estimated 68,524 Guillemots (individuals), 3,749 Kittiwakes (AON), 5,047 Razorbills (individuals) and 208 Puffins (individuals). These are the key species that the compensatory measure would benefit.
  3. As well as the key species, Handa also supports nationally important numbers of Great Skua Stercorarius skua, which numbered 66 pairs (0.8% of the GB population) at the time of designation in 1990. Since then, numbers increased to 283 pairs in 2018, with numbers in 2022 reduced to just 73 AOT. Northern Fulmar Fulmarus glacialis numbered 3,500 pairs (0.7% of the GB population) at the time of designation in 1990 but has reduced to 1,879 pairs. It also supports breeding Arctic Skua (20 AOT, SWT 2021). Other breeding species include European Shag Phalacrocorax aristotelis, Common Eider Somateria mollissima, Red-throated Diver Gavia stellata, Common Gull Larus canus, Herring Gull Larus argentatus, Great Black-backed Gull Larus marinus, Arctic Tern Sterna paradisaea, Oystercatcher Haematopus ostralegus, Ringed Plover Charadrius hiaticula and Snipe Gallinego gallinago (SWT 2021). Handa also supports a range of maritime grassland and heath vegetation.
  4. Seabird species that have formerly bred on Handa but were thought lost include Common Tern Sterna hirundo (last bred successfully in 2002) and Arctic Tern Sterna paradisaea (last bred successfully in 2015). However, tern chicks and fledglings (most likely Arctic) have been seen in July 2022 for the first time in 7 years. Historically Black Guillemot Cepphus grille once bred on Handa and Harvey-Brown & Buckley (1887) reported that ‘the rats had managed to dislodge them’. Also White-tailed Sea Eagles Haliaeetus albicilla once bred on Handa but have not bred since the 1800s (Harvey-Brown & Buckley 1887), although a non-breeding pair was sighted throughout the 2021 season for the first time in many years (R. Potter, SWT, pers. comm).

Site management

  1. Handa Island is managed by Scottish Wildlife Trust (SWT) and is part of the Scourie Estate. The island is managed by one ranger and six residential volunteers during the summer months.
  2. The ranger and volunteer team carry out bird counts and productivity monitoring, with Guillemot, Kittiwake, and Fulmar monitoring plots counted regularly throughout the breeding period, each year. All island counts of Guillemot, Kittiwake, and Fulmar are conducted every 5 years, whilst all island counts for terns and gulls are every 6 years. All island counts of Shag, Arctic Skua and Great Skua are carried out every 1-2 years. Other routinely recorded information includes dates of first eggs and chicks, breeding and migrant casual bird sightings, records of other notable flora and fauna sighted during the season, as well as monitoring rat activity through observations of the rat chew stations for signs of activity (see next section).
  3. The ranger and volunteer team also hold responsibility for ensuring that the paths around the island and the bothy itself, are maintained. They deal with regular visitor trips to the island, undertaking welcome talks and detailing conservation issues, such as ground nesting birds, fragile habitats and relevant health and safety issues. They are also responsible for producing interpretive material and for fundraising and selling souvenirs, in addition to carrying out school visits and guided walks, although these have not been taking place in recent years. However, the aspiration is to return to undertaking these activities in future years.
  4. Over the 2021 season Handa Island was open from the last week of March until the first week of September, with ferry crossings taking place regularly except during periods of poor weather. The island welcomed 6,661 visitors over the course of the breeding season (SWT 2021). Pre COVID-19 pandemic visitor numbers were just below 9,000 for the years 2017 – 2019 (R. Potter, SWT pers. comm.).

Invasive non-native species

  1. The last permanent human inhabitants of Handa left in 1848, and it is considered likely that Brown Rat Rattus norvegicus was introduced during the period between 1848-1867 when the island was farmed for sheep (Stoneman & Zonfrillo 2005). Rats were blamed for reducing numbers of Black Guillemots and for driving Atlantic Puffins ‘off the tops at Handa into more secure crevices in the face and slopes’ (Harvey-Brown & Buckley 1887). In 1904 Harvey-Brown and MacPherson stated that rats ‘simply swarm over the best ground‘(Harvey-Brown & MacPherson 1904). In 1962 Handa Island became a nature reserve, and annual wardens’ reports show that a population of rats persisted, particularly around the coast and at the bothy, the only inhabitable building on the island. As commensals, it is not surprising that their distribution is associated with human habitation. Although most cliff-nesting bird species were either stable or increasing in the mid-90s, it was felt that rats were inhibiting the expansion of Puffin, and the recolonisation of Black Guillemots. It was also felt that other ground or burrow nesting seabirds such as European Storm Petrel and Manx Shearwater might be able to colonise potentially suitable habitat were rats to be removed (Stoneman & Zonfrillo 2005). A three-year study of rats on Handa was carried out between 1994-1996 (inclusive) to better understand the potential impacts of rats on seabirds (Aragundi 1994). The work concluded that the rat population was almost exclusively limited to the coast and the bothy. The deployment of false nests showed that rats could find and decimate nests of ground nesting birds in any suitable breeding habitat. It was thought that Fulmars were not affected by rats, although it was noted that there were no ground-nesting Fulmars on Handa. It was observed that the distribution of Puffins was definitely limited by rats, with 94% of Puffins nesting on the stacks (Stoneman & Zonfrillo 2005).
  2. Brown rat were originally removed from Handa in March 1997 using bait poisoned with warfarin. The eradication were carried out by 12 people comprising SWT staff, volunteers and two qualified climbers. The climbers delivered and distributed bait to areas otherwise inaccessible without ropes, especially at the base of gullies and above the high tide line where rats were likely to forage. Bait was put down burrows and under rocks to avoid secondary poisoning of birds. Where there were no burrows, bait was placed under weighted fish boxes with the handles sawn out to allow rats to enter. Two tonnes of warfarin were brought to Handa. Following initial deployment, 100 kg was stored on the island for back-up baiting (Stoneman & Zonfrillo 2005). This was used in subsequent years to deal with suspected incursions.
  3. Following eradication efforts, a monitoring programme was devised so that staff could routinely check for further incursions. Chewsticks, consisting of lollipop sticks or wooden spatulas, were saturated in margarine oil and stuck firmly into the ground around a small bamboo marker cane. In later seasons the chew sticks were placed within plastic tunnels (thereby excluding rabbits) and sticks were replaced with wax blocks mixed with cocoa powder. These chew stations were checked monthly throughout the breeding season. Chew marks were routinely detected and there was initial confusion as to whether rats had recolonised or whether the marks were made by young rabbits. It was concluded that if rat droppings were absent then the chewing could be attributed to young rabbits. However, the regular appearance of rat droppings in 2005 (coupled with a two of suspected sightings from visitors) provided robust evidence of an incursion in that year (SWT 2005). In 2006 rat activity was also notable, with a rat observed eating a live rabbit outside the ranger’s accommodation. Tracks and droppings were also found on the boardwalk to the toilets. Rat droppings were also found in skua territories, and in August the chewsticks showed severe chewing along the southern beaches. In total rats were sighted on five occasions by visitors and rangers.
  4. Rat activity was also detected in 2007, with rats sighted on two occasions. Scat was found around the bothy area, and evidence of chewing was again found along the southern beaches (SWT 2007). However, by contrast in 2008 there were no rats, and Arctic terns successfully fledged 37 chicks (SWT 2008). In 2009 Biz Bell (WMIL) visited and the chewsticks were replaced with wax blocks, which are a more effective method of monitoring for rats as they show 3d imprints of tooth marks. The wax blocks were placed in boxes so that gulls and rabbits could not access them. Again, there was no evidence of rats in either 2009, 2010 or 2011 (SWT 2009, SWT 2010).
  5. Rats recolonised again in 2012, with signs of rat around the bothy on the warden’s return. Although six days of trapping were undertaken, only one rat was captured (SWT 2012). In 2013 there was further evidence of a growing rat population. Trapping resulted in the capture of 10 rats. The locations of the first chew marks during 2013 were found at the beach adjacent to the islet Eilean an Aigeich, suggesting that this islet and the rocks between the islet and the mainland acted as ‘stepping-stones’ for the rats. Bait stations were stationed on Eilean an Aigeich but were not checked due to lack of allocated budget.
  6. In 2014 SWT carried out trapping, which resulted in the capture of five rats. An additional two rats were sighted. Of these seven rats, three were sub-adults and three were adults. All were male except one (gender unknown). Tail samples were taken from two and tested for poison resistance, which came back negative. Similar trapping was carried out again in 2016, resulting in the capture of six individuals, four adults and two sub-adults, four of which were male and two of which were female.
  7. In 2015 data analysis was carried out using QGIS and excel to better understand abundance (measured as chew detections) and distribution of the rats. The data are included in Table 2.7, and the maps are included as Figure A9 in the Appendix. From this work it was again concluded that the rats were distributed around the coast and near the bothy.
  8. In 2016 it was recommended that trapping should not be undertaken except where rats became an issue, due to the labour-intensive nature of trapping. Live traps must be checked at least once daily and kill traps should be set at dusk and unset soon after dawn. This proved unworkable due to the other demands on warden time during these periods (SWT 2016). During the period between 2016 and the start of the trial of the A24 traps in 2020 trapping was only undertaken at the bothy due to reasons of hygiene, and only after a rat had been sighted. In 2016 traps were set when two rats were sighted around the bothy, both of which were trapped and humanely dispatched (one male, one female, both adults). Eight traps were periodically laid around the bothy throughout the season, but no further rats were seen or caught at this location. No non-target species were caught either. Sightings of rats were noted by wardens periodically on the Flagpole Peninsula and at Boulder Bay, however no trapping was conducted (SWT 2016).
  9. Over the winter of 2016/17 there was a very substantial increase in mean seasonal rat activity, which was calculated as 11 times higher than in 2015, and about six times higher compared to 2016 (Table 1 below, SWT 2017). Kill traps were set around the bothy on multiple occasions but always following a sighting, in response to the recommendations made in 2016. A total of seven rats were caught. The traps were set at dusk and unset in the morning to reduce the chance of by-catch. All people staying in the bothy were warned and shown the location of the traps when they were set. Peanut butter was used as bait and renewed when necessary and traps safely secured in place. The five rats trapped from June onwards were all found between the woodshed and the plantation. All of them were males, mostly adult. The rats were dissected, and vital statistics recorded with all rats found to be in good body condition and on one occasion feathers were confirmed in the stomach contents (SWT 2017). The step increase in rat activity suggests a discrete migration event over the winter/early spring of 2016/17.
  10. In 2018 rat control was only carried out around the bothy, mainly for reasons of hygiene. Kill traps were set around the bothy on multiple occasions and always following a sighting, catching 12 rats in total. Vital statistics were recorded for 11 of the 12 rats trapped, and five of them were also dissected, with all deemed to be in good body condition and none with feathers in their stomachs. One of the dissected females was pregnant with 9 foetuses (SWT 2018).
  11. Since 2018 rat activity appears to be gradually declining, albeit from a very high level. In 2019 the mean numbers of bait stations chewed by rats increased by 2.7%. However, trapping around the bothy (in response to sightings and primarily for reasons of sanitation) was undertaken almost every night throughout the period the wardens were in residence. A total of 24 rats were caught, and of these 22 were dissected, with all deemed to be in good body condition. One dissected rat was found with small feathers in the stomach and two hard small hard pieces that were suspected to be eggshell (SWT 2019).
  12. Rat activity on chew stations reduced slightly in 2020 compared to 2019 (Table 2.7). Similarly nightly trapping around the bothy area, resulted in a much smaller number of rats being captured (six only) indicating that levels of rat activity were lower. However, a decision was made in July to stop trapping around the bothy due to a blackbird being killed in one of the traps. Instead purchase of traps with longer tunnels was recommended. Three of the rats caught were dissected, but none contained bird feathers (SWT 2020). From 21 March to 6 July 2020 Handa was closed to the public due to the national COVID-19 pandemic lockdown. Trips re-opened operating from 6 July to 5 September 2020. It is possible that the slight decrease in rat activity may be due to decreased human activity, as rats are a commensal species.
  13. During the winter of 2020, commencing in October, the Biosecurity for LIFE started a trial of A24 traps on Handa. The project, which will run until 2023, involves trialling the use of 80 self-resetting Goodnature A24 traps to control (but not eradiacte) rats. The A24 traps can re-set up to 24 times meaning that they can be left out during the winter period when accessing the island is difficult. The traps work with a lure specifically designed for rats. The rats enter the chamber to investigate the lure triggering the trap. The body then falls out to be removed by scavengers. The traps do not use poison or other toxic substances.
  14. In addition, seven transects were designed across the island each containing 10 tunnels set at 50 m intervals to monitor the abundance of rats using prefabricated tracking cards. The cards would be placed in each of the 70 tunnels one day, left overnight and collected the following day. The amount of tracking cards that showed rat activity in the form of footprints or scat would be recorded. Six trail cameras were provided by Biosecurity for LIFE, along with the four already on Handa. These ten cameras were deployed to monitor traps in the vicinity of the skua colony to record potential interactions with non-target species.

 

Table 2.6:
Percentage of chew blocks showing signs of rats (chewed/gone) across all monitoring bait stations by month, Handa Island 2015-2021.

Table 2.6: Percentage of chew blocks showing signs of rats (chewed/gone) across all monitoring bait stations by month, Handa Island 2015-2021.

 

Source: SWT 2021

  1. Although A24 traps have been used effectively in New Zealand, on retrieval in March 2022 it appeared as though they had not re-set. The reasons for this are currently unclear but will be investigated over the coming months (R. Potter, Reserves Manager SWT, pers. comm.). Rat monitoring in 2022 revealed relatively little rat activity early season. This is unusual (see Table 2.7) as typically more rats are detected in the early Spring when the natural food supply is less abundant (R. Potter, SWT, pers. comm.). The reasons for this are unclear, although later visits confirmed increased levels of rat activity.
  2. As well as rats Handa Island supports a population of European Rabbit. Historically numbers have varied: rabbits were absent in the late 1990s, but they are now so extensive that control was considered difficult and expensive (SWT 2021). However, since rabbits are not predatory their impacts on seabirds are less clear.
  3. In 2002 the presence of a ‘mystery mammal’ was identified through the appearance of scat. The scat was sent off for identification, and although expert consensus differed, it was generally agreed to be from European Hedgehog Erinaceus europeaus (SWT 2002). Hedgehogs, although unlikely to access sheer cliff faces, can predate the eggs of ground nesting birds such as Puffins, terns and skuas. The hedgehogs occurred again in 2003, with scat occurring in various locations. In September smaller scat was also found too, suggesting that they may have bred (SWT 2003). In 2004, it was advised by Mick Blunt of the Uist Wader Project that systematic large-scale trapping should be undertaken as a matter of urgency. In 2005 the situation become more confusing; there was a lot of scat, some of it from rats and some from hedgehogs (SWT 2005). Two volunteers spent 35 days trapping, lamping and collecting scat. Trapping was undertaken using 13 mink traps in 16 different locations and baited with cat food, mackerel or pierced eggs. A total of 735 trap nights and 22 hour 35 minutes of lamping gave no returns. However, in 2004 a hedgehog was sighted, so this and positively identifiable scat confirm that both rats and hedgehogs were present on Handa in 2005. Hedgehogs were also sighted in 2007, although there were no signs of hedgehog in 2008, 2009, 2010 or 2011. In 2012 Hedgehog scat was found again, although has not been sighted since.
  4. Other invasive mammals that have occurred on Handa include a stoat Mustela erminea in 2008 and in 2020, and an American Mink Neovision vision in 2021. The latter was caught using a mink trap and humanely despatched. There is also considered to be a risk of incursion by House Mouse Mus musculus, though this has never occurred. Even with dedicated effort, maintaining Handa free from invasive mammal species has been, and will continue to be, a significant challenge.

Population trends of key species

  1. Some background information is required on the population trends of the key species on Handa, so that and the benefits of rat eradication in 1997 can be established. This section describes population trends on Handa relative to national trends for Scotland and other relatively local sites.
  2. Many species have been experiencing declines associated with mass failure of sandeel prey during 2004-2008, with recent years showing some degree of recovery.
  3. Although Handa is located in The Minch, the population trends of many species appear to match the more severe declines associated with sites in Shetland and Orkney. It is possible that this is because of its geographical location well to the north of the other Minch sites such as Canna and the Shiants, with which it might otherwise be intuitively compared. However, otolith analysis suggests that the sandeels around Handa share a natal signature with those around Orkney, potentially reaching Handa via larval drift (Gibb et al. 2017), which would account for the severity of the declines Handa has experienced in recent decades, which have occurred despite rat eradication efforts.
Kittiwake
  1. JNCC SMP (Seabird Monitoring Programme) data indicates that Kittiwake numbers increased by around 24% between the late 1960s and the mid-1980s possibly due to the cessation of egg hunting (Cramp et al. 1974). However, catastrophic declines followed at least from 1986 and there are now around 50% fewer birds than in the late 1960s (JNCC 2021). Changes in the marine environment due to warmer sea temperatures, specifically the decline in the abundance of sandeels is thought to be responsible (Arnott & Ruxton 2002). Over the past decade the trend of decline has halted, and there are signs of recovery (JNCC 2021). The data from Handa, taken from counts carried out regularly at designated monitoring plots, reflects these national trends (Figure 2.5).
  2. However, different colonies have declined at different rates with the more northerly colonies in Scotland suffering the greatest declines in the UK. The recent 2018 seabird census survey showed that Kittiwake numbers on Handa had declined by 47% from 7,013 AON in 1999 to 3,749 in 2018. Historic data for the site shows that numbers peaked in 1977 when there were 12,500 AON. Comparison with other islands in The Minch, namely Canna, the Shiants and Mingulay and Berneray showed that other colonies have similarly declined in recent times. Kittiwakes on the Shiants declined by 46% and Kittiwakes on Mingulay and Berneray declined by 70%. The only exception locally was Canna, where Kittiwakes increased by 14%. It is of note that Black rat Rattus rattus was removed from Canna between 2005-2008, which may be the reason for the observed increases. Although rats were also removed from the Shiants, this did not happen until 2015 (the year of the JNCC survey for this site), so the benefits of rat eradication are not reflected in these figures. Although rats were removed from Handa in 1997, they had recolonised by the time of the survey in 2018. It is possible that eradication of rats from Handa may have reduced the rate of decline experienced by Kittiwake, although there is considerable variation in the rates of decline across other local sites. Comparison with the Orkney sites indicates that declines were generally more severe with declines of 47% at West Westray Cliffs, 78% at Copinsay and 84% at Marwick Head (JNCC 2021).
  3. Despite complete breeding failures between 2006-2008, average productivity for Kittiwake at Handa is higher than anticipated at 0.99 chicks fledged per pair, in comparison with the national average 0.83 chicks per pair for Scotland from the same time period (Figure 2.6 below, JNCC 2021). On balance it does not appear that the Kittiwakes at Handa have fared as badly as at other sites. Whether this may be attributable to biosecurity efforts is difficult to prove due to the potential influence of many other factors, but further discussion of available evidence is provided in Section 2.4.6.
Guillemot
  1. The Seabird 2000 Census showed that there is a general trend of increase in UK Guillemot populations since the mid-1980s to the present day. However, Scotland differs in that Guillemot numbers increased between 1969/70 and 2000, and then fell, although there is some evidence of recovery in recent years (JNCC 2021). Guillemot numbers at Handa declined from 112,767 individuals in 1998 to 54,664 in 2016, a decline of 51% (Figure 2.7 below, JNCC 2021).
  2. During the same period other Guillemot colonies in The Minch have also declined: Guillemots on The Shiants declined by 45% and Guillemots on Mingulay and Berneray declined by 32%. Declines in the Orkney colonies were more severe with declines of 58% at West Westray Cliffs, 65% at Marwick Head and 59% at Hoy (JNCC 2021). Although productivity was poor during the period between 2005-2008, coinciding with sandeel shortages, it has generally improved more recently, although at some sites numbers still remain low.
  3. For example, at Shetland productivity between 2011-13 was only 0.14 and at Orkney 0.33. Productivity at Handa was better at 0.68, in line with colonies in NW Scotland (0.58) and SE Scotland (0.70) (SWT data, JNCC 2021). Although productivity is encouraging, the scale of declines associated with sandeel failure is concerning, despite evidence of some recent recovery (Figure 2.7).

Please note that this graph shows population trends based on nest counts taken from regularly visited monitoring plots.  It does not represent total Kittiwake numbers on Handa. Source: SWT 2021

Figure 2.5:
Population trends of two cliff-nesting bird species on Handa Island, 1995-2021

Figure 2.5: Population trends of two cliff-nesting bird species on Handa Island, 1995-2021

Source: SWT 2021

Figure 2.6:
Productivity of three cliff-nesting bird species on Handa Island, 1995-2021

Figure 2.6: Productivity of three cliff-nesting bird species on Handa Island, 1995-2021

Please note that this graph shows population trends based on nest counts taken from regularly visited monitoring plots. It does not represent total Guillemot and Razorbill numbers on Handa. Source: SWT 2021

Figure 2.7:
Guillemot and Razorbill population trends, Handa Island, 1996-2021

Figure 2.7: Guillemot and Razorbill population trends, Handa Island, 1996-2021

Source: SWT 2021

Figure 2.8:
Razorbill population trends based on all island counts, Handa Island, 1996-2021.

Figure 2.8: Razorbill population trends based on all island counts, Handa Island, 1996-2021.