Tagging and Tracking of Seabirds on the West Coast of Scotland

5. Discussion

5.1. Numbers of Storm-petrels tracked and evidence of impacts on welfare and foraging behaviour

Despite restrictions on fieldwork imposed by the Covid pandemic in 2020 and the outbreak of HPAI in 2022, the extended project achieved the stated objectives of tracking a minimum of 10 individuals of breeding Leach’s and European Storm-petrels annually with GPS tags (except for European Storm-petrels in 2024), and the total numbers of individuals tracked across the three years of fieldwork (50 Leach’s and 38 European Storm-petrels) considerably exceeded the initial objectives to track a total of 20 individuals of each species during the course of the initial project (10 birds a year for two years). There was no evidence of any impacts of GPS tagging on either species. There were a few occasions when birds returned to the colony during the egg stage but did not enter the nest to resume incubation which may have resulted from nocturnal fieldwork activity in the colony.

In contrast to GPS tracking, the use of ring-mounted GLS tags to obtain information on migration and wintering areas was not successful. The extremely low tag retrieval rates (1/20 and 0/20 for Leach’s and European Storm-petrels respectively) may have been exacerbated by the inability to attempt recaptures in the year following deployment. Due to fieldwork restrictions on all seabird research imposed by NatureScot as a result of HPAI in 2022, we were unable to attempt retrieval of tags deployed in July 2021 until July 2023. However, the recapture of appreciable numbers of control birds (11/20 Leach’s Storm-petrels and 7/22 European Storm-petrels) and the recapture of two Leach’s Storm-petrels that had suffered leg injuries and lost their GLS tags provides strong evidence that GLS tagging was the principal cause of the low recapture rates. Other researchers have encountered similar problems with the use of ring-mounted GLS rings, which are likely to be due to the use of rings that are slightly too close-fitting. As a result of work conducted during the course of this project, recommendations have been made to all those working on Storm-petrel species globally concerning the size, design and methods of fitting ring-mounted GLS tags. As a result of the development of best-practice procedures, it is likely that GLS tracking of Storm-petrel species in UK will be permitted to resume in future projects.

5.2. GPS tracking of Leach’s Storm-petrels breeding on St Kilda

A total of 78 complete foraging trips were obtained from tracking 50 individual Leach’s Storm-petrels across three breeding seasons (2021, 2023 and 2024). The GPS data revealed a highly consistent pattern of space use across breeding stages (incubation and chick-rearing) and years (Figs 18 – 20). Comparison of within-individual and among-individual space use provided no evidence of individual specialisation of foraging areas: the overlap in core space use among trips by the same individual was similar to the extent of overlap across individuals. The raw tracks and resulting utilisation distributions revealed extensive and consistent use of deep water to the North and northwest of St Kilda, with birds travelling an average of 220 - 360 km from the colony (dependent on year and breeding stage, maximum range recorded = 635km), on trips that lasted 1 - 5 days, and covered a total of 209 – 2,270 km. There was an indication that chick rearing Leach’s Storm-petrels exhibited a dual foraging strategy, as observed in other procellariiform species (Phillips et al. 2023). Some birds (such as BT27476 from nest 45 in 2023, Figure 14 and NT42983 from nest 144 in 2024, Figure 16) alternated shorter duration, short-range trips (presumably to less productive areas closer to the colony), to provision the chick more frequently but at the expense of their own body condition, with longer-duration longer range trips (presumably to more productive foraging grounds), to replenish their own body condition.

Modelling of marine distribution using a step-selection function confirmed the selection of deep water (peaking at around 1,500m) and higher sea surface temperatures during daylight. In this case, lower SST values were more common in the northern part of the species foraging range for example the lowest SST values we observed < 12°C occurred exclusively above 58.75 decimal degrees north. The daytime marine distribution of Leach’s Storm-petrels breeding on St Kilda matches closely the distribution reported from extensive boat-based surveys conducted between 1979 and 1999 in the seas to the northwest of Scotland (Bloor et al. 1996, Pollock et al. 2000). Those studies reported highest abundances over the Wyville-Thompson Ridge and the Faroe-Bank Channel where they may feed largely on plankton, particularly the euphausiid shrimp Meganctiphanes norvegica and the copepod Temora longicornus (Cramp and Simmons 1977). Given the regional dominance of the breeding colony on St Kilda (both in the 1990s and currently) the vast majority of Leach’s Storm-petrels recorded on boat-based surveys in the seas to the northwest of Scotland are likely to originate from St Kilda. The Faroes Islands are believed to host around 1,000 pairs (Frederiksen 2010) and The Flannans and North Rona hold a total of around 1,000 – 3,000 pairs (Mitchell et al. 2004, Burnell et al. 2023). Bloor et al. (1996) and Pollock et al. (2000) both observed that Leach’s Storm-petrels commute between the feeding areas and the breeding colonies during the hours of darkness, which accounts for the absence of daylight records from the vicinity of St Kilda. Here we are able to report for the first time the marine distribution of Leach’s Storm-petrels during the hours of darkness, demonstrating high densities of birds in the shelf seas surrounding, and especially to the NW of, St Kilda (Figure 21C). It is notable that we also found high densities of birds during darkness around the Ymir Ridge, to the South of the Faroes Bank, reflecting birds undertaking multi-day foraging trips remaining in the feeding areas at night. Despite travelling very large distances, and roaming over a vast area (c. 164,000 km², more than twice the land area of Scotland), we found no overlap with any areas leased for windfarm development, due to the highly directional, and entirely pelagic distribution of Leach’s Storm-petrels. The contrast in the extent of terrestrial habitat required for nesting, and marine habitat accessed for foraging is striking. The study plot within which the 50 tracked birds nested was approximately 330 m² in extent. This is an area 500 million times smaller than the extent of marine habitat accessed for foraging.

5.3. GPS tracking of European Storm-petrels breeding on the Treshnish Isles

A total of 48 complete foraging trips were obtained from tracking 38 individual European Storm-petrels across three breeding seasons (2021, 2023 and 2024). The GPS data revealed a variable pattern of space use across the three years (Figures 29 - 31). In contrast to Leach’s Storm-petrel, comparison of within-individual and among-individual space use indicated a high degree of individual specialisation: the overlap in space use among trips by the same individual was considerably greater than the overlap across individuals. As a consequence, multiple trips by the same individual cannot be regarded as independent.

Whilst there were considerable differences in foraging areas among the three study years, the raw tracks and resulting utilisation distributions revealed a usage pattern that was generally restricted to shelf (neritic) seas within 140 km, and to the West, of the colony. However, in 2023 some birds roamed considerably further (up to 480 km distant from the colony), and in a wider range of directions, extending well beyond the continental shelf to the West of the Hebrides, to the North of North Rona, and into the Irish Sea South of the Isle of Man. In 2023, birds were very light, and chicks poorly fed, indicating that food availability was low. In 2024, when only seven birds were tracked, most birds remained fairly local to the colony, and used the same area to the West of Treshnish (used extensively in 2021) but a few individuals roamed more widely, towards the shelf edge and into the Irish Sea. Had a larger sample of individuals been tracked, it is possible that more extreme movements may have been recorded. It is notable that in 2023 and 2024 extensive usage of inshore waters occurred, with birds visiting coastal areas (of Lewis, Skye, Rum, Coll, Tiree, Jura, Islay, N. Ireland and The Republic of Ireland) at night. The significance of this behaviour is unclear, but European Storm-petrels are known to feed on intertidal isopods at night (D’Elbée & Hémery 1998). It is possible that inshore feeding on isopods reflects a scarcity of high-value prey in offshore areas.

Modelling of marine distribution using a step-selection function demonstrated a clear negative association between habitat use and distance from the colony during daylight hours, reflecting lower usage relative to habitat availability at greater distance from the colony. In contrast, during the night, although the association between habitat use and distance from the colony was negative for distances up to 25km (Fig. 33), beyond this range the relationship was less well defined, with increasing confidence limits at greater distances from the colony.. Within our dataset, negative associations between habitat usage and distance from the colony may be easier to detect during daylight due to the greater number of observations obtained during daylight hours. In contrast, during the night, when birds may travel back toward the colony, we recorded fewer observations at greater distances, making it more difficult to precisely estimate habitat usage in those areas. This challenge is further compounded by the short nights in summer, which resulted in fewer night-time observations overall. The reduced precision in estimating nocturnal habitat usage is reflected in the relatively wide 95% confidence intervals observed at distances greater than ~200 km from the colony seen in Figure 33. We also found evidence of a slight peak in usage of areas approximately 100 km from the colony. The reason for this peak is unclear, though it may represent birds utilising an important area for nocturnal foraging. Supporting this, UDs based exclusively on night-time GPS observations highlight an area of relatively intense space use (based on the 25% UD contours) to the south of the Outer Hebrides (Figure 32), which falls within the relevant distance from the colony. This area is also located near the boundaries of the Stanton Banks MPA which encompasses a series of granitic ridges rising from 190 m to within only 30 m of the sea surface and is a hotspot for other marine predators as well as European Storm-petrels (Sheehan & Power 2022, Pommier et al. 2023).

We also found a positive relationship between habitat usage and seabed slope. However, as birds were largely restricted to neritic waters a preference for greater seabed slopes largely refers to the environment experienced within the continental shelf rather than birds targeting the continental shelf break itself. For example, only a few birds were seen foraging beyond the shelf break and therefore we had relatively few observations where birds would experience the higher slope gradients associated with this habitat feature.

Habitat usage was negatively associated with both chlorophyll gradient and NPP. Regarding chlorophyll gradient the negative association suggests that European Storm-petrels were not selectively using areas closest to chlorophyll fronts (which are often co-located with thermal front gradients, Belkin and O’Reilly 2007, Xia et al. 2021). Negative associations between habitat usage and NPP may arise in part because higher NPP concentrations tend to occur close to the coast, which is avoided during daylight. Moreover, closer examination of the data revealed that the majority of observed tracking locations characterised by lower NPP values < 8.1 mg/m³/day were observed in 2021 (624 / 675 filtered locations) with most of those clustered in or around the Stanton Banks MPA where NPP values were relatively low in 2021.

More broadly, it should be borne in mind that currently our assessment of habitat selection represents a population-average response and assumes individuals respond in the same way to the environment. However, it is clear from studies on other seabirds that averaging across individuals can mask important levels of individual variation particularly if some individuals show divergent responses to the same covariate (Trevail et al. 2021, Cleasby et al. 2024). Therefore, a potential expansion of the current modelling work may be a greater consideration of such individual variation, especially considering the degree of individual specialisation we observed for European Storm-petrels.

Previous studies to map the distributions of European Storm-petrels in the waters of the West of Scotland (e.g. Pollock et al. 2000, Waggitt et al. 2020) have not shown clearly demarcated concentrations that might correspond to foraging aggregations of birds from specific colonies. Rather they have tended to find a changeable distribution during the course of the breeding season from May to September, which may reflect the influx of very large numbers of wandering immature birds from July. For example, Pollock et al. 2000 found highest densities of European Storm-petrels in June - August around the Wyville-Thompson Ridge, which is probably too remote from colonies in the Hebrides to be an area used extensively by breeding birds. Similarly, Waggitt et al. 2020 present modelled distributions derived from a large number of at-sea surveys, and show highest densities close to the edge of the continental shelf.

Despite their small size, European Storm-petrels breeding on the Treshnish Isles travelled huge distances and roamed over a vast area (c. 40,000 km², more than half the land area of Scotland). There was considerable overlap with areas leased for windfarm development. Despite the very short duration of tracking of each individual (range 1 – 6 days), 19 of the 38 tracked birds (50%) used at least one of the areas leased for windfarm development. Across all tracked individuals, 3.5% of the distance travelled occurred within lease areas (range 0 – 28.8%). The vast majority (18/19) of individuals that accessed a lease area occurred within the Machair Wind Area, with another bird occurring within the Talisk Area to the North of Lewis. One of the individuals that occurred within the Machair Wind Area also visited the Malin Sea Wind Area.

5.4. GLS tracking of a single Leach’s Storm-petrel breeding on St Kilda

The single Leach’s Storm-petrel tracked from July 2021 to March 2022 using a GLS tag indicated that it departed the colony in mid September, suggesting it bred successfully. It migrated southward following a route to the West of Ireland, rather than through the Irish and Celtic Seas, and then migrated fairly rapidly southward, pausing for a few days to the North of the Canaries, and then heading rapidly past the coast of Mauritania, following the West African coast to the Benguela current, where it spent a month (December) before heading West to waters around St Helena and Ascension. The tag battery failed in March as the bird was returning northward in the mid-Atlantic. This represents the first information on the migration of Leach’s Storm-petrels breeding in UK – there have been no overseas recoveries of any ringed birds from UK colonies. Tracking of 13 individuals from colonies in Canada (Pollet et al. 2019) revealed a trans-Atlantic migration route, to the coast of West Africa, with similar stop-over areas in the Benguela current and the mid-Atlantic around the equator, and additional stop-overs off Mauritania. It seems likely that there is mixing of breeding populations from both sides of the North Atlantic in the migration staging and wintering areas.

5.5. Assessment of HPAI impacts

There was no evidence of any occurrence of HPAI in populations of Leach’s Storm-petrels breeding on Hirta, St Kilda, or European Storm-petrels on Lunga, Treshnish Isles. The very limited number of blood samples taken in 2023 for HPAI antibody assay (n=8 Leach’s and n=1 European Storm-petrels) tested negative for antibodies to any avian influenza virus. We encountered no dead Storm-petrels of either species during the course of fieldwork in 2023 or 2024. One dead Leach’s Storm-petrel was encountered in 2021 (before any outbreak of HPAI among seabirds on St Kilda) at the entrance of a very wide burrow, which was likely shared with, and possibly killed by, a puffin. Whilst we did not undertake any systematic population counts during the course of this project, there was no indication that breeding densities of either Storm-petrel species in our study plots were lower in 2023 or 2024, compared with 2021. Burrow densities of Leach’s petrels in the study area on Hirta appeared higher in 2024 than in 2021.