Scottish Biodiversity Indicator - The numbers and breeding success of seabirds in Scotland 1986-2017

Scotland’s breeding seabirds are of international importance. Seabirds are useful and important indicators for assessing the state of the marine environment. Although they rely on the sea, breeding on land makes them relatively easy to monitor. They respond to a range of factors such as changes in food availability, weather, predation and pollution. Being relatively long lived the numbers of breeding seabirds usually changes slowly over time. Breeding success (the number of chicks produced) provides an indication of food availability (generally fish and other marine species) and other factors such as predation during the breeding season. Breeding success typically varies far more from year to year than breeding numbers.

Evidence

The indicators of breeding numbers for 11 speciesⁱ, and breeding success for 12 speciesⁱ, out of the 24 seabird species that breed in Scotland, are based on estimates from a representative sample of seabird colonies around Scotland. These are monitored as part of the UK Seabird Monitoring Programme, which started in 1986.

The numbers and breeding success of seabirds in Scotland, 1986-2017

Commentary

Included in this indicator are 16ⁱ of the 24 species of seabird that regularly breed in Scotland. All are either Red (European shag; Arctic skua; Atlantic puffin; black-legged kittiwake; and herring gull) or Amber listed as Birds of Conservation Concern (Eaton et al., 2015).

Sandwich tern has been removed from this update due to only one colony contributing data for the whole of Scotland. Out of the 11 species assessed for breeding numbers, Arctic skua has experienced the largest declines (78%). The Northern Isles are their breeding stronghold where a reduction in the availability of sandeels, has contributed to declines in kittiwake and tern populations, which Arctic skuas obtain their food from. Increased predation from great skua has also been linked to their decline (Perkins et al. 2018). Some species trends, although lower, appear to be stabilising possibly at a new level which differs from the 1986 baseline. Foster et al. (2017) showed a link between fishery landings and gull numbers, which led to steep declines and a stabilising of numbers on the Isle of Canna. However, when compared with information from the 1930s, the lower numbers of gulls were more typical than the peaks observed in the mid-1980s. Common tern numbers remained high following on from 2016, like many colonial nesting terns they can respond to favourable breeding conditions rapidly resulting in higher numbers at breeding colonies in some years.

Breeding success across the time series varied for the 12 species assessed (Table 1). It was above the long-term average for black-legged kittiwake and herring gull. Arctic skua; Atlantic puffin; common tern; great skua; little tern and northern fulmar had lower breeding success. All other species were around the long-term average.

Source data and updates

The indicator has been updated with data up to and including 2017. The trend for breeding numbers is based on 11 species and the breeding success trend on the 12 species for which sufficient data are available. Data are derived from the UK Seabird Monitoring Programme (SMP) and its partners. The SMP collects and collates data on breeding numbers, demographic parameters (including breeding success and survival rate), and diet from a sample of colonies around the UK. The data confidence has been reduced to medium as it is over fifteen years since the last national census. The census provides an accurate snapshot of seabird breeding numbers and allows us to benchmark the annual colony counts.

Updates are provided annually and the data are available from JNCC.

UK Indicators

JNCC reports on seabird population trends as an Official Statistic. As Scotland supports the majority of UK breeding seabirds, the UK trend is greatly influenced by trends in Scotland. Seabird abundance in the UK increased between 1970 and 1999 and has since declined.

References

• Eaton, M., Aebischer, N., Brown, A. et al. 2015. Birds of Conservation Concern 4: the population status of birds in the UK, Channel Islands and Isle of Man. British Birds, 108, 708-746.
• Foster, S., Swann, R.L., and Furness, R.W. 2017. Can changes in fishery landings explain long-term population trends in gulls? Bird Study, 64, 90-97.
• Perkins, A, Ratcliffe, N, Suddaby, D, et al. 2018 Combined bottom‐up and top‐down pressures drive catastrophic population declines of Arctic skuas in Scotland. J Anim Ecol. 2018; 87: 1573– 1586

Table 1: Breeding success of seabirds in Scotland 1986 to 2017

Seabird	Average Breeding Success (1986 to 2016)	2017 Breeding Success
Arctic Skua	0.60 (±0.10)	0.15
Arctic Tern	0.11 (±0.04)	0.12
Atlantic Puffin	0.55 (±0.05)	0.47
Black-legged Kittiwake	0.56 (±0.07)	0.65
Common Tern	0.47 (±0.06)	0.40
Great Skua	0.51 (±0.09)	0.33
Guillemot	0.57 (±0.06)	0.54
Herring Gull (natural-nesting)	0.58 (±0.05)	1.01
Little Tern	0.29 (±0.09)	0.13
Northern Fulmar	0.46 (±0.03)	0.41
Northern Gannet	0.69 (±0.06)	0.70
Sandwich Tern	0.25 (±0.06)	0.26

 

ⁱ Data available for 1 = Breeding Numbers 2= Breeding Success  

Arctic skua^1,2; Arctic tern^1,2; Atlantic puffin²;black guillemot¹;black-legged kittiwake^1,2;common gull¹;common tern^1,2;European shag¹; great black-backed gull¹; great skua²; common guillemot^1,2; herring gull^1,2;little tern²; northern fulmar^1,2; northern gannet²; Sandwich tern²

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Indicators and trends

Published: February 2020