Canna seabird studies 2003
Swann RL (2005) JNCC Report 361, ISSN 0963-8091
- Three visits were made to Canna during 2003 to count and ring seabirds, monitor their breeding success and collect food samples.
- The timing of the 2003 breeding season was slightly earlier than average for razorbills and European shags.
- Counts showed that many seabird species are undergoing major declines on Canna. This trend continued in 2003 with European shag, herring gull
and mew gull reaching record lows. Common guillemots and both lesser and great black-backed gulls also declined. Northern fulmar numbers
showed a slight increase as did black-legged kittiwakes, which reached a new record high.
- Breeding success was again exceptionally low in European shags, herring and great black-backed gulls, in which mass failures occurred; for
northern fulmars and black-legged kittiwakes it was above average.
- Totals of 451 fully grown and 2,664 pullus seabirds were ringed with BTO metal rings and 1,207 fully grown birds were retrapped in breeding
- Retrapping of adult common guillemots resulted in 322 birds that were ringed as chicks being located back in colonies for the first time. Two
3-year olds, nine 4-year olds and 24 5-year olds were amongst those caught. The 1996 cohort, of which very few were reported as dead in
their first year of life, showed high return rates. Thirty three razorbills and eight European shags that had been ringed as chicks were
also retrapped in colonies for the first time.
- Fifty-six fish were collected from adult common guillemots. Sprats Sprattus sprattus dominated in number (50%), followed by Gadidae (41%) and
sandeels (9%). Sprats were not significantly different in size from the long-term average. The Gadidae were all whiting Merlangius
merlangus, bar one Trisopterus sp.. European shags fed mostly on sandeels in early July, but on gadoids in late July. Black-legged kittiwake
samples were dominated by 0-group sandeels, with a few gadoids and sprats.
- Predation continued to be a major phenomenon in seabird colonies on Canna, affecting breeding success and overall numbers attempting to breed.
Brown rats Rattus norvegicus were implicated in these declines, particularly with regard to the extinction of the Manx shearwater colony.
This has led in the last few years to devastating declines in both numbers and breeding success in the mixed seabird colonies at the
Nunnery, Lamasgor and Garrisdale. In 2003 there were signs that rat predation was beginning to effect the large colony at Geugasgor on the
north side of the island. Depredated auk and shag eggs were found under boulders and there was much evidence of seabirds switching nest
sites from below boulders to more open sites. This is a feature we have noted at all colonies on the island affected by predation and
appears to be a response to reduce the effect of predation by mammals. Birds were also involved in these high levels of predation:
black-legged kittiwake breeding success on Sanday appeared to be affected by predation from great skuas and peregrine falcons, whilst gulls
depredated shag nests and both young and adult auks at Geugasgor.
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