DURING the breeding season, it became clear that many of Shetland’s seabirds were having poor breeding success. In the recent edition of the Shetland Bird Club newsletter Martin Heubeck, chairman of the club and SOTEAG ornithologist, produced a report which compiles and analyses data and observations on breeding seabirds from around Shetland.
In general, breeding success was patchy, both between species and in comparing different areas. However, low food availability in May and June led to late egg laying, a relatively high incidence of non-breeding pairs and low hatching success. Although conditions improved a little by early July, the increase in food was too late for most species.
Fulmars averaged 0.32 chicks fledged per occupied site – a low average rate. Arctic skuas fared worse – they arrived late, many pairs did not lay eggs and only three chicks are known to have fledged throughout the whole of Shetland. Kittiwake numbers have been in decline for several years and only 60-80 per cent of nests went on to the egg-laying stage with a low proportion of hatching chicks (14-41 per cent). After the first week of July, chick survival rates were relatively high but by mid-July most breeding attempts had failed. On Foula only one kittiwake chick hatched. Arctic terns fledged only a handful of young, arriving back late and abandoning most of their colonies by early June.
Guillemot breeding success continued the decline first detected in 2001 with extensive non-breeding and long periods of incubation leading to eggs being abandoned. Chick loss was also high. There was no specific data on razorbill breeding success but numbers of adults decreased at some study plots. Puffins were not monitored but observers at Hermaness noted chicks coming to burrow entrances during the day in early July, presumably driven by hunger, and being predated by great skuas. Chicks were still being fed in late July at Sumburgh and Foula, reflecting a late breeding season. Tystie breeding success was not monitored but pre-breeding numbers were higher than last year at 10 out of 13 monitoring sites, with the largest increase on Mousa.
Red-throated divers feed inshore on fish and this species also had a very poor breeding season. Many pairs did not attempt to lay and others lost clutches, with relatively few young fledging overall. Great skuas had a more moderate breeding season although numbers and breeding success varied with the highest success rate on Foula, where the chicks were fed mainly on mackerel and there was less cannibalism than in previous years. In general, the great skuas were probably also helped by the abundance of rabbit prey and carrion available.
Shags also did relatively well, with up to 92 per cent of pairs laying at Sumburgh Head, and a high overall proportion fledging broods of three or four young. Although gannets laid several weeks later than average, the colony at Noss has increased 13 per cent since 2003 and enjoyed a high breeding success rate of 0.81 chicks fledged per nest. Breeding success was lower at Hermaness, but gannets can range considerable distances to forage and catch a variety of fish prey which buffers them against the effects of food scarcity which is affecting other species.
Climate and oceanographic changes are the main factors involved in creating food shortages in the North Sea, and the impact on the Shetland colonies is all too evident now.