Isle of May seabird studies in 1999 2000

Abstract

This report covers the breeding season for seabirds on the Isle of May in 1999. In general, 1999 was a poor season for Isle of May seabirds, with many species breeding late and/or unsuccessfully. Only fulmars (and terns which are not included in this monitoring scheme) had a good breeding season.

Shags had a poor year. Many failed to breed and those that did laid late. Overall breeding success (0.33 chicks per incubated nest) was low. Breeding of kittiwakes was late, about 17% of pairs did not complete a nest and many pairs, which did breed, failed during incubation. The average breeding success of 0.20 chicks per completed nest was an order of magnitude better than it was in 1998. However, far too few young are now being reared to keep the Isle of May population stable. Guillemots, razorbills and puffins had one of their worst seasons since monitoring started on the Isle of May, with breeding success estimated at 0.66, 0.52, and 0.58 chicks per egg laid respectively. Fulmars had a good year, with breeding success estimated at 0.47 chicks per incubating pair. Far fewer than normal colour-ringed shags returned (65.8% of those known to be alive in 1998). It is probable that survival between 1998 and 1999 was very low. Return of kittiwakes was also low (73.1%). In addition, few of the birds not seen in 1998 were sighted in 1999 confirming previous concern that survival between 1997 and 1998 had been very low. Survival of guillemots was slightly below the long-term average (return rate 90.2% compared with the long-term mean of 94.8%). However at this stage we cannot exclude the possibility that some birds may have taken a year off. Razorbill and puffin return rates (86.9% and 88.2% respectively) remain moderately high and continue the trend of year to year fluctuations which have been apparent since intensive monitoring began on the Isle of May.

A comparative analysis of long term changes in adult survival rates of seabirds on the Isle of May indicated that over the period 1986-96, the average adult survival of shags was 82.1%, of guillemots was 95.2%, of razorbills was 90.5%, of puffins was 91.6% and of kittiwakes was 88.2%. Shags, razorbills and puffins all had a single year of exceptionally low survival but these years did not coincide. In contrast, kittiwake survival declined significantly over the period and there was evidence that substantial non-breeding occurred in several years. Breeding success of the kittiwake also declined which, given the low breeding success in recent years, gives cause for concern for the Isle of May population. With the current high level of resighting achieved by studies on the Isle of May, it is clear that year-by-year return rates provide a reasonable indication of relative changes in adult survival.