The incidence of nonbreeding by adult Great Skuas and Parasitic Jaegers from Foula, Shetland

Several recent studies of seabirds have found high levels of nonbreeding by experienced adults. By contrast, just 8.9% (range 4-14%) of experienced Great Skuas (Catharacta skua) on Foula, Shetland Islands, deferred breeding between 1989-1996. For Parasitic Jaegers (Stercorarius parasiticus), a corresponding value of 5.5% (range 3-8%) was found between 1993-1994. Only 3% of the territorial pairs of Parasitic Jaegers, including new recruits, failed to lay eggs. Higher incidence of nonbleeding in Great Skuas was recorded in years when fledging production was low. Loss of mate due to death or divorce was the main direct cause of nonbreeding. Loss of territory also was important for male birds. In Great Skuas, more males missed a breeding season than females, but the same did not apply to Parasitic Jaegers. Very young and very old Great Skuas were more likely to defer breeding than mid-aged birds. Evidence is presented that the decline in breeding frequency of old birds was due to senescence as opposed to increased frequency of mate-changes resulting from a high mortality of old partners.