THE INCIDENCE OF NONBREEDING BY ADULT GREAT SKUAS AND PARASITIC JAEGERS FROM FOULA, SHETLAND

Several recent studies of seabirds have found high levels of nonbreedi ng by experienced adults. By contrast, just 8.9% (range 4-14%) of expe rienced Great Skuas (Catharacta skua) on Foula, Shetland Islands, defe rred breeding between 1989-1996. For Parasitic Jaegers (Stercorarius p arasiticus), a corresponding value of 5.5% (range 3-8%) was found betw een 1993-1994. Only 3% of the territorial pairs of Parasitic Jaegers, including new recruits, failed to lay eggs. Higher incidence of nonble eding in Great Skuas was recorded in years when fledging production wa s 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. I n Great Skuas, more males missed a breeding season than females, but t he same did not apply to Parasitic Jaegers. Very young and very old Gr eat Skuas were more likely to defer breeding than mid-aged birds. Evid ence is presented that the decline in breeding frequency of old birds was due to senescence as opposed to increased frequency of mate-change s resulting from a high mortality of old partners.