Breeding of the Fulmar Fulmarus Glacialis

Summary. .1. A study of individually marked Fulmars during three seasons has shown that both sexes can breed in successive years. The evidence for the theory of intermittent breeding is examined and definitions are given of the various categories of birds which compose the population during the breeding season. It is suggested that the supposed non‐breeding adults which are seen, often in moult, in July are failed‐breeders which have lost their egg or nestling. 2. No marked individual changed its nest‐site between years, although the number and identity of sites occupied each July varied considerably. 3. No changes of mate occurred among the six marked pairs which nested in two consecutive years. 4. There is some evidence that both parents remain near the colony during the week before and after hatching. 5. Analysis of the 90 known sites of a colony subject to avian but not wild mammalian predation has revealed a wide range of habitat selection and a strong tendency toward close concentration of nests. 6. There is some evidence that the breeding season, as measured by date of hatching, varies from year to year.