Ecology of the Long-tailed Skua, Stercorarius longicaudus Vieillot, 1819, at Scoresby Sund, East Greenland. Part three: clutch size, laying date and incubation in relation to energy reserves

Energy reserves, gonad development and breeding of Long-tailed Skuas were investigated in East Greenland at Kap Stewart in 1973 and 1974, at Gåseelv and Ugleelv in 1975, and at Kaerelv in 1975. In spring adults arrived along the ice edge and on the tundra with maximal energy reserves, with testes of sub-maximal size and enlarged follicles close to the size at which rapid follicle growth may start. In paired birds the loss of energy reserves in the pre-breeding period decided how far rapid follicular growth proceded and whether none, one or two eggs were laid. In 1973 and 1975 oneegg and two-egg pais had similar energy reserve losses until hatching of the young, regardless of clutch size. Nonbreeding birds in 1974 lost their energy reserves at a faster rate than did breeding birds in 1973 and 1975, weights of laid eggs included. Timing of egg laying was correlated with the disappearance of snow-cover and the experience of the pair and not with body energy reserves. One – egg and two – egg clutches in 1973 and 1975 had the same average completion date (13 June). First clutches were completed between 4 June (Gåseelv 1975) and 5 July (Kap Stewart 1973) and replacement clutches until 14 July (Kaerelv 1979). Testes and largest follicles started to regress at or soon after (failed) breeding. In non-breeders regression started later than in early breeders. All breeding and non-breeding territorial birds developed brood patches, which started to appear in the first days of pair formation. In females, only breeding birds had full-size patches, which were attained at clutch initiation. Males attained full-size patches some days before or after clutch initiation and some of them also without eggs being produced. In early breeders patches started to regress between two and three weeks after clutch initiation, in late breeders within a week. In 1975 about 61 % of the incubation was done by the female