Observations on the breeding biology of Sabine's Gulls (Xema sabini)

Abstract

The breeding biology of Sabine's Gulls (Xema sabini) nesting in a colony with Arctic Terns (Sternaparadisaea) was studied in 1980 at East Bay, Southampton Island, N.W.T. Sabine's Gull nests were initiated over a 1.5-week period, from 23 June to 3 July, and the mean clutch size was 2.77. The mean weight of freshly laid eggs was 22.3 g. Hatching was asynchronous and occurred from 14 to 22 July, following an incubation period of 21 days. Of 53 eggs laid, 44 hatched (83.0%). Chicks showed typical sigmoidal patterns of growth for weight, culmen, and tarsus, and a straight line pattern for ninth primary feather. Growth was rapid, and chicks attained nearly adult proportions by 13 to 15 days of age. The timing and duration of the breeding effort (just over 7 weeks) and the rapid rate of growth of Sabine's Gull chicks are seen as adaptations to the short Arctic summer. The mean internest distance between Sabine's Gulls on the colony was 92.5 m. Nesting was more dense on islands than on the mainland, reflecting the attractiveness of sites least accessible to mammalian predators. An apparent association between breeding Sabine's Gulls and Arctic Terns is reported and two explanations are offered: first, a reliance by Sabine's Gulls on the aggressive behaviour of Arctic Terns or second, coincidence.