Breeding Biology of the Xantus' Murrelet

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Abstract
Xantus'' murrelets (S. [Endomychura] hypoleucus) were studied on Santa Barbara Island, California [USA], from 1975-1979. Clutch initiation occurred from March-June and usually peaked in April. The 2-egg clutch was 45% of the female''s weight. Eggs were laid 8 days apart. Both sexes shared duties during a 34-day incubation period; partners relieved one another every 3-4 days. Eggs were typically left unattended in the interval between laying of successive eggs, immediately after clutch completion, and sporadically during incubation. Breeders and non-breeders of both sexes followed similar patterns of weight change, reflecting similar patterns of colony attendance; murrelets lost weight after the egg-laying period, then gradually gained weight through the rest of the breeding season. The downy young left the nest only 2 nights after hatching, unfed and weighing less than 30 g. Chicks and their parents apparently moved well offshore their 1st night at sea. Even though most murrelets on Santa Barbara nested in concealed rock crevices, irregular nest attendance patterns left eggs vulnerable to predation, resulting in the loss of 44% of the eggs laid. Predation by deer mice (Peromyscus maniculatus) is currently the greatest risk to the breeding of these murrelets on Santa Barbara Island.