The probability of successful breeding by Brent geese in the Arctic appears to be positively correlated with the mass of females when they depart from spring staging areas in North-western Europe. To clarify the mechanism behind this relationship we investigated the significance of flown-in female body stores for breeding over six summer seasons (1990–1995) in the Pyasina Delta in Taimyr, Russia. Female Brent Geese were caught upon arrival on the breeding grounds. Changes in female body mass and the timing and duration of nest recesses were recorded via electronic weighing platforms placed under their nests. Average female body masses declined through the breeding season; from 1464 g at arrival to 1269 g after egg laying and 1066 g at hatching, which was supposed to be close to lean body mass. Thus half of the stores were used between arrival and clutch completion, the other half during incubation. All females foraged during incubation. On average they left the nest 13.1 times every 24 hours, resulting in a daily recess time of 198 min during the first 21 days of incubation. Females were less likely to leave the nest during the last three days of incubation. Leaving the nest was risky: 40% of the Brent Goose nests in the study area lost one or more eggs to gulls during absence of the female. We estimated that 52% of the energy required during incubation was derived from body stores and the rest (48%) from food intake.
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