We investigated fat deposition and the timing of departure of Greater White-fronted Goose Anser albifrons at Lake Miyajimanuma, Hokkaido, which is the largest stopover site in northern Japan, focusing on the differences between seasons and between sexes. We assessed the fat mass of neck-banded geese by observing their abdominal profiles. In spring, the variation in departure dates among individuals was smaller than that in arrival dates, and there was a positive correlation between arrival dates and departure dates in the fall. The factors affecting decision-making relating to the timing of departure are assumed to be the time constraints of migration before breeding in the spring and on the duration of stay in the fall. The spring fat increase in females was larger than in the fall, and had a tendency to be larger than in males in the spring, The seasonal and sexual difference in fat mass increase seems to be explained by the requirement of fat for breeding after departing from the stopover site. This study supports the possibility that migratory birds adjust their fat mass and the timing of departure at stopover sites according to seasonal differences in events occurring after migration, such as breeding.
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