On the floodplain meadows of the Pripyat River in southern Belarus, Ruffs Philomachus pugnax form concentrations of up to 30,000 individuals during spring migration. The Ruffs replenish their energy reserves before continuing their northwards migration. Over eight spring seasons, between late March and middle May 2001–2008, we measured the body mass increase in a sample of 2839 adult and 342 immature males, and 2058 adult and 136 immature females caught in the middle Pripyat near Turov. Body mass increased in two phases in the spring season, rising slowly in the first and faster in the second phase; the breakpoint date between the two phases differed between the sexes (24 April and 3 May, respectively) and showed less variation in females than in males. Adult males gained an average of 1.1 g/day in the first phase and later 2.4 g/day. Immature males gained 0.39 g/day in the earlier phase and 1.7 g/day later. Adult females gained on average 0.56 g/day in the early phase and 1.22 g/day later. In immature females the rate of body mass gain was not significant. The rate of body mass increase varied among years. The water level had a negative impact on the body mass increase in females, but not in males. Presumably, males are less affected by the water level as they have much longer legs and are able to forage in deeper water. According to the estimated flight ranges, both sexes were able to reach at least the breeding grounds in the western part of Siberia in one nonstop flight, except in years with higher water levels. The middle Pripyat is probably the last stopover site before their breeding grounds for at least a portion of these birds.
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