We studied the development of breeding plumage in 641 Ruff Philomachus pugnax males during spring in 2004–2006 in southern Belarus with special emphasis on the growth rate of ruff and tuft feathers, which are the most colourful and spectacular parts of the male plumage. During northward migration, male Ruffs arriving in the end of March or early April showed a mixture of winter and striped feathers. The first adults with fully developed breeding plumage were recorded in the last 10 days of April and the proportion of individuals with breeding plumage gradually increased through spring. However, individuals with at least traces of winter or striped plumage were caught during the whole study period. Birds did not shed the whole winter plumage at once, which is typical for Ruff males in western Europe. Immature (second year) birds were less advanced in developing breeding plumage than adults. Only 20% of all 55 trapped second-year males were in full breeding plumage. During the main migration period there were no annual differences in the growth rate of breeding plumage feathers. Ruff lengths increased 1.6 mm/day in adults and 1.5 mm/day in second-year males, whereas the tuft grew 1.1 mm/day in adults and 1.0 mm/day in immatures. Although growing rates of ruff and tuft feathers were similar in immatures and adults, immature males started to moult into breeding plumage later than adults, resulting in less developed breeding plumage when they departed to the breeding grounds. As second-year males had lower body mass and slower body mass increase than adults, it was proposed that energetic constraints during migration are the main reason of delayed moult into breeding plumage in immature birds.
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