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Abstract

Melanins are common pigments used for colouration in animal tissue and recent evidence suggests that they may be condition-dependent signals used in mate choice. We examined the hypothesis that individuals moulting during migration produce relatively dull feathers compared to individuals moulting on the breeding grounds prior to migration because of the costs associated with overlapping these two activities. We estimated both the colour and moulting latitude of melanin-based feathers in barn swallows (Hirundo rustica). Moulting latitude was inferred from stable-hydrogen isotope (δD) values of individuals that were known to have bred at the same site the previous year. Contrary to expectations, δD values in feathers suggested that most, if not all, individuals moulted their feathers south of the breeding grounds. Furthermore, we found that males moulting at southern latitudes tended to produce darker, more rufous coloured throat feathers compared to males moulting at more northern latitudes, closer to the breeding grounds. However, we found no such relationship in females. Males could face a trade-off between growing new but relatively dull feathers before or just after the commencement of migration versus waiting to grow more rufous coloured feathers well into fall migration or on the wintering grounds. We suggest that this trade-off could produce an honest signal of male quality if only high quality males can afford to migrate large distances on feathers that are greater than a year old.