Changes in body condition from spring migration to reproduction in the Garden Warbler Sylvia borin : a comparison of a lowland and a mountain population

Garden Warbler (Sylvia borin) Science Article 1


Long-distance migrants often arrive on their breeding grounds with large fat stores . Two expl anations for this phenomenon are (1) that large fat stores at arrival are needed to speed up the breeding schedule and to counteract possible time constraints, and (2) that they are used as insurance against adverse weather conditions during the days after arrival. We compared body condition (fat stores, breast muscle volume, body mass) and reproductive state (cloac al protuberance) at spring arrival of two Garden Warbler Sylvia borin populations li ving at different altitudes (200 in asl versus 1500 in asl) under completely different climatic conditions . Newly arrived individual s (predominantly males) of the lowl and population carry slightly smaller fat stores at spring arrival th an their mountain conspecifics . Furthermore, lowland males return with smal ler fat stores than lowland females whereas in the mountains no sex differences seem to exist . For lowland females, which initiate reproductive activities soon after arriv al , the fat storesare most likely used to boost these activities. For the mountain birds, the fatstores are presumably used as an insurance against adverse weather conditions upon arrival. According to the different physiologic al demands of endurance flight an d reproduction, newly arrived birds show rapid changes in their body stores . Individual s recaptured during the transition phase from arrival to reproduction show a significan t decrease of their fat stores within a few days, accompanied by shrinkage of the breast muscle. At the same time, the cloac al protuberance of mal es grows rapidly and females increase their body mass

Widmer M. & H. Biebach 2001, Ardea 89(special issue) : 57-68

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