Widmer M. & Biebach H. (2001) Changes in body condition from spring migration to reproduction in the Garden Warbler Sylvia borin: A comparison of a lowland and a mountain population. ARDEA 89 (1): 57-68

Long-distance migrants often arrive on their breeding grounds with large fat stores. Two explanations 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 (cloacal protuberance) at spring arrival of two Garden Warbler Sylvia borin populations living at different altitudes (200 m asl versus 1500 m asl) under completely different climatic conditions. Newly arrived individuals (predominantly males) of the lowland population carry slightly smaller fat stores at spring arrival than their mountain conspecifics. Furthermore, lowland males return with smaller fat stores than lowland females whereas in the mountains no sex differences seem to exist. For lowland females, which initiate reproductive activities soon after arrival, the fat stores are most likely used to boost these activities. For the mountain birds, the fat stores are presumably used as an insurance against adverse weather conditions upon arrival. According to the different physiological demands of endurance flight and reproduction, newly arrived birds show rapid changes in their body stores. Individuals recaptured during the transition phase from arrival to reproduction show a significant decrease of their fat stores within a few days, accompanied by shrinkage of the breast muscle. At the same time, the cloacal protuberance of males grows rapidly and females increase their body mass