| Summary: | Animal body reserves are often linked with demographic parameters such as breeding success and survival. During breeding season individuals face a trade-off between maintaining body reserves and investing in reproduction. Factors influencing the body reserves of species during breeding season are poorly understood. In this study, we used long-term trapping data from Finnish Constant Effort Sites program to evaluate the impact of sex, migration behaviour, and weather on body reserve index (BRI) of old and young birds during breeding season in 20 species. Our main interest was to study how variation in weather conditions influences the BRI of breeding passerine birds. Weather variables did not explain BRI of adults. However, we found that BRI of young birds was weakly negatively connected with mean temperature, but this may have low biological importance. BRI of adult males increased towards the end of August, but female BRI showed a seasonal decline throughout June and July. Breeding females face the peak in physiological stress later thanmales, probably because females lay eggs, and often invest more in incubation and brood rearing thanmales. The seasonal decline caused females to have lower BRI thanmales at the end of the breeding season. This can cause females to be less prepared for the approaching autumnmigration thanmales. Our findings suggest that there are sex-specific changes in BRI during the breeding season, whichmay have carryover effects on other life history events. Peer reviewed
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