Migration is energetically costly, and many passerines prepare for and maintain migration with hyperphagia and increased fuel or fat reserves. During spring migration, as they approach their breeding grounds, passerines may deposit fat in excess of what is needed to complete migration. Individuals may carry excess fuel reserves as insurance against potentially poor environmental conditions in early spring (insurance hypothesis). If this is true, individuals arriving early at northern stopover locations or their breeding grounds should have greater energy reserves than later arrivals. Alternatively, passerines may arrive in spring with excess fat to help offset the demands of breeding (breeding performance hypothesis). Given the energetic requirements of egg production, females may arrive with greater reserves than males if excess fat directly or indirectly offsets breeding costs. We analyzed the energetic condition of 12 warbler species mist-netted during migration from 1999 to 2012 at Braddock Bay Bird Observatory, Monroe County, New York, USA. This northern stopover location is near the breeding range (in relation to total migratory distance) for most of the parulid species we examined and, therefore, is a likely location to show carryover effects between migration and breeding. In 11 of the 12 species, energetic condition was greater in the spring than in the fall for both sexes; and in all 12 species, condition was greater in females than in males in both seasons. Contrary to the insurance hypothesis, condition increased with arrival date for most species during spring migration. Although better condition in females supports the breeding performance hypothesis, the presence of this difference in both seasons suggests that additional factors influence energetic condition in parulids. Given that males arrive in better condition in the spring than when they depart in the fall, individuals of both sexes may carry excess energy reserves during spring migration to potentially use for reproductive efforts.
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Vol. 133 • No. 3