Migratory landbirds are in decline across North America as factors like habitat loss and climate change pose significant challenges across the annual cycle. Migration is a period of especially high mortality as birds must navigate new landscapes to find suitable foraging habitat and deal with ecological barriers, predation, and competition along the way. Stopover sites where birds can rest and refuel are crucial for successful migration. Refueling performance is linked to a variety of factors including season, migratory distance, habitat quality, and body condition upon arrival. Here, we used plasma triglyceride to investigate how season, progression of season (ordinal day), and breeding latitude influenced the stopover refueling performance of 7 migratory songbird species during fall and spring migration at an inland site in the Connecticut River Valley in western Massachusetts. We found that Swainson’s Thrushes (Catharus ustulatus) and White-throated Sparrows (Zonotrichia albicollis) had greater refueling performance in spring, while Gray Catbirds (Dumetella carolinensis) showed increased refueling performance in fall, suggesting that birds refuel differently at the same site and experience different pressures between seasons during migration. Progression of season did not influence refueling performance for most species in either season, but in fall the refueling performance of Gray Catbirds increased as the season progressed, while the refueling performance declined as the fall season progressed for Yellow-rumped Warblers (Setophaga coronata). We used stable isotopes to examine how breeding latitude influenced refueling performance for a subset of Gray Catbirds and found that, contrary to our expectations, Gray Catbirds nearer to their breeding destination in spring had greater refueling performance, suggesting that birds may try to improve their body condition directly before arriving at their breeding grounds to increase their reproductive competitiveness. Our results show that refueling performance of songbirds passing through inland stopover sites varies across species and can be influenced by a variety of factors including season and breeding latitude.

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