Mass gain and stopover dynamics among migrating songbirds are linked to seasonal, environmental, and life-history effects

Abstract

During migration, birds must stopover at refueling sites to replenish energy stores, with the rate of refueling linked to stopover length, migration speed, and the timing of critical life stages, such as breeding. Under optimal migration theory, birds are expected to maximize fuel intake and minimize stopover length (the time minimization hypothesis). Within a multi-species framework, we demonstrate that time minimization behavior is context-dependent, as refueling rate, stopover length, and departure decisions vary between seasons, among species with different ecological traits (e.g., diet and migration distance), and in response to density-dependent factors and prevailing environmental conditions. Using long-term banding data (10 years) from southwestern British Columbia, Canada, we investigated the ecological processes shaping stopover dynamics among 5 warbler (Parulidae) and 5 sparrow species (Passerellidae). Specifically, we assessed variation in fuel deposition rates (FDRs) as well as the associations between mass gain, stopover length, and departure probability. FDR was greater in insectivorous warblers during spring migration and in species migrating long distances. FDR responded positively to temperature and habitat productivity, but negatively to precipitation events, particularly for insectivorous, long-distance migrants during spring migration. Similarly, density-dependent refueling rate was only observed in insectivores and long-distance migrants during spring migration, where higher densities of foliage-gleaning insectivores were associated with a lower FDR. Stopover duration was more closely associated with body condition upon arrival and subsequent mass gain than direct environmental effects, especially during northward spring migration, providing support for time minimization and a mass threshold influencing departure decisions. These results highlight that refueling rates and stopover quality vary among species depending on their life-history strategies, with particular implications for long-distance, migratory insectivores, which may be most susceptible to climate shifts.