Urbanization and migratory status shape nonbreeding space use behaviors in a North American songbird.

Abstract

Background: Understanding how and why animals use space is core to virtually all aspects of ecology, from influencing individual fitness, to predicting impacts of global change. Yet, most of what we know comes from studies focused on the breeding season, which represents just a fraction of the full annual cycle. Further, most of these studies have been conducted on species considered obligate migrants or residents, which limits our ability to understand the full scope of how species, including facultative migrants or species with high variability in migratory behavior, use space in response to changes in their environment.

Methods: We used high-resolution satellite transmitters from 2020 to 2024 to track adult individuals from multiple populations of American Robins (Turdus migratorius) to quantify patterns of space use behavior and movement during the nonbreeding period. We analyzed endogenous (sex, body size, breeding origin, and migratory status - whether a bird was a migrant or not) along with exogenous factors (local weather, land use) to test competing hypotheses on how life history and the environment shape nonbreeding space use.

Results: Across 85 individuals, we found that robins displayed large variation in nonbreeding space use behaviors, ranging from strong site fidelity to complete nomadism, and nearly half (~44%) switched behavioral strategies within-season. Migratory status, in combination with the degree of urbanization on the nonbreeding grounds, were the strongest predictors of nonbreeding movement compared with climatic conditions or other endogenous differences such as body size or sex.

Conclusions: Our results reveal strong variation in nonbreeding space use among and within individuals of a single species. The variation we documented in American Robins likely typifies the flexibility inherent within species considered facultative migrants who also encompass large ranges-a feature which may be adaptive for keeping pace with highly heterogenous environments across a large geographic distribution. Studying the full annual cycle space use needs of species that exhibit inter- and intra-population variation in migratory behavior may be key to determining which species, populations, or individuals may win or lose out as environments increasingly become more unpredictable in the face of ongoing global change.