Ground-nesting by arboreal American robins (Turdus migratorius)

Animals with dependent and vulnerable young need to decide where to raise their offspring to minimize ill effects of weather, competition, parasitism, and predation. These decisions have critical fitness consequences through impacting the survival of both adults and juveniles. Birds routinely place their nest in specific sites, allowing species to be broadly classified based on nest location (e.g., ground- or tree-nesting). However, from 2018–2020 we observed 24 American robin (Turdus migratorius) nests placed not on their species-typical arboreal substrates or human-made structures but on the ground at a predator-rich commercial arbor in Illinois, U.S.A. This behavior does not appear to be in response to competition and did not affect nest daily survival rate but was restricted to the early half of the breeding season. We hypothesize that ground-nesting may be an adaptive response to avoid exposure and colder temperatures at sites above the ground early in the breeding season or a non-adaptive consequence of latent robin nest-placement flexibility.