Mass Mortality in Migrating American Robins (Turdus migratorius) in Virginia, USA: Data Beyond a Diagnosis.

Abstract

While investigating an American Robin (Turdus migratorius) mass mortality event in February-March 2022 in southwestern Virginia (USA), we collected parasites, investigated gizzard contents, and described flock demographics. We recovered ectoparasites (ticks, mites, lice) from 32/83 (38.7%) individuals. Ten of 83 (12.0%) American Robins presented with scaly mites (Knemidocoptes) and varying levels of disfiguring keratin growth on their legs. We report that 57.1% of individuals harbored endoparasites and 90% of intact ventriculi contained seeds, pulp, or husks from American holly (Ilex opaca) berries. Although we were unable to test the berries directly, these findings lend credence to the conclusion that these birds died from consuming fermented holly berries. This mortality event provided an unusually large sample no longer collected systematically and provided baseline natural history data for a common migratory species. These data are valuable to researchers exploring changes in flock dynamics and parasite load due to contemporary stressors such as habitat and microhabitat modifications caused by anthropogenic impacts like climate change.