Migration is thought to be costly such that challenges faced during migration likely affect how birds migrate through direct selection on migratory behavior. Survival throughout the annual cycle and conditions that affect the survival costs of migration are therefore critical to our understanding of the causes and consequences of migration. Estimating daily survival throughout the annual cycle of migratory birds has only recently become possible. We used over a decade of GPS tracking data from 186 migratory Buteo lagopus to quantify causes of mortality as well as demographic, seasonal, and spatial variation in daily survival and how survival consequences of migration changed during an outbreak of highly pathogenic avian influenza virus. The cost of migration via direct mortality varied between avian influenza and baseline study periods. Surprisingly, mortality risk during typical spring migrations was lower than in all other seasons, but mortality risk during the avian influenza outbreak was higher during both spring and fall migrations. Additionally, we found increased mortality risk in females during the avian influenza outbreak but no sex differences during baseline periods. Hence, the relative cost of migration was altered by the virus. During baseline periods, mortality risk was highest during late fall migration and winter, coinciding with increased anthropogenic‐related mortalities at lower latitudes. However, the number of deaths caused by avian influenza (n = 11) in a single year equaled the total number of deaths caused by anthropogenic‐related factors (n = 11; primarily collision trauma and persecution) across all 12 study years. Overall, our study highlights how added stressors such as disease outbreaks and anthropogenic factors may exacerbate existing seasonal constraints by directly increasing mortality risk.
Paprocki et al. (Sun,) studied this question.