More than mortality: Consequences of human activity on migrating birds extend beyond direct mortality

Birds must contend with an array of anthropogenic threats during their migratory journeys. Many migrants are killed due to encounters with artificial light, introduced species, pollutants, and other anthropogenic hazards, while survivors of these encounters can suffer longer-lasting negative effects. The nonlethal effects of anthropogenic threats on migrating birds are less well understood than direct mortality, yet both potentially contribute to population declines. For example, building collisions frequently kill migrating birds, but the numbers of migrants that survive with an impaired ability to fly, refuel, or navigate to their destination on time is not well understood. Though not immediately fatal, such injuries can lead to delayed mortality and, ultimately, reduced lifetime reproductive success. Furthermore, migrants are likely to encounter multiple threats on their journeys, which can interact synergistically to further reduce fitness. For instance, light pollution attracts and disorients migrants, increasing the likelihood of window strikes, and surviving birds may be more vulnerable to predation from introduced predators. While considerable attention has focused on the lethal effects of anthropogenic threats, here, we review nonlethal effects of eight types of threats during migration, their interactions, and the pathways through which they can exert fitness costs. In doing so, we identify knowledge gaps and suggest areas for future research. In the absence of more information, we propose that the greatest reduction in the cumulative lethal and nonlethal impacts of anthropogenic hazards will be achieved by addressing threat types, like artificial light at night, that interact with and compound the impact of additional threats. Direct mortality from anthropogenic sources is recognized as a key driver of population declines, but a full understanding of the impacts of human activity on migrating birds must include the cumulative and interacting effects that extend beyond immediate mortality en route to influence overall migration success and lifetime fitness. Lay Summary & BULL; Migrating birds around the world face a wide variety of threats from human activity and many species are declining significantly. & BULL; Many migrants are killed by these threats, while those exposed but not immediately killed may experience changes in their condition, migration timing, orientation, or route. & BULL; Nonlethal effects of threats during migration, such as sensory pollutants and introduced species, have not been well-studied but are likely contributing to population declines by lowering future survival or reproduction. & BULL; We review support for the nonlethal impacts of eight anthropogenic threats, describe how they can cause nonlethal fitness costs for migrating birds, and identify knowledge gaps. & BULL; Reducing threats that compound the impacts of other threats, such as artificial light at night, will help to minimize the cumulative impacts on migratory birds. & BULL; Fully capturing the costs of human activity on migratory birds requires understanding both lethal and nonlethal impacts of encounters with anthropogenic threats during migration.