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

Abstract

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 8 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.

Anthropogenic mortality threatens the survival of Canarian houbara bustards

“…Besides mortality due to natural causes, it is important to determine mortality due to human activities, in order to establish sustainable conservation programs 3 , 4 . Predation by domestic or feral cats Felis silvestris catus , road-kills and collisions with power lines are among the causes of anthropogenic mortality with the greatest impact on vertebrate species 5 – 10 . As an example, Loss et al 7 estimated that these two factors represented respectively the first, third and fourth leading causes of bird mortality in the USA and Canada, with annual losses of respectively more than 5000, 200, and almost 50 million individuals.…”

Biomass burning in the Neotropics is exposing migrating birds to elevated fine particulate matter concentrations

“…Anthropogenic activities and their environmental and ecological consequences have generated an array of challenges for migratory bird populations across their annual life cycles (Lees et al, 2022; Nemes et al, 2023). A unique risk factor faced by nocturnally migrating passerine (NMP) bird species during seasonal migration is the disorienting influence of artificial light at night (ALAN; Burt et al, 2023).…”