Direct anthropogenic bird mortality: Analysis and synthesis of individual threats as a baseline for understanding cumulative impacts
Understanding and reversing the widespread declines of bird populations requires estimation of the magnitude of all mortality sources. Habitat loss, climate change, and other stressors indirectly cause bird mortality through intermediate mechanisms. However, a tremendous number of birds are also directly killed by numerous sources of anthropogenic mortality. Compared to indirect stressors, direct mortality sources are characterized by relative clarity of cause and effect. The study of direct anthropogenic mortality therefore has potential to lead to better understanding of animal population responses to anthropogenic stressors and to mitigation measures that effectively target the stressor and reduce mortality. We conducted several systematic reviews to estimate the amount of U.S. bird mortality caused by direct anthropogenic threats. Our objectives are: (1) to synthesize these and other similar estimates in order to provide a broad picture of the direct threats to bird populations, and (2) to summarize research needs that must be addressed to understand how mortality cumulatively affects bird populations.
Annual U.S. bird mortality from different direct mortality sources varies from billions (cat predation) to hundreds of millions (building and automobile collisions), tens of millions (power line collisions), millions (power line electrocutions; communication tower collisions), and hundreds of thousands (wind turbine collisions). Notably, there is striking agreement between this and a similar threat ranking for Canada. We identified two categories of research needs that must be addressed to understand avian population responses to direct mortality, those that apply to individual field studies and to large-scale data syntheses. For the former category, needs include: (1) increased focus on randomly selected study units, (2) sampling for all species potentially killed, (3) sampling year-round, and (4) quantification of biases that cause underestimation of mortality. For the latter category, needs include: (1) improvement of geographic and seasonal coverage of the collective body of mortality research, (2) identification of correlates of mortality variation, and (3) development of approaches that clarify the degree to which mortality from different threats is cumulative versus additive. A shift beyond quantifying gross mortality estimates to also assessing population level impacts is needed; however, we conclude that much of the data required to make this shift is currently lacking. Further development and application of rigorous data collection and modeling approaches will improve understanding of the impacts of direct bird mortality. Such developments will also transfer to other animal taxa and to evaluating indirect threats, such as habitat loss and climate change.