Seasonal Exposure to Hemispheric Conservation Challenges Influences Population Trends of Migratory Warblers (Parulidae)

Human activities have profound impacts on ecological systems and biodiversity worldwide. Estimating wildlife response to those activities across broad spatial scales is challenging, yet effective conservation measures require an understanding of where and when these activities are contributing to population declines. We investigated the relationship between exposure to human activities (conservation challenges) and population trends of migratory warblers (Parulidae) that breed in the United States and Canada. Our four-stage approach (1) summarizes the literature describing the sensitivity of species to conservation challenges; (2) estimates weekly species' exposure across the full annual cycle; (3) quantifies spatial correlations among species' exposure to conservation challenges; and (4) quantifies the relationships between seasonal exposure to conservation challenges and warbler population trends. Exposure during post-breeding migration, followed by the stationary non-breeding season and pre-breeding migration, explained the most variation in warbler population trends. Within the post-breeding migratory season, the conservation challenges with the greatest total seasonal exposure values were roads, light pollution, communication towers, forest management, and livestock management. During this season, species undergoing the steepest declines were associated with exposure to light pollution, communication towers, urban and suburban areas, livestock management, and agriculture. Notably, exposure to conservation challenges during the breeding season was not an important predictor of warbler population trends. Our results provide hemispheric, full annual cycle information for prioritizing conservation investments and additional research for warblers, specifically the importance of addressing challenges to which birds are exposed during post-breeding migration. More broadly, this framework can be used to assess spatiotemporal conservation challenges impacting migratory species trends globally.