A Framework to Model Current and Future Nonbreeding Distributions for Four Declining Nearctic-Neotropical Migratory Forest Birds to Inform Conservation Planning

The full-annual cycle of a Nearctic-Neotropical migratory bird consists of a summer breeding period in North America, southward migration in autumn, a winter stationary nonbreeding period in the Neotropics, and northward migration in spring back to breeding areas. In nonbreeding periods, conservation strategists lack understanding of environmental factors affecting species distributions, stationary nonbreeding distributions, and migratory airspace and stopover habitats. Successful full-annual cycle conservation planning requires knowledge of these critical components to reverse the decline of imperiled species. We focused our efforts on 4 Nearctic-Neotropical migratory forest birds experiencing population declines: Cardellina canadensis (Canada Warbler, CAWA), Setophaga cerulea (Cerulean Warbler, CERW), Vermivora chrysoptera (Golden-winged Warbler, GWWA), and Hylocichla mustelina (Wood Thrush, WOTH). In chapter 1, we quantified current (2012 to 2021) and projected future (2050) suitable climatic and land use/land cover conditions as components of stationary nonbreeding distributions. Multi-source occurrence data and covariates from 3 global coupled climate models (CMCC-ESM2, FIO-ESM-2-0, MIROC-ES2L) and 2 shared socioeconomic pathways (SSP2-RCP4.5, SSP5-RCP8.5) were used in an ensemble modeling approach to predict distribution responses to climate and land use/land cover change. Our findings suggest that 3 of 4 focal species will experience distribution contraction, upslope elevational shifts in suitable conditions, and limited latitudinal and longitudinal shifts. We proposed two conservation strategies for directing limited resources to important stationary nonbreeding areas involving currently suitable landscapes and landscapes likely to be suitable in the future. Chapter 2 utilized a three-scale approach to delineate migratory airspaces, identify high-use stopover landscapes within migratory airspaces, and assess habitat and protected area characteristics within selected stopover landscapes. At the continental scale, we quantified autumn and spring migratory airspace in a modified three-stage framework by estimating migratory connectivity, developing randomized least-cost paths, and incorporating telemetry movement data to produce generalized additive mixed model prediction surfaces. At the regional scale, we predicted autumn and spring stopover landscapes for each focal species in an ensemble modeling framework with 2014 to 2023 eBird occurrence records and global covariates. The stopover landscapes were used in a summation exercise to identify two high-use stopover hotspots for case studies where we characterized habitat and protected area status at the local scale. Our findings at the continental scale suggest 3 of 4 focal species cross the Gulf of Mexico during both migratory seasons. At the regional scale, stopover landscapes in the eastern U.S. revealed an urban effect with other high-use stopover areas in riparian habitats and the Appalachian Mountains. Neotropical stopover landscapes were mostly associated with the highlands of Central America. At the local scale for the two case studies, Neotropical migrants used riparian forests at lower elevations in the Upper Mississippi River watershed, and evergreen broadleaf forests and croplands at lower elevations in coastal Honduras. However, our findings that many areas within stopover hotspots are unprotected draw attention to the need for coordinated conservation action in places that often overlap with multiple political jurisdictions.

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