eBird Data Highlight Shifts in Wetland Resources Structuring Waterfowl and Shorebird Abundance.

Recent findings documenting rapid drying in some wetland ecosystems raise concerns over the sustainability of flyway habitats supporting migratory waterbirds. To improve our understanding of these potential impacts, we combined newly available data documenting 40 years (1984-2023) of wetland surface water hydrology in the western U.S. with eBird relative abundance maps to identify emerging bottlenecks in habitat availability. Assessments were made using an ensemble of shorebird and waterfowl species (hereafter waterbirds) representing diverse life histories tied to wetland ecosystems within the region. A machine-learning approach was applied to identify wetland factors important to structuring individual species abundance as a spatial framework to assess changes in essential ecosystem functions aligned with seasonal distributions (i.e., breeding, post-breeding migration, nonbreeding, pre-breeding migration). Inundated wetlands accounted for only 0.3% of land cover within the study area. "Wetland area" (measured as surface water extent), semi-permanent wetlands, and littoral saline lake wetlands were the primary factors structuring bird abundance. Waterbirds exhibited patterns of density dependence to offset resource scarcity by aggregating within landscapes encompassing the most predictable and abundant wetland habitats. Functional wetland losses, caused by persistent declines in surface water, overlapped with species' annual cycles signaling decreasing availability and greater uncertainty in waterbird habitats. Losses were highest in semi-permanent wetlands, with declines of 19%-48% over the past 20 years. While these effects were based on a selection of representative species, impacts were emblematic of associated waterbird guilds reliant on concurrent wetland environments. To address rapid change in wetland resources, we encourage integrating our approach into waterbird management strategies to conserve the ecological processes that support flyway function in North America and worldwide.