Molting Strategy Influences Vulnerability to Climate Change in Migratory Birds

AbstractMolting, the process by which birds replace their feathers, is a necessary annual event with major ramifications for fitness. However, few studies have described threats to birds during molt. Here, we combine long-term climate and bird occurrence datasets to investigate the potential for ongoing anthropogenic shifts in precipitation volume and phenology in southwestern North America to influence bird populations that molt in the region. Our analyses of eBird records reveal intraspecific variation in molting location, which may heighten resistance to interannual variation in precipitation. However, we demonstrate widespread declines among molt-migrant populations following weak and late monsoons, suggesting that this variation is insufficient to buffer many against the current rate of environmental changes. We hypothesize that deviations from historical precipitation regimes increasingly deprive birds of sufficiently predictable resources to supply molt, elevating mortality. Finally, we present associations between sensitivity to precipitation variation and recent population trends, demonstrating that anthropogenic shifts in resource availability during molt have already contributed to population declines and pose a growing threat to western North American birds. Overall, our study demonstrates that anthropogenic shifts away from historical patterns of resource availability may compromise the self-maintenance and recovery of individual organisms, representing an overlooked threat to biodiversity.