Regional variation in the strength of breeding vs non-breeding factors limiting natural populaitons of a declining migratory bird
Most species of conservation concern are not systematically declining across their entire range but instead display complex spatial patterns characterized by regions of steep declines and others that are stable or even increasing. Ensuring the long-term persistence of these species requires understanding the drivers of this demographic variation and prioritizing management actions to strategically address the most urgent threats to regional populations. Achieving these goals is particularly challenging for migratory species due to their large geographic ranges, long-distance movements, and complex annual cycles. We used a novel method for delineating natural populations to identify demographically-distinct regional populations of a declining Neotropical migratory songbird, the Wood Thrush (Hylocichla mustelina). We then used remotely-sensed forest cover and climate data, along with information about migratory connectivity, to model annual variation in Wood Thrush population abundance as a function of environmental factors experienced across the full annual cycle. This model allowed us to quantify the extent to which recent regional population declines have been driven by breeding vs. non-breeding season threats.
Our analysis revealed 17 distinct populations that differ in key demographic attributes (e.g., abundance and trend) and indicates the loss and degradation of non-breeding habitat has played a major role in range-wide Wood Thrush declines. However, regional variation in the magnitude of population declines was not related to threats experienced during the non-breeding period. Instead, our model indicates the degree of habitat loss on the breeding grounds has been the primary driver of regional variation in population trend. These results support theoretical models showing that migratory bird populations are simultaneously limited by both breeding and non-breeding habitat and underscore the complexity of prioritizing full annual cycle conservation actions for these species. Successful Wood Thrush conservation will likely require a mix of non-breeding habitat protection, which is expected to have diffuse impacts on most populations, and more targeted protection of breeding habitat for high-priority populations. Achieving the optimal mix of management actions will require additional research but our results provide a foundation upon which to base these decisions.