Cerulean Warbler Full Annual Cycle Ecology: Filling in Critical Knowledge Gaps

Cerulean Warblers (Setophaga cerulea) are a declining migratory bird species of conservation concern that breed in mature hardwood forests of eastern North America and spend the stationary non-breeding period in the tropical Andes of South America. To reverse their >50-year population decline, a full annual cycle conservation strategy is needed. However, several important knowledge gaps have limited our understanding of this species’ full annual cycle ecology, including migration ecology, response to forest management on the breeding grounds, and basic ecology during the stationary non-breeding period in Andean forests. From geolocator data, we found a moderate pattern of migratory connectivity in which most Appalachian breeding population spent the ~6-month stationary non-breeding season in Colombia/Venezuela, while most of the birds from the Ozarks and Great Lakes regions travel to Ecuador/Peru. We documented a pattern of multiple extended migratory stopovers events in Central America, which highlights the importance of forest conservation in this migratory bottleneck. On the breeding grounds, eastern hardwood forests have greatly changed in structure and composition due to centuries of anthropogenic disturbances. Even-aged secondary forests with less natural disturbance have replaced forests with natural heterogeneous canopy structure required by this species. While recent forest management strategies designed to emulate natural disturbances have temporarily increased cerulean densities, we found that increases were sometimes ephemeral. Territory densities were negatively related with stand basal area and midstory cover and positively related with overstory cover. Management to delay midstory encroachment and increased basal area could include periodic prescribed burning or pre-commercial thinning and may prolong effectiveness at attracting Cerulean Warblers. In the Andes, deforestation has resulted in widespread habitat loss and fragmentation of montane forests, where ceruleans spend approximately half of their annual cycle. We estimated stationary non-breeding home ranges, documented high density, low detectability, a male-biased sex ratio, a tendency to be in mixed-species flocks with ~3 ceruleans per flock, and relatively high interannual survival compared with apparent weekly survival in mature secondary forest. Our findings help to address critical knowledge gaps and can be used to inform a better and more targeted full-annual cycle conservation approach for ceruleans throughout their extensive range.