A new study from researchers at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) reveals that environmental conditions in migratory birds’ winter homes can impact their ability to survive spring migration and the breeding season. The study, published in Current Biology, is the first to link winter conditions with migration survival. Reduced rainfall and diminished vegetative productivity in the Caribbean non-breeding habitats of Kirtland’s warblers and black-throated blue warblers resulted in fewer birds surviving spring migration. For Kirtland’s warblers, poor winter habitat quality also led to reduced survival in the subsequent breeding season.
With bird populations declining, understanding the factors influencing their survival is crucial. The Caribbean is expected to become drier in the future due to climate change, posing additional challenges for migratory bird species like warblers. Lead author Nathan Cooper, a research ecologist at NZCBI, emphasizes the importance of prioritizing conservation measures on the most drought-resistant non-breeding grounds to mitigate the impact of climate change on migratory birds’ survival. The study highlights the need to consider the full annual cycle of migratory birds when implementing conservation efforts.
To investigate how environmental conditions on non-breeding grounds affect survival during spring migration and breeding, researchers utilized tracking data from Kirtland’s warblers. GPS tags, which can track individual birds continuously, are too heavy to attach to most songbirds, so researchers relied on radio transmitters instead. By tracking the same individual birds tagged in their non-breeding habitat in the Bahamas, the team was able to estimate survival rates during migration and its relation to environmental factors for Kirtland’s warblers. Additionally, a statistical technique was used to estimate migratory survival rates for black-throated blue warblers based on capture-recapture data.
Both species showed lower survival rates during migration compared to stationary periods, with poor winter habitat conditions leading to lower survival rates during spring migration and sometimes during the subsequent breeding season. This is the first direct evidence of a carry-over effect on survival during migration. By accurately estimating survival during migration versus stationary periods, researchers can pinpoint when and where migratory birds are under threat and take appropriate conservation measures. A holistic approach is needed to effectively protect migratory bird species, as it is unclear whether deaths during migration are the primary source of mortality or just one of many factors.
To further understand the dangers faced by Kirtland’s warblers throughout their annual cycle, the researchers have received a grant to build full annual-cycle population models. This will help scientists determine how migration fits into the broader picture for warblers and could provide valuable insights for the conservation of other declining migratory species. By considering the entire migratory cycle and the impact of environmental conditions on survival, researchers aim to develop effective conservation strategies to ensure the long-term survival of migratory bird species facing various threats.