A recent study indicates that the environmental conditions in the winter habitats of migratory birds can significantly impact their survival during spring migrations and their breeding season.
A recent study conducted by researchers at the Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) reveals that the conditions in the winter habitats of migratory birds can significantly influence their survival during spring migration and the breeding period.
Although it’s been understood for a while that the quality of winter habitats affects the timing of migration and reproductive success in migratory birds, this new research published in Current Biology is the first to connect winter habitat conditions directly to migration survival. The data analysis for both the Kirtland’s warbler and the black-throated blue warbler showed that lower rainfall and reduced plant growth in their Caribbean winter habitats led to fewer birds successfully making it through spring migration. Additionally, for Kirtland’s warblers, the poor quality of their winter habitat also impacted survival during the following breeding season.
With bird populations declining, it’s essential to understand what factors affect their survival throughout the year. Climate change is projected to make the Caribbean drier in the years to come, which could pose even greater challenges for migratory bird species like these warblers.
“If the quality of winter habitats continues to degrade over the next fifty years due to climate change, we now know that this will harm birds’ chances of surviving spring migration,” stated Nathan Cooper, the study’s lead author and a research ecologist at NZCBI. “This information can help us prioritize conservation efforts in the most drought-resistant winter habitats.”
Beginning in 2017, Cooper and his colleagues aimed to explore how conditions in non-breeding habitats influence survival during spring migration and breeding activities. However, most songbirds are not suitable for GPS tracking devices as they are too heavy. Researchers instead use lighter radio transmitters, which only provide location data within a 6 to 10-mile radius, making it difficult to accurately pinpoint where an individual might breed across vast areas of North America.
However, Kirtland’s warblers presented a unique case because almost the entire population breeds in a relatively confined area in Michigan. This allowed researchers to follow the same birds tagged in their winter habitats in the Bahamas.
The research team analyzed three years’ worth of radio tracking data from 136 tagged Kirtland’s warblers to gauge their migration survival rates in relation to environmental conditions.
In addition to estimating Kirtland’s warblers’ survival rates, the researchers also indirectly assessed the migratory survival rates for black-throated blue warblers using a statistical method developed by Scott Sillett, a co-author of the study and a wildlife ecologist at NZCBI, in 2002. The team refined Sillett’s techniques to analyze 14 years of data from the black-throated blue warbler’s breeding grounds in New Hampshire and non-breeding grounds in Jamaica, linking survival rates to environmental factors.
Despite the different approaches used, both bird species showed decreased survival rates during migration compared to non-migratory periods. The study confirmed that adverse winter habitat conditions, such as drought or reduced plant life, resulted in lower survival rates during spring migration and, in some cases, the breeding season that followed. This marks the first direct evidence of how these conditions affect survival during migration. The specific breeding grounds of Kirtland’s warblers enabled researchers to validate the indirect methods used for estimating migration survival.
“To effectively conserve migratory birds, we must identify the times and locations throughout their yearly cycle when they are most at risk,” remarked Cooper. “Accurately estimating survival during both migration and stationary periods is the first step in understanding when, where, and how birds face mortality throughout the year.”
Cooper emphasized the importance of a comprehensive approach to protecting migratory bird species. It’s still unclear if migration-related deaths are the primary cause of mortality or part of a broader range of factors. Fortunately, Cooper, along with authors Clark Rushing from the University of Georgia and Scott Yanco from the University of Michigan, received a four-year grant from the U.S. Fish and Wildlife Service’s Great Lakes Restoration Act. This funding will help develop annual population models for Kirtland’s warblers, which can assist scientists in understanding how migration threats fit into the bigger picture for these warblers, a strategy that may also benefit other declining migratory species.
