Category: Press Materials, Press Release

Title: Study Reveals Bird Distributions are Shifting South – not North – due to Changes in Tropical Precipitation

WASHINGTON – Changing climatic conditions, specifically rainfall declines on tropical non-breeding grounds, have reshaped migratory birds’ geographic distribution and abundance, according to a new paper released today. Researchers from Georgetown University, University of Maryland, Baltimore County, the Cary Institute of Ecosystem Studies, and the Smithsonian Migratory Bird Center published a study in the peer-reviewed scientific journal Proceedings of the National Academy of Sciences (PNAS) that examines the role of shifting tropical rainfall patterns in altering migratory bird ranges on the temperate breeding grounds.

The study assessed 30 years of data on the ecology of a long distance migratory bird, the American Redstart, collected on the non-breeding grounds at the Font Hill Nature Preserve in Jamaica. Researchers found birds were less likely to survive long migrations following dry seasons, indicating that increasingly dry conditions in the Caribbean make longer spring migrations riskier. Following wet seasons, there was no effect of migration distance on survival. Considering the conditions in the Caribbean have been getting progressively drier, long distance migrants are becoming increasingly disadvantaged, and as a consequence, the current breeding origin of this population is more than 500 km south of their origins in 1990. These results support a mechanism of contemporary changes in species distributions under global climate change, which challenges a common view of climate-induced northward range shifts.

“Climate change is having increasingly severe impacts on many migratory animals. The Caribbean, in particular, is experiencing drier and less predictable environmental conditions during the winter months, and this trend is projected to worsen. This is especially concerning given that many migratory songbirds spend their non-breeding seasons in the Caribbean, so it’s crucial to understand the implications of this drying trend on their behavior and population dynamics,” said lead author Bryant Dossman, postdoctoral fellow at Georgetown University. “Our study demonstrates that these drying conditions have significant impacts on the American redstart, influencing their survival rates from one year to the next, and individuals with longer migration distances are at a more significant disadvantage.”

“The prediction that breeding ranges are shifting north because of rising temperatures ignores the fact that most migratory birds spend the majority of the year on the non-breeding grounds where rainfall is most critical. Our results emphasize the need to understand how events, like climate change, throughout the year drive the basic ecologies of these wide ranging species,” said senior author Peter Marra, dean of the Earth Commons Institute at Georgetown University. “Our findings clearly indicate that climate change, in this case drying conditions in the tropics, is causing increased mortality of this long-distance migratory species, and perhaps others.”

The recent findings highlight the challenges that lay ahead in protecting and restoring declining bird populations. In 2019, Marra and other researchers published research in Science Magazine revealing that North America has lost almost 3 billion birds, a loss that cannot be attributed solely to climate change on the breeding grounds. This new study provides insights into the subtle and intricate ways in which climate change operating throughout the year can affect species like migratory birds. The impact of changing rainfall patterns in places such as the Caribbean remains an understudied aspect of how climate change impacts biodiversity.

According to Dossman, “While a warming climate on the breeding grounds could be driving some species northward, the drying trend in the tropics seems to be having the opposite effect. This push and pull of a changing climate on migratory species occupying temperate-tropical areas, underscores the importance of studying these animals throughout the year to better understand the subtle but profound consequences of our changing climate.”



Georgetown’s Earth Commons Institute is a hub for environmental and sustainability innovation, research and education to accelerate action on the most pressing issues of our earth. Assembling a team of interdisciplinary experts, researchers, leaders and students, the Earth Commons is transforming the university into a living laboratory and developing scalable solutions for a greener, more sustainable world.