Climate change is causing many species to shift ranges. Understanding how and why species shift ranges in response to climate requires knowledge of population dynamics and the factors that drive them. These processes are particularly complex for long-distance migrants, who are impacted by conditions they encounter in multiple locations.
A good example of this is migratory birds, who fly between winter non-breeding grounds and summer breeding grounds. Climate change impacts in their winter range have the potential to influence survival and abundance as far away as their summer ranges. How can we tell if or how climate impacts in one range are impacting population dynamics in the other range?
Dossman, B.C., Studds, C.E., LaDeau, S.L., Sillett, T.S. and Marra, P.P. 2023. The role of tropical rainfall in driving range dynamics for a long-distance migratory bird. Proceedings of the National Academy of Sciences 120(52): e2301055120.
Recent research tries to answer this question by using tracking data from the American redstart, a common North American songbird that breeds across much of the eastern United States and Canada and overwinters in the Caribbean and Central and South America. A Jamaican population of redstarts was monitored from 1990 to 2019 through a combination of tagged individuals and stable isotope analysis of tail feathers to determine their breeding origins in North America. Their survival and migration distance was then linked to climate conditions in the tropical non-breeding grounds, specifically precipitation.
The results provide evidence that the amount of rainfall in the tropics during the winter has a significant impact on survival and migration distance. In dry years, birds that migrated longer distances (i.e. further north) were significantly less likely to survive than those that migrated shorter distances, with a difference in survival rate of almost 40%.
In contrast, in wet years, there was no significant difference between birds that migrated longer distances than those that migrated shorter distances, with a difference in survival rate of only 13%. It’s likely that the risks of migration are magnified over long distances following dry winters, making the birds that breed at more northerly sites less likely to survive.
This has resulted in a major shift in patterns of migratory connectivity over the past 30 years. During that time period there has been an increase in the frequency of dry years in Jamaica, meaning that fewer birds are surviving to breed in more northern latitudes.
The result is that the breeding population has shifted south – more than 500 km south since 1990. Abundance has shifted as well, with an increase of over 50% since 1990 for more southerly breeding populations, according to Breeding Bird Survey routes. Contrary to most conventional climate change predictions that predict poleward range shifts, the change in tropical rainfall patterns has led to a southward expansion of American redstarts.
This shift in connectivity highlights the complex factors that influence range dynamics, and how climate change can have long-range impacts. Linking environmental conditions in non-breeding ranges to population dynamics in breeding ranges represents a step towards understanding how and why migratory populations shift. This can then lead to better predictions of how migratory species might respond to climate change.
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