Drought in the Tropics Alters American Redstart Migration
A long-term study reveals that rainfall in tropical forests has quietly redrawn the migratory routes of this striking warbler.
Climate’s unseen hand
The American Redstart Setophaga ruticilla has long been a symbol of spring’s arrival in North America, flashing its orange-and-black plumage through woodlands from Florida to Canada. But new research published in *Global Change Biology* reveals that where these birds breed has been far from stable. Over the past century, the redstart’s breeding origins have drifted north and south by hundreds of kilometres – not because of conditions on the breeding grounds, but because of changing rainfall in their tropical wintering areas.
Using museum specimens dating back to the 1870s and modern feathers collected across the Americas, Henry Stevens and colleagues at Georgetown University and the Smithsonian analysed hydrogen isotopes locked into tail feathers – tiny chemical fingerprints that record where each feather was grown. Their results show that populations wintering in drier regions, such as Mexico’s Yucatán, have shifted their breeding origins steadily southwards, while those wintering in wetter regions, such as the Andes, have shifted northwards. These movements closely tracked rainfall trends in the birds’ wintering habitats.
Following the rain
The study builds on earlier work in Jamaica showing that drought reduced food supplies for redstarts, leading to poorer body condition and lower survival among birds that migrate farther north to breed. Over time, this selective pressure caused a southward contraction of the Jamaican population’s breeding origin by around 500 km. Stevens and colleagues found this same mechanism echoed across the species’ range: in regions where rainfall has declined, redstarts now tend to originate from more southerly breeding areas.
The researchers examined five wintering regions across the Neotropics and compared feather isotopes across three broad time periods – early 20th century, late 20th century, and modern. Across most regions, periods of prolonged drought corresponded with a southward shift in breeding origin; wetter years saw northward expansion. These shifts occurred on surprisingly short timescales, sometimes within just two decades, and could reach more than 1,000 km in magnitude.
Rewriting migration’s map
The implications reach far beyond one species. Most models predicting how migratory birds will respond to climate change assume that breeding distributions move steadily north as temperatures rise. This new study challenges that assumption. Instead, redstart populations appear to ‘breathe’ in and out with rainfall patterns across their tropical wintering range – a finding that underscores how events outside the breeding season can shape population dynamics thousands of kilometres away.
“To understand where migratory birds will breed in the future,” the authors write, “we must first understand how rainfall, drought, and survival interact across the full annual cycle.” The results highlight that migratory connectivity – the geographic link between a bird’s breeding and nonbreeding sites – is not fixed but fluid, reshaping itself as climate pressures shift between hemispheres.
Beyond redstarts
The findings echo wider patterns seen in other long-distance migrants. Similar isotope analyses of Blackpoll Warblers have revealed northward shifts in breeding origin linked to climate trends, while drought-linked southward shifts have been seen in other tropical-wintering songbirds. The message is clear: changes in rainfall thousands of miles from a bird’s nesting grounds can reverberate through its entire life cycle, altering migration routes, breeding success, and even long-term population stability.
For the American Redstart, whose bright flashes enliven both Caribbean mangroves and Canadian maples, the shifting patterns of rainfall across the tropics are redrawing its migratory geography in real time – an unseen but powerful fingerprint of a changing climate.
October 2025
Read the full paper here
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