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Instinct and gradual increases in daylight have long bee thought to be the driving factors behind bird migration. Recently, gradual shifts in seasonal temperature averages have been suspected to have an impact.
Seasonal rainfall, a new study suggests, may trump all of these triggers—and changes in tropical wintering habitats may have a disruptive effect on many of North America's birds.Researchers studied American redstarts because they aggressively defend their territory and tend to return to the same areas year after year. Pete Marra, a research ecologist at the Smithsonian Conservation Biology Institute's Migratory Bird Center, explained:
These behaviors made it relatively easy to keep track of individual birds over multiple years and document changing spring departures. Each individual was fitted with a unique combination of colored leg bands.
By observing the redstarts at their wintering grounds in Jamaica over a span of five years, researchers noticed a correlation between the number of insects available and the timing of the birds' spring departure.
The insect populations were linked directly to the amount of seasonal rainfall.
"Our results support the idea that environmental conditions on tropical non-breeding areas can influence the departure time for spring migration," said Colin Studds, lead author of the study, "we found that the same birds changed their spring departure from one year to the next in relation to the amount of rainfall and food in March."
Read more about migrations:
Climate Forced Migration: Pathways Of Future Conflict
Climate Change Puts Bird Migration on Ice
Epic Leatherback Turtle Migration Tracked by Satellite for First Time!
Africa's Great Migration of Wildebeest Shows "Swarm Intelligence" (Slideshow)