Migratory birds can partially offset climate change but it comes with a cost

Climate change-induced habitat degradation is causing havoc with bird migratory dates. According to a new study, birds can partially compensate for these changes by delaying the start of spring migration and finishing the journey faster. Yet, the method has a cost- a decrease in overall survival.

The findings by researchers from Cornell University, the University of Maryland, and Georgetown University are published in the journal Ecology.

"We found that our study species, the American Redstart, can migrate up to 43% faster to reach its breeding grounds after delaying departure from wintering grounds in Jamaica by as much as 10 days," said lead author Bryant Dossman. He led the study while a graduate student at Cornell and is currently a postdoctoral fellow at Georgetown. "But increased migration speed also led to a drop of more than 6% in their overall survival rate."

Tactics for speeding up migration can include flying faster and making fewer or shorter stopovers to refuel along the way. Though migrating faster helps compensate for delayed departures, it can't entirely make up for a lost time. In general, for a 10-day delay, Dossman says individuals can recover about 60% of the lost time, but that means still arriving late on the breeding grounds.

Jamaica has become increasingly dry in recent decades and that translates into fewer insects, the mainstay of the redstart diet. Now, it takes the birds longer to get into conditions for the rigours of migration, especially from poorer-quality habitats. At the same time, plants are greening and insects are coming out sooner on the breeding grounds--also because of climate change.

"On average, migratory songbirds only live a year or two, so keeping to a tight schedule is vital. They're only going to get one or two chances to breed," said Dossman. "Longer-lived birds are less likely to take the risk of speeding up migrations because they have more chances throughout their lives to breed and pass on their genes."

The study is based on 33 years of American Redstart migration departure data at the Fort Hill Nature Preserve in Jamaica. Senior co-author Peter Marra, director of the Earth Commons--Georgetown University's Institute for Environment & Sustainability--oversees the study site. Using this historical data in tandem with automated radio tracking and light-level tags, scientists compared the redstarts' expected departure date with their actual departure date in recent years to see how it's changed.

"The behavioural shifts documented in this research remind us that the manner in which climate change affects animals can be subtle and, in some cases, able to be detected only after long-term study," shared Amanda Rodewald, a co-author on the paper as well as the Garvin Professor and Senior Director of the Center for Avian Population Studies at the Cornell Lab.

"Understanding how animals can compensate is an important part of understanding where the impacts of climate change will play out," said Marra. "In this case, we may not lose a species entirely, but it is possible that populations of some species may go extinct locally due to climate change."

What happens on the redstart wintering grounds carries over into the breeding season. Though the redstart population is stable and increasing in much of its breeding range, detailed eBird Trend maps show the species is declining in the northeastern United States and southern Quebec, Canada.

"The good news is that birds are able to respond to changes in their environment," Dossman said. "They have some flexibility and variation in their behaviours, to begin with, but the question is, have they reached the limit of their ability to respond to climate change?"

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