Scientists explore how migratory birds decide when to take off

Migratory songbirds typically fly under the cover of darkness, but the reason for their takeoff timing has long eluded researchers. Now, a pair of studies suggests the birds rely on the onset of dusk and the suggestion of fair skies ahead as signs for taking flight.

Published in the journal Movement Ecology, the studies used radio tracking devices and analyzed data from nearly 400 songbirds belonging to nine migratory species, including the yellow-rumped warbler, American redstart and Bicknell’s thrush.

Like most other migratory birds, these species migrate at night, probably in a bid to avoid predators, stay cool and set aside the daylight hours for foraging. Nevertheless, the exact reasons they start their flights when they do have long been subject to debate, with some scientists speculating that they time their departures to maximize night flight time or that they rely on celestial objects, such as visible stars, to cue flight.

To try to better understand birds’ timing choices, researchers decided to conduct their studies where migrations begin instead of at stopover points along the way. This enabled them to decrease the effects of variables such as fuel availability that could affect birds’ flight decisions.

The radio data revealed a much narrower migration takeoff window than expected. Ninety percent of the birds that the researchers identified as likely to be setting off on their seasonal migrations did so within 69 minutes of dusk.

“This is a really tight window, which suggests there is something important about this time of night,” said Nathan Cooper, a research ecologist for the Smithsonian’s National Zoo and Conservation Biology Institute and the papers’ first author, in a news release. “And we argue, this consistent departure time early in the night is about maximizing night flight time.”

A second analysis revealed that migratory songbirds are good meteorologists. When the researchers looked at four of the species more closely, they found probability of departure was “related to changes in atmospheric pressure, almost completely regardless of species, season, or location.” They were most likely to depart when atmospheric pressure had risen over the previous 24 hours - an indication of fair weather in the days ahead.

Nevertheless, the weather is probably just one of many factors that prompt the birds to migrate, the researchers said, citing other research that suggests migration timing also depends on sex, age and celestial cues.