Why Migrating Birds Sprint in Spring and Fly Slowly in Autumn

It turns out that migrating birds may sprint in spring, but take an easier route in autumn. Scientists have examined passerine birds, also known as perching birds, and have found that depending on the season, the birds will either migrate more quickly or more slowly.

Learning about bird migration is important for conservation efforts. By knowing when and where these birds travel, scientists can better recommend areas that need to be protected. In order to learn a bit more about migration, the researchers used a tracking radar to measure the speed of birds flying over Falsterbo Peninsula, a bird migratory hot spot in south-western Sweden. Over three years, the scientists examined seasonal differences for passerine migrants.

So what did they find? There was a seasonal difference when it came to airspeed. This was more noticeable among short distance migrant birds. It's possible that these birds fly faster in the spring because they have a greater urgency to reach their breeding grounds first and to choose the best territories, mates and other resources.

That's not all that the researchers found, though. Short distance migrants have higher ground speeds than airspeeds in both seasons. It's therefore obvious that these birds make use of wind assistance. In contrast, long distance migrants travel with airspeeds exceeding ground speeds, which results from flying in headwinds.

"These results indicate surprisingly fine-tuned seasonal modulation of airspeed and responses to wind," said Cecilia Nilsson, one of the researchers, in a news release. "Associated with different behavioral strategies, passerine birds thus are adapted to different levels of time selection pressures during spring and autumn migration."

The findings reveal why passerine birds move more quickly in one season while flying more slowly in another. By waiting for nights with good wind conditions, the birds can save energy while prolonging their migration.

The findings are published in the journal Behavioral Ecology and Sociobiology.