Field Notes

Birds Flying Backwards

It’s spring. Why are some birds flying south?

Eric Walters was standing along the shore of Lake Michigan in the mid-1980s when he saw what he thought was an “historic avian event.” Although it was balmy in Chicago, thousands of songbirds were heading south. It was his first experience of the phenomenon known as reverse migration.

Walters, the founder and former president of the Illinois Ornithological Society, subsequently discovered that, under the right conditions, it is fairly common to observe birds flying the “wrong way” in the spring. He has even followed flocks of southward migrators in his car several times to find out if they were just taking short hops around the neighborhood. He has pursued several North Shore birds as far south as Jackson Park on Chicago’s South Side.

Scientists are divided as to what causes reverse migration. There simply hasn’t been much study of the underlying causes. But at least a few Chicago-area birders seem to think it is becoming a more common sight.
One scientist who has studied reverse migration carefully is John Richardson, senior vice president of LGL environmental research associates in King City, Ontario. He studied migration patterns under a grant from the Canadian Air Force, which was trying to reduce bird strikes with aircraft by creating a predictive model. From radar data, he found that birds that engaged in reverse migration tended to do it on days when a cold front causes local wind direction to swing around from the north or northwest, perhaps accompanied by snow. He also observed that birds migrating in these patterns rarely flew fewer than 100 miles in the “wrong” direction.

“If you’re noticing an increase in reverse migration in Chicago,” he says, “it might have to do with climate change. Spring migration tends to be earlier now — European studies have established that to my satisfaction — and birds that migrate early are more likely to experience bad weather.”

Geese and robins, on the other hand, may be responding to a different cause of reverse migration, according to Laura Erickson, senior editor at the Cornell Laboratory of Ornithology. Work done at Cornell shows that these birds prefer to follow a 37° F isotherm (those lines on the weather map that divide temperatures). “Below that is where the biggest numbers of them tend to gather in the spring,” she notes. If they have to, they’ll reverse the direction of their migration in order to return to their comfort zone. “And the way the temperature has been yo-yoing, that’s going to affect their movement greatly.”

But perhaps the simplest explanation is the best. Walters believes that much of the reverse migration along the lakefront is done by birds that have simply “overmigrated.” Flying at night and aiming for Michigan and points north, they simply miss the turn at the lake. Finding themselves over open water in the morning, they head to land, then follow the shoreline south around the lake, just as motorists do.

“The best conditions in which to observe reverse migration come after a 10 to 18 mile-per-hour southwest wind in the evening from mid-April to mid-June with no precipitation,” Walters says. “The next morning, we’ll usually have a real show on the lakefront.” Walters suggests going early to any lakefront park but particularly recommends Montrose Harbor, Waukegan Harbor, and the Evanston campus of Northwestern University.

— Nancy Shepherdson