‘Ancestral memory’ influencing migratory habits of thrushes

Sachi Schott
LSLBO Assistant Bander

Banded birds receive a leg band stamped with an ID number. However, the odds that a given bird will be seen again are very small. A lot of bands must be put on a lot of birds before we can expect to hear from one again.

LSLBO has banded a record number of Swainson’s thrushes this fall, and this week we received the exciting news that one we banded on Aug. 6 was found 2,000 kms away in Wisconsin! This is the second Swainson’s banded by LSLBO to be encountered elsewhere.

Swainson’s thrushes winter in western South America, so it seems strange one would travel all the way east to Wisconsin. A more efficient route would be to fly directly south through the central U.S. Why would these thrushes follow such an indirect pathway?

Part of the answer lies in the distant past during the last Ice Age. During this time the boreal forest and the animals in it were restricted to two ice-free areas to the southeast and southwest of the 49th parallel. As the glaciers began to melt, the boreal forest expanded in their wake, spreading first north and then west in a pattern scientists have uncovered by studying fossil pollen and plants.

Today, Swainson’s thrushes that breed in Alberta will still migrate thousands of kilometres towards their ancestral breeding grounds in the southeast before travelling to their wintering grounds.

Weather patterns and habitat distribution must also influence Swainson’s migration. Still, it is wonderful to think about how their behaviour reveals the shape of a former landscape, if only as an ancestral memory in the mind of a bird.

A combination of inclement weather and a lot of sharp-shinned Hawk activity has made for a quieter than usual week at the station, as the presence of predators means that we must be careful about capturing birds.

Meanwhile our first night of owl banding was delayed by rain, but the nets are up and we hope to be catching Northern Saw-whet Owls soon.

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