Tuesday, March 12, 2013


This isn't Capistrano, but our swallows are on their way back from their wintering grounds. The first to come are Tree Swallows (Tachycineta bicolor), which winter in southern US and Mexico. They probably winter the farthest north because they are programmed to migrate so early. In fact, a very few birds sometimes winter as far north as Ridgefield National Wildlife Refuge in southwestern Washington.

Tree Swallows are cavity nesters. But they can't excavate their own cavities like a woodpecker can, so they depend on natural cavities, old woodpecker holes, and nest boxes that we put up for them. They like to nest over water when they can, as predators that can climb trees to prey upon swallow eggs and young don't inhabit flooded wetlands.

Thus nesting cavities for them are quite limited, and there is fierce competition for them among swallow pairs. The earlier a male swallow returns to its breeding grounds, the more likely it will find an unoccupied cavity. Once a cavity is occupied, it's an uphill battle for the next bird that shows up to try to take it over, and so first come, first served.

Thus our Tree Swallows start trickling back to western Washington by late February, and the trickle becomes a river by the beginning of April. By that time, pretty much all available cavities are taken. Of course many of these birds continue on north, all the way to Alaska.

This surprisingly early arrival, well before our real spring, comes with a downside. The weather can be wretched at this time of year, cold and windy and rainy. Swallows are aerial insectivores, and their prey can be very hard to come by under such conditions. There is no doubt that in an especially bad spring, some of the birds succumb to starvation. There is no point in trying to raise a brood of young until conditions get better, so they don't attempt that until considerably later, some time in April or May.

Male and female Tree Swallows look the same, beautifully iridescent blue above and snow-white below, but with an interesting caveat. First-year males look just like older birds, but first-year females are recognizably different, dull brown on the back. There are scarcely any birds in the World with a unique first-year female, but the Tree Swallow is one. The adaptive significance of this first-year plumage is poorly understood.

Dennis Paulson

1 comment:

J. Carter said...

"...a very few birds sometimes winter as far north as Ridgefield National Wildlife Refuge in southwestern Washington."

We have them in a backyard box in Washougal, WA this year!