For the Birds Radio Program: Tree Swallow

Original Air Date: April 9, 2010 Rerun Dates: April 22, 2015; April 2, 2012; April 29, 2011

Tree Swallows are literally gutsy birds–their relatively long intestines allow to digest more kinds of food than other swallows, so they can feed on berries if it gets too cold for insects, but harsh springs still take a toll.

Duration: 4′33″


On the last Saturday of March, I drove up to Montezuma National Wildlife Refuge, along the Erie Canal at the top of Cayuga Lake. My plan was to look at waterfowl, but I was thrilled to see two or three Tree Swallows as well. Tree Swallows are invariably the first swallows to arrive in the north, thanks to their long intestines. If we have a temporary return to wintry temperatures making flying insects disappear, Tree Swallows can digest berries, unlike their relatives. In some years, late frigid spells do take a large toll of Tree Swallows, so I always see my first ones with both joy and trepidation. But the warm weather held, and when I went back on April 4, dozens of them sparkled with iridescence in the sunshine. Pairs perched atop every nest box while others flew in and out of tree cavities. We’re expecting some freezing nighttime temperatures this weekend in Upstate New York, but I’m hoping that this week’s warm weather brought out so many insects that the swallows have replenished their body fat after their long migration, helping them to survive the cold snap.

People trying to attract bluebirds often dislike Tree Swallows, which are rather assertive when they have their heart set on a piece of real estate. My mother-in-law usually gets both species nesting in her field—usually if you put out several boxes, the birds can work out a reasonably amicable settlement. Bluebirds are territorial enough that they space themselves pretty widely and you don’t usually see them squabbling amongst themselves. Tree Swallows are very sociable—their winter and migratory flocks can number in the hundreds of thousands. During the nesting season, squabbling and jousting are the very nature of Tree Swallow society. Tree Swallows mainly remain with their mates through the nesting season, but there’s quite a bit of hanky panky—in some populations, as many as 50 percent of all broods of young have chicks fathered by a male other than their mother’s mate. Fortunately, male Tree Swallows never demand paternity tests, and seem perfectly happy contributing to the support of the young in their nest, regardless of whether they’re the father of all of them or not.

Tree Swallows spend much of their life in the air. They glide more than most other swallows do, and their typical flight is fairly slow, from about 5 to 19 miles per hour, though they can speed up to over 30 miles per hour in short bursts when necessary. During territorial battles, they sometimes tumble into the water—one female was observed to swim, using a butterfly-type stroke with her wings, for over 50 meters, to where she could finally climb out of the water onto emerging vegetation.

Because they spend so much time in flight, I guess it makes sense that Tree Swallows have occasionally been seen scratching their heads while on the wing. When perched, they pull their foot up over their wing to scratch their head, but in flight, they pull the foot under the wing. I love that ornithologists record such details in the literature.

Tree Swallows are seen so often at nest boxes and cavities that many people assume they sleep in them. But except for females incubating eggs or brooding tiny nestlings, Tree Swallows virtually never sleep in cavities or nest boxes except during extreme cold, usually in early spring. They usually sleep in trees or on marsh vegetation. During the breeding season they fly to their nest site at first light, and spend most of their time nearby, giving us a long day of exciting entertainment and beautiful views.