Nature News | Marlene A. Condon Apr 30, 2021
The male Tree Swallow is one of the most gorgeous animals that I have ever laid eyes upon. Its back is a stunning glossy blue-green that contrasts with its pure white undersides.
Tree Swallows are birds of open country (fields in the East and prairies out West) where they can easily forage for insects that are in the air from spring through fall. Yet these birds normally nest in the cavities of trees. Where snags (dead trees) are not readily available, they will take advantage of bird boxes that have been set out for bluebirds.
Thus, people’s fondness for bluebirds has helped Tree Swallow populations to increase as well. Chickadees, titmice, and wrens are some of the other species of birds that might also take up residence in a box built to bluebird specifications.
If you are alert and can remember to check the sky late in the afternoon during the month of March, you might spy Tree Swallows returning from their winter journey south. While many species migrate at night, Tree Swallows move during daylight hours.
Tree Swallows feed upon berry or berry-like fruits, which allows them to return to their nesting grounds earlier in spring than many other insect-eating birds. If cold weather lingers and prevents the emergence of insects, Tree Swallows will not starve.
However, when the weather has warmed and insects are active, a male Tree Swallow will escort his mate (he usually has a new one each year) around the territory he has staked out. After the female has chosen from the potential nest sites, she gets to work on building a nest while the male watches over her, helping very little with the nest construction.
Composed of grasses, plant stalks, pine needles, moss, and other plant matter, the nest will usually be lined with chicken or goose feathers after the first eggs are laid. A study several years ago suggested that these feathers provided warmth for the developing chicks that helped them to grow more quickly. Those chicks from nests without feathers took longer to mature.
From four to seven eggs are laid and then incubated by the female for 14 to 15 days. Both parents feed the nestlings that will be ready to leave the nest after about three weeks. But if these baby birds are in a bluebird box, they may be trapped and will die.
Unlike bluebird nestlings, Tree Swallow chicks need help to climb out of boxes. Therefore, if you think your bluebird boxes might be used by Tree Swallows, either make a “ladder” of little blocks up the interior wall containing the exit hole, or attach a small piece of hardware cloth to the interior wall below the hole. You could also roughen up the inside surface of the wall containing the exit hole, which will make it easier for any young bird to make its way up and out of the box.
Please spread this information to anyone putting out bird houses to help keep Tree Swallows safe!