Reflections in Nature: Tree swallows are the first swallows to return in spring

My diary entry for April 30, 2022: “When returning from our weekly trip to the landfill, I took the Steam Hollow Road. Mary Alice always likes going on this road during all of the different seasons. We stopped at the lake, where we watched several pairs of Canada geese that had already staked out their territories.

“Quite a racket was raised when the pairs got too close to each other. Then, we saw a beaver swimming in the lake and also what seemed as hundreds of tree swallows flitting over the lake while catching insects.”

Later that day, a friend told me had seen tree swallows fighting with bluebirds at the blue birdhouses that he had erected.

In past diaries, I have usually recorded my first sighting of tree swallows by the middle of April. Tree swallows are the first swallows to return to our area in the spring. If freezing temperatures suddenly cause insects to become dormant, tree swallows, unlike other swallows, are able to switch from a diet of insects to a diet of seeds in order to survive.

The earliest date that I noted for seeing tree swallows in Bradford County was on April 5, 1998, and the latest date was April 27, 1993. I read in my bird book that the tree swallow migration continues until the end of May.

The swallow belongs to the family Hirundinidae, which is from the Latin word hirundo, meaning swallow. There are 79 species of swallows worldwide. Although swallows are often mistaken for swifts, they are not related. The swallows are related to the martins. Swallows are not on the wing as often as swifts, however, along with martins, they probably spend more time in daylight flight than any other passerines (songbirds).

Swallows and martins are especially adapted to aerial life and show no close ties to any other bird group. The swallows are slender, sleek and have long pointed wings. When in flight flying swallows hold their rather large mouths fully open to enable hundreds of insects to be scooped from the air. Swallows have short, weak feet, and unlike the swifts, which cling to vertical rock walls, the swallows prefer to land on wires or rooftops. Normally, these birds do not perch in the leafy part of trees.

The tree swallow’s scientific name is Iridoprocen bicolor, with the genus name coming from two Latin words originating from the Greek word Iris, meaning bright color, and prokne, who was the mythical daughter of Pandion that was changed into a swallow. The species name bicolor is also Latin and means two colors, due to the tree swallow being either a steely blue-black or green-black above and pure white below.

The tree swallows spend the winter months along the southern border of the United States. Unlike the robins and red-winged blackbirds that migrate with the amount of light in a day, the swallows migration is influenced by the weather, which makes the variation of their arrival date in our area. Even the famous (cliff) swallows that return to the San Juan Capistrano Mission are not quite accurate. Although they do return each spring, the date depends on the weather and not a date on the calendar.

The tree swallow is the first swallow to return and the last to leave our area. Usually, advance scouts will be the first arrivals in the spring and most often will be seen flying over a body of water. These early scouts will often suffer from the cold and lack of food.

Our North American swallows are among the earliest migrants in the fall. After gathering in large flocks — often seen sitting on utility wires — they soon disappear. In Europe, it was once believed that the swallows hibernated in the marsh mud.

According to the Atlas of Breeding Birds in Pennsylvania, tree swallows are more common in the northern part than the southern part of the state. Tree swallows were recorded at a wide range of elevations and in all physiographic sections of Pennsylvania. Although they are often seen near water, the tree swallow is found in both forested and non-forested areas. No correlations have been shown on the bird distribution between the river and stream patterns of Pennsylvania.

After arriving in the spring, the male will court a female by pursuing her in flight, while doing aerial gyrations. Unlike many songbirds, the tree swallow will choose a new mate each year, with some males having two mates at the same time.

The tree swallows are the only species of swallows in Pennsylvania to nest in tree cavities. Unlike other swallows, they prefer to nest alone. They are aggressive and will defend a radius of approximately 15 yards of their nests, which are usually nesting at least 30 yards from each other.

The nest, which consists of grass and straw and lined with feathers, preferably white ones, is built by the female. The male will sometimes help in nest construction. In Pennsylvania the average date for egg laying occurs on June 2, however, eggs have been found in nests as early as May 6. Four to six white eggs are laid, with incubation taking 13-16 days. The young will leave the nest in 16-24 days. Only one brood is raised each year.

At times a tree swallow can be seen fighting for a birdhouse erected for bluebirds. For those witnessing this event, it can be upsetting because the swallows are quite aggressive and could succeed in driving the bluebird off. To prevent this conflict, erect two boxes approximately twenty yards apart. If you’re lucky, tree swallows will move into one box and bluebirds in the second. Twenty yards is too close to allow another swallow family. The tree swallows will live in harmony with other birds at that distance, however.

Bill Bower is a retired Pennsylvania Game Commission Wildlife Officer. Read his blog and listen to his podcasts on the outdoors at www.onemaningreen.com.


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