Bird of the Month: Tree Swallow
Written by Andy McCormick
Andy McCormick, Volunteer and Former Board President of Eastside Audubon
Tree Swallows are acrobatic flight masters. The adults show a sharp contrast between their metallic blue back and white chest and belly.
Right-angle turns and swooping runs define the flight of a Tree Swallow. As it turns in flight, its metallic blue back glints in the sun. The next turn reveals its pure white belly. The swallow settles into a long glide and may even fly at the surface of the water. The blue above and white below make a distinct separation just below the swallow’s eye providing a good field mark for birders to distinguish the Tree Swallow from the smaller Violet-green Swallow which has white that curls up behind its eye. Also separating the two swallows is that the white patch at the rump of the Tree Swallow does not form saddlebags as it does in the Violet-green Swallow (Dunne).
The common Tree Swallow shares the genus Tachycineta with eight other swallows in the Americas including the Violet-Green Swallow which is also common in North America. The genus name describes these fast fliers and is derived from Greek takhus, swift, and kineo, move. The species epithet, bicolor, is from the Latin for having two colors, referencing the dark upper part and the white under part. Swallow is from the Anglo-Saxon swalewe (Holloway).
TRee swallow
Scientific Name: Tachycineta bicolor
Length: 5.75”
Wingspan: 14.3”
Weight: 0.7 oz (20 g)
AOU Alpha Code: TRES
AERIAL INSECTIVORE
Tree Swallows spend most of their daylight time in the air seeking and catching flying insects. Specifically, they eat many types of flies, beetles, winged ants, bees, wasps, dragonflies and damselflies. They are able to follow insects to high altitudes and are also frequently seen to capture prey off the surface of the water. The varied diet of Tree Swallows gives them an advantage over other swallows. By adding vegetable matter including bayberries and some seeds to make up about 20% of their diet, Tree Swallows are able survive in poor weather conditions and at higher latitudes during winter months (Winkler et al).
Tree Swallows usually nest near water where insects may be abundant but are also comfortable near meadows if they can locate a cavity in a tree, rocks, or other substrate including the occasional bluebird nest box. The female builds a feather-lined cup nest of grass, weeds, moss, or pine needles. Interestingly, the eggs are pink when first deposited but soon fade to white. The young hatch out in about two weeks and fledge three weeks after that (Kaufman).
MIGRATION TIMING
Tree Swallows arrive in the Pacific Northwest in mid-March to early April making them one of the earlier southern migrants to the area. This schedule allows them to begin nesting a bit earlier than other swallows and gives them access to more possible nest sites. Researchers have documented that spring temperatures and insect emergence is happening up to two weeks earlier than in the past, and this early arrival may help Tree Swallows maintain their traditional timing of arriving when insects are more abundant. However, early arrival also puts Tree Swallows at risk from exposure to cold weather. Not withstanding their ability to eat seeds, some studies estimate that nestlings may die from hypothermia or starvation in periods of low insect abundance (Winkler et al).
Tree Swallows are complete migrants and except for some areas in the southern United States they leave the breeding area during winter months. Ebird maps suggest that west coast breeding Tree Swallows follow a coastal migration route, but little research has been conducted on west coast Tree Swallows. Studies in the eastern United States indicate that Tree Swallows migrate in moderate-sized flocks over evenly spaced leaps from one roost location to the next one. However, in fall Tree Swallows will mass in the hundreds of thousands before moving farther south (Winkler et al).
DISTRIBUTION AND MANAGEMENT
With the world population estimated at 20 million individuals, Tree Swallows are common in North and Central America. They breed from Alaska across Canada and south through New England, the Great Lakes, Rocky Mountains and along the Pacific coast. The wintering grounds stretch from the southern United States through Mexico and Central America to Panama.
Despite the widespread population of Tree Swallows and some increase in their numbers in the southern portion of their range, there has been a gradual decline in their population since the 1980s. There are similar disturbing population trends among other aerial insectivores such as flycatchers suggesting a correlation between insects and bird population declines. Current research is showing that reductions of formerly abundant insect species has changed the makeup of insect populations (van Klink et al).
Photo credit: Penelope Kipps
References available upon request from amccormick@eastsideaudubon.org.