Creature Feature: Tree Swallow

Tree Swallows - Photo Rodney Campbell

Tree Swallow in Profile

The Tree Swallow (Tachycineta bicolor ) is one of our earliest, hardiest insect-eating migrants. Their scientific name means ‘swift-moving two-colored’ in reference to their quick acrobatic flight and their dark iridescent backs contrasting with their white bellies. These beautiful blue-green swallows winter in areas around the Gulf of Mexico and start heading north in late March- early April. Their liquid song and chattering around tree snags is one of the heralds of Spring in the Midwest. They have also developed a devoted following of human researchers; see the Tree Swallow Projects research page for examples.

Field Marks

As their name implies, Tree swallows are ‘two-toned’: greenish above and white below. The back color can vary: adult males have an iridescent blue-green, while the females are a bit grayer. First-year juveniles don’t have this color, and stay a dull brownish until their first Spring; some females will stay brownish even into their second year. As a result, the adults are fairly easy to pick out from any swallow swarm in eastern North America; they’re the only swallows with the “green above, white below” color combination. Identifying juveniles takes practice, as they look a lot like Rough-winged Swallows, but have a clearer throat and different behaviors. Tree Swallow flight is also different from other swallows, with fewer wingbeats and more gliding, especially over water.

Close Relatives

Tree swallows belong to a widespread, successful family, the Hirundinidae, which includes swallows and martins. These aerial insectivores have reached nearly all habitats and continents, and are conspicuous diurnal migrants. The Tree Swallow genus, Tachycineta , includes some neat relatives, including Violet-green Swallows ( T. thalassia ), Bahama Swallows ( T. cyanoviridis ), and Mangrove Swallows ( T. albilineata). All share the same general coloration – greenish above, white below – and all nest in tree cavities that have been abandoned by other birds. For Violet-green swallows in western North America, this has meant nesting in forest snags in the foothills and mountains, and commuting to feed on the wetlands and grasslands of nearby tree-less valleys and plains. Mangrove swallows of Mexico and central America specialize in coastal marsh/swamp areas, favoring mangrove snags or other trees killed by coastal flooding.

Range

Tree Swallows nest over a broad swath of North America, basically nesting up to treeline in Alaska and Canada. All they really need are snags with holes (or nest boxes) and a steady supply of insects. Interestingly, they’re not common nesters in the southern states; they’re rare below the Carolinas and mostly absent from the southern coastal plain. Possibly the lack of big insect ‘blooms’ in these less seasonal areas makes nesting challenging for them. They migrate south in large flocks that assemble at staging areas; even so, the mortality of migrants and wintering birds is high (see Butler: Population Dynamics and Migration Routes of Tree Swallows).

Their winter range is impressive, covering most of the Caribbean and coastal Gulf of Mexico, but even stretching up into the Carolinas. This is much further north than most other swallows, and is mostly due to some neat Tree Swallow behaviors. In cold weather, they’ll often forage for aquatic insects, slowly flying over the surface of ponds and lakes to pick off surface bugs. Apparently a lot of aquatic larvae time their emergence to miss the hordes of summer insectivores……but they can’t avoid hardy Tree Swallows. These swallows also have a unique ability to switch to some fruits when insects are in short supply. I’ve seen large flocks of Tree Swallows descending on myrtle bushes in Florida and the Carolinas; they’ll land or hover next to the bushes, picking off berries.

Nesting

Two male Tree Swallows argue for possession of a nest box (Photo: Earl Harrison)

We know a lot about Tree Swallow nesting because they’re one of the cavity nesters that will readily accept nest boxes. This has allowed researchers to set up long-term studies of these birds. Males arrive very early in Spring, sometimes in February, to compete for nest cavities with other birds and swallows. This competition has selected for aggressive, fearless birds, and Tree Swallows will often attack birds, mammals, and even humans that approach their nest cavities too closely. Oddly enough, Tree Swallows won’t repeatedly attack bluebirds, and some bluebird box managers have used this trait to their advantage. They put pairs of boxes close together. One of the pair gets colonized by Tree Swallows, who will invariably drive off any House Sparrows, Starlings or other birds that try to use the other box….except bluebirds. The Tree Swallows wind up being unwitting guardians of the Bluebird nest.

Once established at a cavity, the male will perch there and sing a liquid cascade of notes, an unusually beautiful song for a swallow. This is an advertisement to lure a female, who will carefully inspect the cavity before giving the male a thumbs-up or thumbs-down (if they had thumbs!). Once the pair is established, she’ll lay 4-6 eggs in the cavity, usually in early – mid April. It takes 14-16 days for the eggs to hatch, and a further 16-24 days for the nestlings to fledge, depending on food supplies. Egg laying is asynchronous, which means first layed, first hatched. This means that there is a spectrum of sizes in a swallow brood, and if food becomes restricted, the smaller birds will be out-competed for food and likely perish. Activity around Tree Swallow nests can be frantic in May and early June, as both parents rush in to feed youngsters and out to forage for insects for their hungry brood. If a nest is destroyed or attacked, the pair can nest again, but usually they can only raise one brood in a season.

Locally

Tree Swallows are widespread, if not abundant, in open fields and wetlands of central Ohio. Hardly any large local park doesn’t have a few nest boxes for them, and box arrays at most of the Metro Parks almost guarantee some there. Parks with sizeable wetlands, like Glacier Ridge, Battelle-Darby, 3-Creeks, and Pickerington Ponds, are especially good places to see them. You can also find them around most of the large reservoirs near Columbus. Hoover Reservoir is especially blessed, since its shallow north end has a large number of dead snags, and the birds can be easily seen around Galena (Area M and N) and Oxbow Island.

One of the neat spectacles of late Summer is the gathering of adult and juvenile Tree Swallows into large flocks, usually in late July and August. These flocks often contain other species, and usually form in areas where food is abundant, often at managed wetlands that don’t dry out in late summer. Locally, Big Island Wildlife Area and the Hebron Fish hatchery (near Newark) can have flocks in the hundreds or even thousands. These flocks will migrate as a unit and even merge with other groups to form huge flocks on the wintering grounds along the Gulf coast. Our residents leave by the end of August, but there is often a pulse of migrating Canadian birds that comes through in mid September-early October. After that, we just have to do without these beautiful birds until next March.