Tree Swallows: A Brief Introduction to One of the First Neotropical Migrants of the Year
By Chris Byrd
As we near the month of March here in the Puget Trough, the onset of spring migration is near. Bird species that wintered in the southern United States, Central and South America will begin their yearly migration to northern latitudes for the breeding season arriving in our region anywhere from the end of February until early June. Tree swallows are one of our earliest arrivals, and in their own way, herald the transition of winter into spring.
Identification of adults: Tree swallows can be identified by their iridescent blue-green backs white bellies, long tails with a moderate fork and broad wings in comparison with other swallow species. Differentiation from similar species:
Tree swallows have dorsal coloration (back) that extends below the level of the eye,and forms a distinct boundary between the iridescent back and white belly. The similar looking violet-green swallow can be differentiated from tree swallows by the white that wraps around behind the eye, and the white patches that extend from the ventral (bottom) portion of the swallow onto the rump. Violet green swallows also have shorter tails (during the down stroke of the wing the tail of violet green swallows appears shorter than the wings) with greater forking and thinner wings.
Biology Basics:
Range: Tree swallows winter in the southern United States, Mexico and Central America as far south as Guatemala.
Habitat: During the winter tree swallows inhabit a variety of open habitats. During migratory periods open waters with emergent vegetation become important as night roosts, although trees and even powerlines can suffice. The habitats preferred during the nesting season are similar to those of the wintering period, with the addition of the need for cavities (often in trees) for nesting.
Diet and Behavior: Tree swallows are often found in large flocks over water fields, foraging for flying insects. Unlike most swallow species, tree swallows are not entirely dependent on insects; instead they are able to incorporate plant matter (for example the berries of Myrica californica). The ability to use an expanded suite of food resources may explain their ability to overwinter further north, and migrate earlier than other swallows.
Nesting: Tree swallows are a cavity nesting species. Being unable to excavate their own cavities tree swallows rely on (and often overlap in habitat with) primary cavity excavators such as woodpecker species. Although cavity abundance can be a limiting factor for many secondary cavity users tree swallows have the ability to use a wider variety of natural cavities (beyond those excavated by other spe ies) to fulfil their nesting requirements. Most nests are in standing dead trees over water, stumps and in live aspen trees with cavities excavated by sapsucker species.
The nest itself is an open cup composed primarily of grass. A unique feature of many swallow nests is the inclusion of large feathers, with the rachis (quill end) of the feather embedded in the nesting material, and the vane of the feather covering the eggs. Some studies have suggested that white feathers may be preferred over other feather colors.
Conservation and Stewardship: A great way to help tree swallows on your property is to provide nest boxes. If you wish to include nest boxes remember that placement is very important. Improperly placed boxes can create population sinks where mortality exceeds breeding success. Some tips for tree swallow nest box placement include:
1) Place boxes on poles in open areas.
2) Keep the boxes 100 feet apart
3) Place the boxes 50 yards from the edge of the water, 100 yards from buildings,
50 yards from tree and shrub cover, and 25 yards from roads.
These recommendations help minimize predation, human impact, and nest site competition from other bird species. As winter draws to a close, keep your eyes on local ponds and wetlands. If you do, you may be fortunate to catch a glimpse of our first tree swallows.