Tree Swallow, Fernhill Wetlands, Forest Grove, Oregon on 12 April 2008 by Greg Gillson.
The Tree Swallow is generally the first swallow to arrive in the Pacific Northwest in spring. In fact, in northern California it regularly winters in small numbers in the Central Valley and along the coast. West of the Cascades in Oregon and Washington it arrives in late January. Additional birds arrive throughout the Pacific NW through March and into April, when they can be found over many ponds hawking insects and inspecting nest boxes placed there for them or inspecting holes in trees over water for suitable natural nest sites. Most migrate south from August to October, but there are some mid winter reports in the Pacific NW, primarily along the coast.
Tree Swallows are widespread across North America. In summer they occur from the edge of the treeless tundra in Alaska and across northern Canada south to just the northern edge of southern California, Arizona, New Mexico, Texas and the Gulf States. In winter, as discussed, they occur along both US coastlines from Virginia to Florida to Texas, and southern California and along the Colorado River in Arizona, and from there south throughout Mexico to Guatemala, Honduras, and also in Cuba.
The Tree Swallow is a metalic blue above with blackish-brown wing and tail feathers and white underparts. They show a black mask. Females are duller than males. Juveniles and first year birds can appear quite brownish, but show the darker mask. The photo above shows an adult male in brilliant breeding plumage.
A similar bird in the West is the Violet-green Swallow. The back of Violet-green Swallow, seen at close range, is a shocking bright lime green. The side of the face is white (lacking the black mask surrounding the eye that Tree Swallow shows). In flight, the Violet-green Swallow has shorter, more triangular wings, and white rump patches that almost meet at the base of the upperside of the tail. Violet-green Swallows are much more likely to make their home in nest boxes in your backyard than Tree Swallows--unless you live right on the water.
Juveniles and dull first year birds might be mistaken for Bank or Rough-winged Swallows. Indeed, some Tree Swallows show a hint of dusky breast band. But the dark, well-defined mask is a primary clue to identification if well seen. Bank and Rough-winged Swallows do not nest in nest boxes, so any brownish swallows doing so are likely Tree (or Violet-green) Swallows.