Tree swallows have blue-green back plumage and a black eye mask. Their undersides are white. Note how long the primaries are on this bird, extending to beyond the tail when folded. (G. Thomas Bancroft)

A clear sweet high pitch whistle came through the air. It sounded a little like a whine that moved into a gurgle and then to a chirp. The soft sound began again after a short pause. The sun was out and the air cool as I walked along the dike at Wylie Slough in Skagit County. It was only the third week of March, and I was surprised to hear the song of a Tree Swallow at this time in Washington. I scanned the branches for this small insectivores bird. They call from a perch near a possible nesting cavity.

The white underside of the Tree Swallow shows clearly as this bird grips to a small twig. (G. Thomas Bancroft)

An iridescent green streak shot from a branch, its long pointed wings propelling the bird forward. The swallow began to do acrobatic twists and turns as it attempted to catch insects on the wing. The bird flew out over the marsh, only a few feet above the brown vegetation, looped around a dead tree that rose out of the shallow water, twisting to the right and out of sight.

Wylie Slough is near the outlet of a Skagit River branch. A few years ago, this area was restored to tidal wetlands, removing dikes that had allowed part of it to be farmed, reestablishing tidal flow, and creating habitat for salmon and wetland birds. The dike runs several miles through this wetland and provides excellent viewing of the restoring wetlands.

A tree swallow giving its high pitch song near a nest box along Wylie Slough in Skagit County, Washington. (G. Thomas Bancroft)

The tree swallow flew in past my head, only a dozen feet away, and landed on a dead branch, looking away from me. The bird began to preen its feathers, twisting its head to grab individual feathers with its delicate bill and pull them through, straightening the barbules, making sure the feather functioned properly. The swallow crunched its neck in contortions to preen feathers along its back.

A tree swallow peers down from a perch on a dead snag at Wylie Slough along the Skagit River. (G. Thomas Bancroft)

A second bird alighted just inches from the first and began to sing softly, a mated pair. A small wooden bird box was attached to the trunk of a red alder 10 feet off the ground. The pair had appropriated this location for a future nest. Several woodpecker holes were in the dead trees, and this pair could choose to use one of them. I left the pair resting on the branch and continued down the dike.

A nesting box hung from another tree at eye level, and I stopped to watch if this one had a prospecting pair. Two minutes later, a swallow swooped low over the open water beyond the tree, twisting a few times, before flying onto a knot above the box. It sat, staring down at me, bending its head to the side as if it wasn’t sure what I was. The bird stayed only a second before dashing off in the opposite direction, but in less than a minute it returned, quickly followed by a second one. One bird flew down from the perch to hang on the side of the box, sticking its head in and out of the hole, but never fully entering the cavity, before it flew out on a foraging trip.

Over the next week or so, many more tree swallows will return to this wetland. The abundant dead trees in the recently flooded fields should be full of woodpecker holes and would provide many places for swallows to nest. If sites are available, tree swallows will nest in dense numbers, just defending the cavity and a small space around it. I stopped to listen to a bird give its high pitch song, thinking that spring is rapidly progressing in Washington. In two days, it will be the spring equinox.

A tree swallow sits crosswise on a dead branch. The blue-green feathers of its head glisten in the sun and its white breast feathers so their shoft texture. These insectivorous birds use keen eye-sight to catch flying insects. often foraging over wetlands. (G. Thomas Bancroft)