This is Bird Note.

A flash of glittering dark blue cuts through the gray of late February,

as a male tree swallow glides low over a cat-tail marsh.

The tree swallows liquid notes hint at spring's approach.

Where a broad green pasture meets the forest edge,

there is a second, even brighter flash of blue.

A male bluebird alights on a fence post.

The intense blue of its back glints in the welcome sunlight of late winter.

Tree swallows and all three North American bluebirds in the welcome sunlight of late winter.

Tree swallows and all three North American bluebird species are among the earliest northbound migrants to arrive, heralding spring a month before the equinox.

Both species will nest only in cavities, such as old woodpecker holes or man-made nest boxes. The supply of such

specialized nest sites is limited. Competition is intense. By arriving so early, tree swallows

and bluebirds improve their chances of finding unoccupied cavities, and they may fiercely guard

a nest site until April, before actually nesting.

Both species benefit greatly from nest box programs, but it's crucial to put up only nest boxes

with very specific hole sizes that encourage these flashy blue migrants, but deter non-native starlings and house sparrows.

To learn about how to provide just the right home for tree swallows and bluebirds,

come to our website, birdnote.org.