In February I noted that the tree swallows (Tachycineta bicolor) had returned to Modoc County, a little earlier than usual this year. Recently these iridescent blue and white birds have been courting and showing interest in the tree cavities and nest boxes where they raise their young.
Courtship among tree swallows involves pursuit intended to display the flying proficiency and expertise of the male. Pairs often remain together but sometimes change mates at the beginning of the breeding season. Tree swallows can be monogamous or a male may have two or more mates at once. Although this native of the Americas congregates in huge flocks during the non-breeding season, tree swallows build their nests alone or perhaps in the vicinity of one or two other nests. Occasionally individuals who do not have mates or birds who have lost their nest assist in caring for the young of other pairs.
The female gathers the materials and constructs the nest without assistance from the male. He will occasionally gather building materials but this appears to simply be for show. The male’s job is defense. The nest is an open cup of pine needles or grass in the tree cavity or nest box. This is unlike some other birds, for example, the acorn woodpecker, who do not build any sort of nest but rather simply lay the eggs on the base or floor of the cavity. The nest is lined with the feathers of other bird species, particularly waterfowl. The tree swallow prefers white feathers, when available. The feathers are often arranged with the tips curling up over the edge of the nest. This provides a warm home for the chicks, which helps the youngsters develop and mature rapidly. In addition, the feathers help keep the numbers of ectoparasites, like mites, low. The female incubates the 2 to 8 white eggs for 13 to 16 days. Upon hatching the tree swallows are featherless, blind and helpless. Occasionally as they get older the chicks do peek their little heads out of the cavity hole, but without a nest cam they cannot be observed unti they fledge and leave the nest.
The tree swallows are busy building nests everywhere. I took these pictures at Ash Creek Wildlife Area, Baum Lake and on the Lower Hat Creek (California).