Bluebird Housing & Nesting
Nest Building: The breeding season begins anytime from mid-March to early April. The male initiates selecting the nest site by "showing" the female several possible sites. The female may begin to build nests in several sites, but eventually she decides on a site and concentrates her efforts there. It usually takes four to six days to build the nest, but this varies with the time of season, weather, and the age and experience of the breeding pair.
The nests, which are built in woodpecker holes, dead or rotting trees, and in nest boxes, are composed mainly of dry grasses, rootlets, and weed stems. Some nests are built entirely of pine needles. The cup is usually lined with fine grasses, rarely with hair and fur. Males may carry nest material to the nest, but they do not participate in the actual building of the nest. Rather, they spend much time guarding their mates during this time to prevent them from mating with other males.
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Eggs can be laid as early as late March or as late as early July, depending upon the weather and latitude. Females may begin to lay eggs one or two days after the nest is completed, but some females wait a week or more. One egg is laid each day, in the morning. The average clutch has three to five eggs but as many as seven have been reported. Clutch sizes tend to be smaller for younger females and for second broods of the breeding season. The eggs are smooth and glossy and are sky blue or white in color. Because all eggs laid by a single female are the same color, the presence of an odd-colored egg in a clutch may indicate that another female has laid her egg in the nest, a practice known as egg dumping.
Eggs can remain un-incubated for awhile and still be viable; however, once incubation begins, it must be continuous. Females generally begin to incubate the day the last egg is laid. The incubation period is 12 to 14 days but can be longer in the case of extreme or prolonged cold weather.
Nestlings hatch within one or two days of each other, and the female broods the nestlings for a few days. Both adults tend the young. The nestlings begin to thermoregulate, or regulate their own body temperatures, when they are about six days old, and females then decrease the amount of time they spend brooding. Nevertheless, the females may continue to brood at night during cold weather. The young leave the nest after 16 to 22 days, but they remain dependent upon their parents for food and protection for three to four weeks.
Eastern Bluebirds raise two broods per season. Pairs may build their second nests on top of the first nest, or they may nest in an entirely new site. The male continues to tend the fledged young while the female begins to re-nest. Young from the first brood will reportedly help raise siblings from the second brood.
More About Bluebirds
Eastern Bluebirds are sexually dimorphic; that is, males and females look quite different. Males have bright blue heads, tails, backs, and wings. The sides, flanks, and throat are chestnut red. The underparts are also chestnut red from the chin down to the belly, but the belly is white. Although there is much variation in their plumage, females are generally less colorful than males. They have light gray-blue heads, dull brown backs, and blue tails and wings. There is a slight white ring around the eye. In winter, the female’s upper breast turns a pale reddish-brown.
Distribution and Breeding Habitat
Eastern Bluebirds can be found east of the Rockies, throughout the eastern United States and Canada, and down to central Mexico. Some populations are year-round residents, but others migrate to more southerly latitudes for the winter. Eastern Bluebirds are found in suburban and rural habitats containing sparse vegetation and scattered trees or other perches. They typically nest in fields, meadows, and orchards, avoiding both densely wooded and congested residential areas.
Eastern Bluebirds eat a variety of invertebrates, including caterpillars, butterflies, moths, grasshoppers, katydids, and spiders. They also feed on wild fruits. Primarily ground feeders, they prefer feeding and nesting in areas with short, sparse vegetation, which affords a clear view of ground-dwelling insects. Pair Formation and Territoriality Eastern Bluebirds are monogamous. Pairs generally stay together throughout the breeding season, and pairs may breed together for more than one season. Some birds, however, may switch mates during a breeding season to raise a second brood. Both sexes defend territories; however, the males tend to defend territory edges while the females primarily defend the nest site.
Winter Movement and Dispersal
Families flock together until fall, when they merge with other family flocks. Some, but not all, bluebirds residing in the northern portions of the range migrate to southern latitudes, but those residing in southern latitudes tend to be residential. Adults tend to return to the same breeding territory year after year, but only a small percentage (three to five percent) of young birds return to their natal area to breed.
Source: Cornell Lab of Ornithology