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The Eastern Bluebird, Sialia sialis, is a small thrush found in open woodlands, farmlands and orchards, and most recently can be spotted in suburban areas. It is the state bird of Missouri and New York.
This species measures 16–21 cm (6.3–8.3 in) long, span 25–32 cm (9.8–13 in) across the wings and weigh 27–34 g (0.95–1.2 oz). Adult males are bright blue on top and have a reddish brown throat and breast. Adult females have lighter blue wings and tail, a brownish throat and breast and a grey crown and back. Eastern Bluebirds are found east of the Rockies, southern Canada to the Gulf States and southeastern Arizona to Nicaragua.
The bright blue breeding plumage of the male, easily observed on a wire or open perch, fluttering down to the mowed grass to capture a grasshopper, cricket or beetle makes this species a favorite of birders. The male's call includes sometimes soft warbles of jeew or chir-wi or the melodious song chiti WEEW wewidoo (Sibley, 2000).
Approximately two-thirds of the diet of an adult eastern bluebird consists of insects and other invertebrates. The remainder of the bird's diet is made up of wild fruits. Favored insect foods include grasshoppers, crickets, katydids, and beetles. Other food items include earthworms, spiders, millipedes, centipedes, sow bugs and snails.
Fruits are especially important when insects are scarce in the winter months. Some preferred winter food sources include dogwood, hawthorn, wild grape, and sumac and hackberry seeds. Supplemental fruits eaten include black raspberries, bayberries, fruit of honeysuckle, Virginia creeper, Eastern Juniper and pokeberries. Bluebirds feed by perching on a high point, such as a branch or fence post, and swooping down to catch insects on or near the ground. The availability of a winter food source will often determine whether or not a bird will migrate. If bluebirds do remain in a region for the winter, they will group and seek cover in heavy thickets, orchards, or other areas in which adequate food and cover resources are available. If you want to attract these species, put crunched-up peanuts in a feeder about four feet off the ground.
Eastern bluebirds are very social birds. At times they gather in flocks of a hundred or more. However, they are territorial during the breeding season and may continue to defend a feeding area throughout the winter. Mating occurs in the spring and summer months. A mature female will typically raise two broods each season. Nests are constructed in trees within abandoned woodpecker holes or other cavities that provide adequate protection (usually several feet above ground). Construction of the nest is done primarily by the female and takes approximately 10 days to complete. These nests are small, cup-like structures that are lined with grass, feathers, stems, and hairs. Each female lays 3 to 7 (average 4 to 5) light-blue or, rarely, white eggs. The female incubates the eggs, which hatch after 13 to 16 days. The young are altricial at hatching, meaning they cannot care for themselves upon hatching. The female broods the chicks for up to 7 days after hatching. Fledglings then leave the nest 15 to 20 days after hatching.
Both parents cooperate in raising the young, which they feed a diet consisting almost entirely of insects. Several studies have revealed that some young will stay around the nest to help raise another brood. Fledglings are grayish in color with a speckled breast. The blue color becomes much more prominent and the speckles on their breast disappear as they mature. Bluebirds may begin breeding the summer after they are hatched.
Eastern Bluebirds can live for 6 to 10 years. The record lifespan for a bluebird was 10 years and 5 months. However, a majority of bluebirds die within their first year of life. Starvation and freezing can threaten young bluebirds, but most threats come from other animals, including humans. Natural predators of eggs and nestlings can include eastern chipmunks, flying squirrels, American black bears, fire ants and raccoons. Bluebirds of all ages (including adults) are threatened by rat snakes, racers and American kestrels. Introduced species such as European starlings, house sparrows and domestic cats pose a major threat to bluebird nests as well, with the cat being a serious predator of adult bluebirds and the other birds being competitors for nesting sites. Non-nesting adults face predation with all native species of falcon, owl and most varieties of hawks (particularly in the Accipiter genus). When approached by a predator, male eastern bluebirds make a song-like warning cry. If a male is not present, a female will begin to sing, hoping to attract a protective male back to the territory. Both males and females will also flick their wings and warble when predators are nearby but losses are often heavy when a persistent predator finds their nest.
Habitat and importance of nestboxes in conservation
Eastern bluebirds prefer to nest in woodlands where cavity holes excavated by a previous species will serve as their home. These woodlands must be near clearings or meadows because this is the preferred hunting ground of the species. River or creek access is an added benefit and preferred. Keep these things in mind when placing a nestbox on your property.
Determined bluebird lovers may wish to pay attention to nestbox size (overly large boxes can invite deadly raids by non-native Common Starlings). The entrance hole should be 1 1/2" diameter for Eastern Bluebirds, and the box should have good ventilation. Active and/or passive pest control is also greatly useful against non-native House Sparrows—these are small enough to enter bluebird boxes and kill bluebirds, and destroy nests. Although doing well now, Eastern Bluebird populations declined to a level raising extinction fears by the 1960s, and in large part, the volunteer intervention of bluebird lovers in Eastern North America brought the species back by implementing the above. Such practices and the organizations promoting them remain important for stable populations today. An activist writes, "The most significant factor in the recent population recovery is volunteerism - by young and old - people like you - doing their part by putting up and monitoring nestboxes, spreading the word, and encouraging others to get involved."
- BirdLife International (2012). "Sialia sialis". IUCN Red List of Threatened Species. Version 2012.1. International Union for Conservation of Nature. http://www.iucnredlist.org/apps/redlist/details/106006344. Retrieved 16 July 2012.