Bluebirds, sometimes referred to as “gems of nature”, have received a lot of attention and help from people in the past 35 years or so. Once commonly found throughout eastern North America, Eastern Bluebird numbers dropped by as much as 90% during the time period of 1930 – 1990.
Why did Eastern Bluebird numbers drop? There were a variety of reasons for this decrease in numbers. Forestry practices, such as clearcutting, removed many trees which had the holes (cavities) that Eastern Bluebirds use for nesting. For the trees with cavities that did remain, Eastern Bluebirds were faced with competition from two new birds, the European Starling and House Sparrow. These birds were introduced into the United States and are successful in outcompeting bluebirds for nest sites and will sometimes kill bluebird nestlings and adults. Another highly effective, non-native predator on bluebirds are cats. It is estimated that as many as 3.7 billion songbirds are killed annually by cats across the United States. Much open grassland habitat (prairie or savanna) was converted to agricultural use or for residential development. During the 1930’s, production of pesticides and their use in agricultural production began to expand. This led to the poisoning of the insects that bluebirds feed upon, particularly crickets and grasshoppers. Birds suffered secondary poisoning by eating these animals.
What has been done to help bluebird populations to recover? In 1978, the North American Bluebird Society was founded by Dr. William Zelezny. The Society’s goal is to increase the populations of North America’s three bluebird species (Eastern Bluebird, Western Bluebird and Mountain Bluebird) and other cavity-nesting birds. Many Iowans share a concern for Eastern Bluebirds, the only species of bluebird that nests in Iowa, and are working to increase their numbers on our landscape.
The Eastern Bluebird is a bit smaller than an American Robin but larger than most of our sparrows. Males have a beautiful blue back and rusty colored throat and breast. Females are a bit duller in color, but do show a bit of blue in their wings and like the male, have a blue tail. Youngsters are gray in color with a speckled breast and bits of blue on their wings and tail.
Eastern Bluebirds are tied to open habitats with a few trees, larger shrubs or fence posts for nesting and perching. This includes orchards, pastures and croplands near wooded area, but in more urban environments, places such as golf courses, cemeteries and parks are used. Eastern Bluebirds use perches when hunting for their favored prey, crickets and grasshoppers. Other items in their diet include beetles, worms and spiders.
Up to a third of Iowa’s Eastern Bluebirds overwinter locally, although most leave for more southern states. Eastern Bluebirds do not have insects available during Iowa’s winter and instead feed on the fleshy fruit from plants such as red cedar, Virginia creeper, sumacs, and hackberry.
Eastern Bluebirds are cavity nesters although they don’t dig their own holes, but use abandoned holes left by woodpeckers or squirrels. These holes keep the bluebird adults and young safe from many predators, especially hawks, owls, crows and jays. With the clearing of forests, loss of orchards, and conversion of fence posts to steel from wood, Eastern Bluebirds have lost many nesting sites and have begun to rely upon nesting boxes provided by people.
In early spring, male Eastern Bluebirds return to set up territories and attract females upon their arrival. The earliest arriving males pick out the best nesting areas, and set up and defend a territory several hundred feet wide to secure access to food for their mates and young. When females arrive, they inspect one to many areas and may start nests in several of these before finishing the one nest that will be used.
Females do most of the nest building, and all of the egg-laying and incubation. The cup-shaped nest is made from materials that are nearby and can include items such as dry grass, strips of dry bark, pine needles, cattail fluff, twigs, straw, and rootlets. Each of the typically four to six pale blue, bluish-white, or rarely white eggs, is laid a day apart. The male feeds the female during the 12-14 day incubation period which begins on the day that the last egg is laid. The young, called nestlings, typically hatch within a 24-hour period with no feathers and eyes closed. To keep the nestlings warm, the female broods (sits atop) them during their first week, particularly at night and during rainy or cold weather. Males are attentive parents and share feeding duties with their mates. At about four days, the nestlings open their eyes and are covered with gray down. By the time they are 16-20 days old, the young fledge (leave the nest). The parents continue to feed the young, now called fledglings, while the fledglings improve their flying and hunting skills. At first, the fledglings will fly only a short distance and try to land on a fence post or tree branch where they’ll be fed by one or both of their parents. Within a week or two, the fledglings become better flyers and are able to catch their own food.
As soon as the young leave the nest, the parents begin the construction of a new nest or tidying up of the existing nest, to prepare for their second family. In some cases, youngsters from the first hatch will help in raising the young from the second. In especially good years, with plenty of insects to eat and good weather, bluebirds may even raise a third family in a season.
The various families raised by the parent birds flock together until the fall. Most of Iowa’s Eastern Bluebirds then head south to Missouri, Arkansas, Mississippi, and Texas, and return to Iowa the following spring.
Rebecca Christoffel. Christoffel Conservation - March 2014.
Special thanks to Rita Goranson (Member of the Iowa Ornithologists' Union) for helping make this revision possible.