Friday, March 10, 2006
Eastern Bluebird (Sialia sialis)
"Dull indeed would be the man that did not feel the thrill awakened by the first glimpse of brilliant color in the orchard and the cheery warbling notes borne to our ears on the first gentle breath of spring!"(1) My first (ever) bluebird sighting, two females and a male, was at Towle Field on April 29, 1990. In the 1980s, naturalists and former Carlisle residents Donald and Lillian Stokes installed nest boxes at Towle Field and Foss Farm. Many of these boxes are still in use.
Bluebirds are "cavity nesters" that build their nest either in natural cavities or in nest boxes.
In the nineteenth century, when open fields and orchards were common in New England, bluebirds preferred to nest in the cavities of old apple trees. However, changes to the natural landscape greatly reduced the numbers of these handsome birds. Agricultural fields have largely disappeared to housing developments, and those fields that remain on conservation lands face gradual encroachment of trees and shrubs. We have seen only one instance in Carlisle in which bluebirds used a natural cavity as a nest site. It was built in a snag located in a swamp and had been previously occupied by a woodpecker. All the other bluebird nests we have seen were built in nest boxes. Beaver damming often benefits bluebirds and other cavity nesters because flooding kills trees, creating potential nest sites.
Bluebirds face stiff competition from other birds (and other animals) for nest sites. Tree swallow, house wren, European starling, and house sparrow also use nest boxes. The last two species are non-native species introduced from Europe; their nests may be legally removed to encourage bluebirds and other native birds. The Stokes' book (2) and the web site of the North American Bluebird Society (3) have plans for nest boxes. Tricia Smith constructed several boxes using plans from (2) at the corner of Indian Hill Road and Concord Street and her boxes at the corner of Indian Hill Road and Concord Street have been very popular with bluebirds. Places to see bluebirds in Carlisle are areas with large fields and nest boxes: Towle Land, Foss Farm, and the Cranberry Bog. Bluebirds raise two and sometimes three broods, and a pair may start the first brood as early as March. Very cold and wet weather can kill an entire brood by starvation; this happened in early May last year at Towle Field during a prolonged cold and rainy period. The nest is usually built of fine grass or pine needles, and the female typically lays four or five eggs.
Approximately 70% of the bluebird's diet consists of insects such as crickets, grasshoppers, beetles, weevils and caterpillars. Other small invertebrates such as spiders, centipedes and sowbugs are also eaten. Bluebirds also eat the fleshy fruits of dogwood, red cedar, sumac, bayberry, Virginia creeper, bittersweet, poison ivy, and other plants. In winter when insects are scarce, fruit constitutes most of the bluebird diet. Bluebirds often forage by flying from a low perch such as a fence post to the ground to capture prey, a strategy called "ground-sallying" or "hawking."
Many other creatures also use bird boxes. We often find white-footed mice, sometimes as many as five, inside during cold weather. One year, we discovered a flying squirrel using a Towle Field nest box. Last fall, I found many jumping spiders (4) living in nest boxes, especially in those boxes where a house wren had built its stick nest. Paper wasps often attach their nests to the inside of the roof; rubbing soap or wax on the surface will discourage them. If you have nest boxes, the old nest and any other debris should be removed in early spring in order to make them available to birds.
The writer thanks Jim Fenton of Haverhill, Mass., for permission to use his bluebird photograph.
1. A.C. Bent, Life Histories of Familiar North American Birds: http://birdsbybent.netfirms.com/ch21-30/bluebird.html
2. D. & L. Stokes, The Complete Birdhouse Book, Little, Brown & Co., 1990.
3. North American Bluebird Society nest box plans: www.nabluebirdsociety.org/boxspecs.htm
4. Biodiversity Corner: Jumping Spider, Carlisle Mosquito, June 18, 2004.
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