Every measurement on this page — floor sizes, entrance-hole diameters, and mounting heights for wrens, bluebirds, swallows, and purple martins — is reprinted word for word from the U.S. Forest Service’s species accounts and the Department of Agriculture’s classic farmers’ bulletin, cited at the end.

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The wrens and the bluebirds

House wren — Troglodytes aedon — L 4¼″

Habitat: House wrens are common nesters in shrubbery and brush throughout the northern two-thirds of the United States, but they winter in the southern states. They range from the plains to timberline throughout the Rocky Mountains. They are commonly found along the edges of woods, swamps, fields, and in orchards.

Nest: House wrens are aggressive in their nesting habits and will drive other birds from cavities. Nests have been found in a variety of sites including 2-inch pipes used to brace fence posts, nesting boxes, natural cavities, and downy woodpecker holes (Sutton 1930). Nests are usually less than 10 feet above the ground. Diameter of the entrance hole in nesting boxes should be 1 inch.

Food: House wrens are capable of eating large quantities of insects and arthropods, which constitute 98 percent of their diet. Animal items include beetles, caterpillars, bugs, grasshoppers, and ants (Bent 1948).

Brown-throated wren — Troglodytes brunneicollis — L 4″

Habitat: Brown-throated wrens inhabit oak forests, mostly in desert ranges, but can be found up to elevations of 8,000 feet in southern Arizona.

Nest: This wren uses natural cavities or old woodpecker holes in tree trunks or limbs. Like the house wren, it will sometimes occupy recesses about buildings and nest boxes (Pough 1957).

Food: We could not find published information on food habits but the diet is probably insects similar to that of other wrens.

Winter wren — Troglodytes troglodytes — L 3¼″

Habitat: Winter wrens inhabit coniferous forests of spruce, fir, and pine, and underbrush in woodlands in eastern and western United States and Canada.

Nest: Winter wren nests, made from twigs and leaves, are built near the ground in exposed roots or fallen logs or in rocks and crevices (Bent 1948, Wetmore 1964).

Food: Details on food items were not found in the literature, but the diet is probably adult and larval stages of insects (Bent 1948).

Bewick’s wren — Thryomanes bewickii — L 4½″

Habitat: Bewick’s wrens are common and widespread in the West, but uncommon and local in the Appalachians and Ozarks. They are usually found in farmyards, brushlands, fencerows, and suburban areas. Bewick’s wrens are fairly common in the pinyon-juniper forest type, and in mesquite-willow-cottonwood associations along southwestern streams.

Nest: Nests of the Bewick’s wren can be found in a multitude of places. Most nests are cup-shaped and can be either open or closed above. They are usually located in cavities close to the ground (Miller 1941). Nest sites include natural cavities, woodpecker holes, knotholes in fallen trees, fence posts, tin cans, bird boxes, and deserted automobiles (Bent 1948).

Food: Bewick’s wrens, like other wrens, eat large numbers of insects that are injurious to vegetation. Ninety-seven percent of the diet is insects, including primarily hemiptera and coleoptera. In the South they are credited with eating boll weevils (Bent 1948).

Carolina wren — Thryothorus ludovicianus — L 4¾″

Habitat: Carolina wrens are common in forest types with thick underbrush throughout the eastern United States. The number in northern populations fluctuates widely depending on the harshness of winter conditions.

Nest: Carolina wrens are quite universal in their choice of nesting sites. These wrens prefer nesting sites that are fairly well enclosed, but they are not totally dependent upon cavities. They are well adapted to habitat conditions provided by man, but also nest in the woods where they prefer tangles and brushy undergrowth. Nests have been found in natural cavities, mailboxes, newspaper cylinders, old hornet nests, and bird houses (Laskey 1948, Nice and Thomas 1948).

Food: Animal food, mostly insects, makes up 93 percent of the Carolina wren’s diet. Of this, beetles, caterpillars, and moths comprise the largest portion. The 7 percent vegetable material is mostly seeds taken in the winter. Since the Carolina wren feeds mostly on or near the ground, deep snow is detrimental to survival. They will visit feeding stations if placed near brush piles (Bent 1948).

Eastern bluebird — Sialia sialis — L 5½″

Habitat: Under natural conditions, eastern bluebirds prefer to use cavities in savannah-like habitats east of the Great Plains (Rustad 1972). They are an edge species and therefore do not live in dense woods or in closely built residential sections of town (Thomas 1946). Like purple martins, bluebirds have taken advantage of nest boxes provided in areas around farms, near open fields, and in orchards.

Nest: Eastern bluebird nesting sites (snags) are often eliminated because of their unsightliness or interference with cultivation. When available, eastern bluebirds nest in old woodpecker holes, hollows of decayed trees, and crevices of rocks (Pearson 1936). They will readily take to hollows in wooden fence posts or correctly sized and placed nest boxes (5 × 5 × 8 inches high with a 1.5-inch hole located 6 inches from the bottom). Boxes should be placed 5 to 10 feet above the ground at the edge of a forest opening or field.

Food: Eastern bluebirds consume 70 percent animal matter and 30 percent vegetable matter. Vegetable intake increases to more than 50 percent in December and January, but is completely lacking in May. Animal matter includes grasshoppers, crickets and katydids, various coleoptera, moths and caterpillars, some hymenoptera and hemiptera, as well as various other invertebrates and small vertebrates. Vegetable matter is mostly wild fruits (Bent 1949).

Western bluebird — Sialia mexicana — L 5½″

Habitat: The western bluebird is most abundant in open ponderosa pine forests of the Transition Zone, but may also be found in oak woodlands, pinyon-juniper, mixed conifer, and subalpine forests.

Nest: Nests are usually in old woodpecker holes, but this bird also uses natural cavities. Nests have been reported in oak, sycamore, and pine trees. In Monterey County, California, nests were found from 5 to 40 feet above ground in pine stumps or trees (Bent 1949). This bluebird, like the eastern, also readily nests near humans in bird houses.

Nest boxes should be 5 × 5 × 8 inches with a 1.5-inch entrance hole located 6 inches from the floor. Boxes should be placed 5 to 10 feet above ground near forest openings or meadows.

Food: Beal (Bent 1949) examined the contents of 217 stomachs and found 72 percent animal material (grasshoppers 21 percent, caterpillars 20 percent, useful beetles 9 percent, other beetles 16 percent, ants 5 percent, other hymenoptera 1 percent) and 28 percent vegetable material, mostly wild fruits, including elderberries, mistletoe berries, blackberries or raspberries, prunes, cherries, and a few weed seeds.

Mountain bluebird — Sialia currucoides — L 6″

Habitat: The mountain bluebird nests in nearly all timber types of the Rocky Mountain region, and is reported from 800 to 11,000 feet elevation in Idaho (Burleigh 1972). However, this species usually ranges from 7,000 to 11,000 feet in open forests or near forest edges.

Nest: The mountain bluebird usually nests in natural cavities or in old woodpecker holes but will also use man-made structures. Nests have been reported in fir and pinyon snags and aspen trees (Burleigh 1972, Bent 1949). We recorded six nests in the White Mountains of Arizona ranging from 12 to 35 feet above ground in ponderosa pine snags.

Five of these were in abandoned woodpecker holes and one was in a natural cavity. Nest boxes should be similar to those for other bluebirds.

Food: This is probably the most insectivorous of the bluebirds. Studies indicate that nearly 92 percent of the diet is animal material, including miscellaneous beetles, weevils, ants, bees, wasps, cicadas, stinkbugs, negro bugs, assassin bugs, jassids, flies, caterpillars, grasshoppers, locusts, and crickets (Bent 1949). Vegetable items include currants, grapes, elderberries, sumac seeds, mistletoe berries, hackberry seeds, Virginia creeper seeds, and cedar berries.

The swallows and the purple martin

Violet-green swallow — Tachycineta thalassina — L 4¾″

Habitat: Ponderosa pine affords the favorite habitat for violet-green swallows (Bailey and Niedrach 1965), but they are also found in aspen-willow and spruce-aspen forests. They prefer open or broken woods or the edges of dense forests.

Nest: Violet-green swallows nest in holes, cavities, and crevices in a variety of situations. Where birds are abundant, the demand for nest sites is sometimes greater than the supply, and practically any available cavity may be used. These swallows have been reported to use old nests of cliff swallows and even burrows of bank swallow (Bent 1942).

Winternitz (1973) reported violet-greens using old woodpecker holes in live aspen as nesting sites, but in Arizona, we found them nesting primarily in old woodpecker holes in ponderosa pine snags. We found one in the dead top of an aspen, 5 in dead tops of ponderosa pine, and 26 in ponderosa pine snags. Nest heights ranged from 16 to 80 feet and averaged 43 feet.

Food: Apparently, the diet of this species is exclusively insects taken on the wing. It includes leafhoppers, leaf bugs, flies, flying ants, and some wasps, bees, and beetles (Bent 1942). In Colorado, Baldwin (pers. comm.) found that insects made up 99 percent of the stomach contents of six violet-green swallows. Flies were the most abundant insect found. Scolytid beetles, seed and leaf bugs, miscellaneous insects, and a few spiders were also found.

Tree swallow — Iridoprocne bicolor — L 5″

Habitat: Tree swallows breed throughout North America from the northern half of the United States north to the limit of tree growth. They are migrants throughout the Central and Southern states and winter primarily in Central America.

Nest: Tree swallows prefer to nest in natural cavities and old woodpecker holes—usually near water. The lack of natural cavities, competition for existing cavities, and the availability of nest boxes, have resulted in a shift in nesting preferences to nest boxes in the eastern United States (Bent 1942, Low 1933, Whittle 1926). Bluebird boxes and purple martin houses are frequently used.

Tree swallows are not colonial, but will nest within 7 feet of each other, if there are adequate meadows, marsh, or water area available for feeding (Whittle 1926). Woodpecker holes in aspen, spruce, and pine are the most common nest sites in the West (Bailey and Niedrach 1965).

Food: This species is the first of the swallows to arrive in the north in the spring, and the last to depart in the fall. Because tree swallows can subsist on seeds and berries, they are not as dependent upon insects as are other swallows. They are partial to waxmyrtle and bayberry where these are available.

Plant food proportions in the diet are 1 percent in spring, 21 percent in summer, 29 percent in fall, and 30 percent in winter (Martin et al. 1951, Forbush and May 1939).

Purple martin — Progne subis — L 7″

Habitat: The natural nesting population of purple martins prefer open woodlands or cutover forests where suitable snags remain. Purple martins have been reported in oak, sycamore, ponderosa pine, Monterey pine, spruce, and fir forests of California (Grinnell and Miller 1944). In the Southwest, the purple martin breeds in the ponderosa pine belt and in the saguaro cactus desert.

Nest: The western purple martin has not adapted to nesting in boxes as well as the eastern form (Bunch 1964), and much of the western population depends upon holes made by woodpeckers, usually in tall pines in relatively open timber stands (Bent 1942). Martins also nest in old woodpecker holes in saguaro cactus. We have recorded 21 nests near Cibecue, Arizona, all in ponderosa pine snags.

Nests ranged from 25 to 35 feet above ground. Nest compartments in martin houses should be 6 × 6 × 6 inches with an entrance hole 2½ inches in diameter 1 inch above the floor. The boxes should be 15 to 20 feet above ground.

Food: The purple martin feeds on the wing, and nearly all the diet is insects, although some spiders are taken (Beal 1918). Johnston (1967) examined the stomach contents of 34 martins collected in April, May, June, and August in Kansas. Beetles, true bugs, flies, bees, and wasps were the important food items. Although the purple martin has been credited for feeding on large numbers of mosquitoes (Bent 1942), it was not documented by the two food habit studies mentioned.

Why the government told farmers to put up boxes in the first place

Seven common species of swallows are found within the limits of the United States, four of which have abandoned to some extent their primitive nesting habits and have attached themselves to the abodes of man.

In the eastern part of the country the barn swallow now builds exclusively under roofs, having entirely abandoned the rock caves and cliffs in which it formerly nested. More recently the cliff swallow has found a better nesting site under the eaves of buildings than was afforded by the overhanging cliffs of earth or stone which it once used and to which it still resorts occasionally in the East and habitually in the unsettled West.

The martin and the white-bellied, or tree, swallow nest either in houses supplied for the purpose, in abandoned nests of woodpeckers, or in natural crannies in rocks. The northern violet-green swallow, the rough-winged swallow, and the bank swallow still live in practically such places as their ancestors chose.

Field observation convinces an ordinarily attentive person that the food of swallows must consist of the smaller insects captured in mid-air or picked from the tops of tall grass or weeds. This observation is borne out by an examination of stomachs, which shows that the food is made up of many small species of beetles which are much on the wing; many species of flies, together with large quantities of flying ants; and a few insects of similar kinds.

Most of these are either injurious or annoying, and the numbers destroyed by swallows are not only beyond calculation but almost beyond imagination.

Unlike many other groups of birds, the six species of swallows found in the Eastern States extend in a practically unchanged form across the continent, where they are reinforced by the northern, or Pacific-coast, violet-green swallow.

It is a mistake to tear down from the eaves of a barn the nests of a colony of cliff swallows, for so far from disfiguring a building they make a picturesque addition to it, and the presence of swallows should be encouraged by every device. It is said that cliff and barn swallows may be induced to build their nests in a particular locality, otherwise suitable, by providing a quantity of mud to be used by them as mortar.

Barn swallows may also be encouraged by cutting a small hole in the gable of the barn, while martins and white-bellied swallows will be grateful for boxes like those for the bluebird, but placed in a higher situation.

Nest box dimensions at a glance

Source

Scott, Evans, Patton & Stone, Cavity-Nesting Birds of North American Forests, USDA Agriculture Handbook 511 (1977), species accounts. F. E. L. Beal, Some Common Birds Useful to the Farmer, USDA Farmers’ Bulletin 630, revised edition (1926), “The Swallows.” Both United States federal publications. Public domain. Project Gutenberg #49172 and #62678.

Species accounts reproduced verbatim and in their original order. Illustration placeholders and footnote markers have been removed; the species name lines are set as headings; wording, spelling and punctuation are otherwise as printed, including each account’s literature citations.