Several species of cavity-nesting birds will nest in martin housing. The task of identifying a nest can be challenging, especially if the tenant of a mystery nest is not observed entering or leaving the cavity. Below are descriptions and photos of some common cavity-nesting birds, their nests, and eggs.
Purple Martins build their nests out of small twigs, straw, bark, and mud; and line the nest bowl with green leaves. They build a flat nest, only about 1 to 3 inches high and some nests feature a mud dam, or lip, at the front of the nest. Adult and subadult nests can vary. Subadults—the less experienced nest builders—will generally have a more sparse nest and any only contain a few pieces of nesting material and green leaves. Eggs are pure white and measure about 24.5 x 17.5mm.
Tree Swallows build nests out of straw and fine grasses, lining the cup with feathers. Eggs are pure white and measure about 18.7 x 13.2mm.
Eastern Bluebirds build their nests out of fine grasses, straw, and wheat stalks. The eggs are almost always pale blue (5% of bluebirds lay white eggs, a genetic trait) and measure 20.7 x 16.3 mm.
House Wrens build bulky nests out of twigs, usually topped off with a cup of grasses, plant fibers, feathers, and hair. Male House Wrens build several ‘dummy’ nests entirely of twigs, and the female chooses one to complete with the cup of grasses. The eggs are white but thickly speckled with cinnamon-brown dots and measure 16.4 x12.7 mm.
While Barn Swallows are not cavity nesters, they are often confused with Purple Martins. Barn Swallows will nest in barns, under decks, porches, bridges, and docks. Nests are built out of mud and lined with feathers and grass. Eggs are white, with brownish speckles and measure 18.8 x 13.5 mm.
House Sparrows build bulky nests from straw, weeds, grasses, and trash. The nest is lined with feathers, hair, and string. House Sparrow nests will fill the entire compartment, leaving only a circular tunnel where they enter. You will oftentimes see the nests spill out of the entrance. Eggs are white or greenish white, dotted with grays or brown, and measure 22.8 x 15.4mm.
Starlings, like House Sparrows, build bulky, slovenly nests of coarse grasses, weed stems, straw, twigs, corn husks, cloth, and feathers. Eggs are similar in color to robin eggs, but larger, pale blue or green, unmarked, and measure about 29.2 x 21.2mm.