Nests and eggs that may show up in bluebird nestboxes

Cavity Nester Nests, Eggs and Young Photos and Bios:

Descriptions (click to jump to): Ash-thoated Flycatcher, Bewick's Wren,Eastern Bluebird, Mountain Bluebird, Western Bluebird; Black-capped Chickadee, Carolina Chickadee, Carolina Wren, Cowbird, Eurasian Tree Sparrow, Great Crested Flycatcher, House Finch, House Sparrow, House Wren, mice, Mountain Chickadee, Nuthatch, Oak Titmouse, Prothonotary Warbler, Purple Martin, (European) Starling, Tree Swallow, Tufted Titmouse, Violet-green Swallow

One of the easiest ways to identify a nest is by the eggs. You can also gain clues by the shape of the nest and materials. There are several guides to nests and eggs. To identify eggs, it is helpful to know where they were found, color, markings, gloss, size, and shape.

Be aware there is often considerable variation in a nest of a particular species, depending on the individual, circumstances, location and available materials. Timetables vary. You may be able to identify the nester by watching what birds go in and out of the box with nesting material. But other birds may sit on or check out a box - with my birdcam, over a two day period I saw a bluebird, Starling, House Wren and Titmouse all sitting on or checking out a box claimed by a Black-capped Chickadee. Other birds will enter a box being used by another species: e.g., House Sparrows and House Wrens for the purposes of attack; or other birds like bluebirds or Tree Swallows out of curiosity. If you see a parent feeding nestlings, you can be pretty sure. (There have been a few documented cases of birds feeding nestlings of other species though!) Usually Bluebirds, House Sparrows, House Wrens, and Tree Swallows are very visible (sitting on top of box, etc.) You may never see Chickadees or Titmice until the nestlings hatch and require constant feeding.

• Eastern Bluebird: Neat, cup shaped, woven nest typically 100% fine grass or pine needles. Cup may be in the back of the box. Occasionally bits of fur or a few feathers, or even some hair (e.g., from a horse). Fairly deep, often cylindrical nest cup - usually 3-4" deep, with the cup portion 2.5" in diameter and about 2.25" deep. Eggs are powder blue (no dark spots), sometimes white. See photos.
• Mountain Bluebird: Nest of grass, weed stems, pine needles, twigs, rootlets, bark, and, sometimes, wool, hair, or feathers. Eggs are smooth, glossy, unmarked, and are pale blue, bluish-white, or, rarely, white. Photos.
• Western Bluebird: Collection of grasses, weed stems, and, sometimes, hair and feathers. Routinely add ribbons, strips of cellophane, thin bark and leaves to their nest, which may cause some to confuse it with a House Sparrow nest. See photos by Linda Violett. Linda has also found straw wrappers (common), 8-track tape, shoestring, a large pink costume feather, flowers (Bougainvillea is "in" in 2007), plastic newspaper ties, polyester batting, leaves, cigarette filters, and plastic rings in WEBL nests. Eggs are pale blue, bluish-white, or white. See photos.
• House Sparrow: Loose jumble of odds and ends, including coarse grass (with seed heads), cloth, feathers, twigs and sometimes litter (e.g., clear plastic, cigarette filters). Mid-summer nests sometimes contain bits of green vegetation (mustards or mints.) Tall nest, often with tunnel like entrance, particularly when built outside of a nestbox. In a nestbox may have more of a cup shape, and may be built up to cover sides of box. Eggs are cream, white, gray or greenish tint, with irregular fine brown speckles, shell is smooth with slight gloss. See photos and more photos. The background color can vary, the color of the spots can vary, the thickness of spotting can vary, and the size can vary. NOTE: House Sparrows are non-native invasive pests, and are not protected by law. They will attack and kill adult bluebirds (sometimes trapping them in the nestbox), and destroy eggs and young.  House Sparrow nests, eggs, young, and adults may be legally removed or destroyed. It is better to have no box at all than to allow House Sparrows to reproduce in one.
• Tree Swallow: Nest of grass or pine needles, usually lined with feathers. Feathers often placed to curl up over eggs. Flatter cup (about 2" diameter, up to 1.5" deep) than bluebirds. (Note: the start of a Tree Swallow nest can look like a House Sparrow nest - don't remove it unless you're sure!) Occasionally contain mosses, rootlets, aquatic vegetation, and other plant materials. Some trash possible: cloth, paper, plastic, tinsel, cellophane, rubber bands, birchbark, often white. Eggs are pure white (may appear pinkish when first laid up to 4 days) with a pointy end. See photos.
• Violet-green swallow: Straws, dry grasses, lined with abundance of feathers. White oval eggs, no markings. (Indistinguishable from Tree Swallow.)
• Black-capped Chickadee: Downy nest with moss base, topped with fur and soft plant fibers. Female may cover eggs with moss/fur when leaving the box.  Very thin-shelled white/cream eggs with light brown/reddish speckles, dots or blotches, little or no gloss, spots may be concentrated more on the wide end of the egg. See photos.
• Carolina Chickadee: Nest base of moss, sometimes with strips of bark, thickly lined with grass, plant fibers, fur, hair. Eggs are ovate to rounded ovate, white, finely marked with reddish-brown dots, spots or blotches, often concentrated on the larger end, little or no gloss. See photos.
• Mountain Chickadee: Nest of soft materials, with base of rotten wood chips or lichen, moss or grass, topped with loose fur or hair from mammal scat, owl pellets or other sources, (feathers?). Distinct cup molded in fur, then plugged with a blanket of looser fur used to cover eggs when not being incubated. Eggs are ovate, smooth shell with little or no gloss, pure dead white to entirely marked with reddish brown dots which may be concentrated on the larger end. Less heavily marked than Black-capped Chickadee eggs. See photos.
• Tufted Titmouse: Downy nest of moss, fur, and soft plant fibers. Occasionally primarily crumpled up dried leaves with grass, and a bit of snakeskin, cellophane, bark strips, etc. Cup may be padded with hair, fur, bits of string, or cloth. May have earwigs living under moss.  Eggs are white with rose/mauve speckles, little or no gloss, more evenly distributed than chickadee. See photos.
  • Oak Titmouse: A base of moss, next fine grass, then a thick topping of fur. The cup is deep and usually is filled with a wad of fur as a "blanket" to cover eggs while the hen is laying. Eggs are white, smooth and non-glossy or slightly glossy. Some have tiny pale reddish-brown speckles evenly and sparingly scattered over the egg.
  • Juniper Titmouse:
• House Wren: Messy nest of coarse twigs (often with cottony spider cocoons), lined with fine fibers and downy feathers, usually filling box. Males may build eggless "dummy nests" in nearby boxes to reduce competition.  Tiny glossy white eggs, often tinted pink/buff, with numerous pinkish brown/reddish brown/brown specks that generally form a ring on the larger end of the egg. See photos.
• Carolina Wren: Nest is a bulky, somewhat messy mass of debris like leaves with some coarse hay/grass, twigs, moss, little roots, weed stalks; strips of bark, plastic or even snakeskin; generally domed with tunnel like entrance; and lined with feathers, animal hair, Spanish moss, wool, and fine grasses. Eggs are white/pale pink or rosy tint/light gray (larger than other wren eggs); usually with heavy brown/reddish-brown flecks often concentrated at larger end. Little or no gloss, unlike House Wren. See photos.
• Bewick's wren: (not found in CT).  Bulky nest with a deep cup of feathers, hair, moss and dead leaves on a base of sticks, chips/leaf debris. May have a wider variety of material in th ebase, and moss or dry leaves in the cup. May be a little more "organized" looking than a Carolina Wren nest. Eggs are white with irregular brown, purple/gray spots/dots often concentrated in a ring on the larger end. Little or no gloss, unlike House Wren.
• Eurasian Tree Sparrow: Domed structure of loosely intertwined (not woven) stems of dried grass, straw, pine needles and rootlets, a few sticks, surrounding a cup lined with softer materials such as feathers, fur, flower parts, waste paper, bits of cloth, string, and green leaves. Entrance is on side. May fill box to top. 4-7 eggs, 0.8" eggs are white to pale gray, heavily marked with dark brown (sometimes purplish or grayish) spots/small blotches/speckles, darker than a House Sparrow. Markings usually concentrated around broad end. Smooth, slightly glossy. Great variation in size, shape, and color. Currently only found in MO and IL.
• Nuthatches occasionally use Eastern Bluebird nestboxes.
  • Brown-headed nests may start with dried leaves, or be lined with pine seed husks, inner bark strips, wood chips, grasses, cotton and feathers. Eggs smooth (almost no gloss) with evenly distributed fine reddish-brown dots/small spots/blotches.
  • Red-breasted nest of bark shreds, grass, moss and feathers. Eggs white, heavily or sparingly spotted and dotted with reddish brown, little or no gloss. Eggs similar to white-breasted nuthatch but smaller. May dab globs of pitch around hole.
  • Pygmy Nuthatch: Nest cup of bark shreds, fine moss, grass, plant down, fur, hair, cocoons and other soft fibrous materials, most incorporate feathers. May also contain snakeskin and bits of cloth (wool or cotton) and papery material from wasp nests, string, grass blades. Never include conifer seed wings (see Brown-headed Nuthatch). May also "caulk" nest site cracks with hair and feathers. Eggs are short subelliptical to short-oval, little or no gloss, white, unevenly and sparingly speckled or finely spotted with chestnut-red, reddish/purplish brown, with heavier markings often at the large end. See photos.
  • White-breasted nest base may be about 1/2" of bark (flakes and strips) and pellets of dried earth or lumps of mud. Matted nest of bark shreds, small twigs, grasses, rootlets, with a little fur, hair, feathers, cellophane, cigarette butts. Cup may be saucer shaped. Shell smooth, very little gloss, white (can be creamy or pinkish-white), usually heavily marked with light cinnamon brown/red/ lavender/gray speckles and spots, often denser at larger end. Subelliptical to short subelliptical. See photos.
• Great crested flycatcher: Bulky nest constructed of twigs, leaf litter, pine needles, bark, moss, lichen, grass, and rootlets, usually about 10-18" high (most are 12-15"). Off to one side a cup is lined with finer hair, feathers or fur. A piece of snakeskin or cellophane is often found in the nest. Eggs (larger than a HOSP egg), moderately glossy, ivory/cream/pink, and densely streaked/scrawled with fine purple, gray, red, brown, or olive markings often concentrated on the larger end of the egg. See photos.
• Ash-Throated Flycatcher: Messy, typically with rootlets, grass, weed stems, and dry cow or horse dung in the nests, and a cup lined with fine grass and matted hair/fur. Unlike Great Crested Flycatchers, they seldom add snakeskin to their nests. Eggs are oval, smooth, slightly glossy,creamy white to ivory, and sometimes pinkish white background color, heavily marked with fine or heavy streaks or blotches in browns, purples, and grays (fewer markings than GCFL eggs.)
• Prothonotary Warbler: Mosses, lichen, rootlets, small twigs and dry leaves, strips of bark. Usually neatly rounded, cup shaped hollow, smoothly lined with fine grasses and sedges, leaf stems and feathers. Eggs: Oval, smooth, somewhat glossy shell, with a creamy or slightly yellow tinge. Boldly and liberally spotted/blotched with reddish brown and purplish gray spots and splotches over the entire egg.
• Purple martin: Nest of grasses, twigs, bark, paper, leaves, string; nest cup lined with fine grasses, decorated with fresh green leaves. Eggs slightly glossy, pure white, larger than Tree Swallow eggs.
• Woodpeckers may enlarge the entrance hole and roost in bluebird nestboxes.
• Gray or red squirrels may occupy boxes with holes larger than 1.5 inches.
• Flying squirrels may also use boxes.
• Mice may breed or roost in a nestbox. Next constructed of a variety of materials, such as grass, leaves, hair, feathers, milkweed silk, shredded bark, moss, cotton, or shredded cloth. No nest cup. Rodent droppings are generally evident. See photos.
• House finch: rare reports of using bluebird nestboxes. Often nest in wreaths on doors. Nest of twigs, grasses and debris. Eggs typically oval, smooth shell, slight gloss, pale blueish green with sparing black spots/dots.
• European Starling: The nest is bulky and slovenly. The cavity is filled up with grass, weed stems, twigs, corn husks, dried leaves, pine needles, etc, with a depression near the back. Feathers, rootlets, paper, plastics, cloth, string etc. may also be added. The cup lining may include feathers, fine bark, leaves, fine grass etc. Some nests also have fresh green plants (thought to work as fumigants against parasites and pathogens) like yarrow in them. May occupy boxes with holes larger than 1.5 inches. The slightly glossy eggs are pale bluish- or greenish-white (rare reports of eggs with fine reddish-brown spots), and are slightly smaller and darker than a Robin's egg. See photos and Biology.
• Cowbird, Brown-headed: Cowbirds do not build their own nest - they lay eggs in the nests of other birds, depending on the host to incubate and raise their young. Eggs are oval with variable shape, with a moderately glossy, granulated shell, white or grayish white, evenly dotted with brown or reddish brown, sometimes with heavier markings at the large end. Similar to a House Sparrow egg. The eggs of the Bronzed cowbird are pale bluish-green and have no markings. See Biology.

NOTE:  It is illegal to disturb a nest with eggs of any bird except House Sparrows, starlings and pigeons, which are not protected. Empty House Wren nests (sticks only, no nest cup or eggs) can be removed.

• (Bird ID) The Sibley Guide to Birds, Sibley, David Allen
• (Nest ID) A Field Guide to the Birds' Nests: United States east of the Mississippi River (Peterson Field Guide Series), Harrison, Hal H.
• (Nest ID) A Field Guide to Western Birds' Nests, Harrison, Hal H.
• (Nest ID) A field guide to the nests, eggs, and nestlings of North American birds, J. Colin James, O. Harrison
• Online guide to adult birds, where you enter location and whatever info you have (like color) and it shows you photos

FOR MORE INFORMATION:

• How to ID birds nests, Cornell Lab of Ornithology, Birdhouse Network 
• Egg ID - The Provincial Museum of Alberta, Eggs-A Virtual Exhibition
• Linda Violett's web site - birds that may be seen in Mountain Bluebird boxes
• Mountain chickadees - photos of adults and checks
• Nestbox specifications for various species
• Photos of wrens and other birds, songs - Patuxent Bird Information Center
• Urban Western Bluebird nests - which sometimes contain trash
• Stokes Birds at Home
• Weird Eggs Found in Nestboxes (photos)
• White Bluebird Eggs
• Wildbird.com
• Wild Birds for the 21st Century - Eggs of American Birds (plates)
• Not all blue eggs are bluebird eggs, Sialis.org

PHOTO CREDITS: Many thanks to the following people who allowed me to use their awesome photographs: Bluebars (from the Bluebirding Forum), Dick (BBnut) from PA (from the Bluebirding Forum), Pam from New Jersey, Susan Costa, Shelly Harris of Oklahoma. Steve Hewlett, Leslie McCulloch, Tracy Powell, Leah Solliday from Florida, and Photography of bluebirds and snowy owls. BZ is me - Bet Zimmerman from CT.

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