Bird Nest Identification: Shape, Materials and Height

NestWatch asks observers to record the nest's composition and structure, whether the materials are mud, feathers, sticks, mosses or grasses, and whether the shape is a cup, a dome, a platform, a hanging pouch or a bundle of sticks. It also asks for the location: ground level, height in a tree or shrub, placement on a building, and the surrounding habitat. Its filters for narrowing species are nest type (cup, platform, cavity), region, habitat and substrate.

That order works because shape is coarse and reliable, materials are distinctive, and height and substrate are what finally separate two birds that build the same shape.

NestWatch's own focal-species archive covers 46 species chosen for being common, widely distributed and convenient to monitor, which is a realistic size for a backyard shortlist.

• American Robin: grasses, twigs and debris worked into a solid mud foundation, lined with fine grasses and plant fibers, on a horizontal branch usually 5-25 feet up, or on a ledge of a house, barn or bridge. The mud is the mark.
• Northern Cardinal: twigs, weeds, grass, bark strips, leaves and rootlets lined with fine grass or hair, well hidden in dense shrubs, vines or low trees 3-10 feet up, built by the female.
• House Finch: an open cup of grass, weeds, fine twigs and leaves in a wide range of sites, especially conifers, palms, ivy on buildings, cactus and holes in structures, averaging about 12-15 feet up.
• Barn Swallow: a cup of mud and dried grass lined with feathers, built by both sexes in open buildings, under eaves, under bridges or docks.
• Eastern Bluebird: a loosely woven cup of weeds, twigs and dry grass lined with finer grass, sometimes animal hair or feathers, inside a cavity or a birdhouse 2-20 feet up.

A cup in a shrub with no mud and no box is most often a cardinal or a finch; the site and the lining separate them.

Hanging pouch. The Baltimore Oriole weaves a pouch of plant fibers, bark strips, grapevines, grass, yarn, string and Spanish moss, with the rim firmly attached to a slender drooping branch near its end, usually 20-30 feet up. Nothing else common looks like it.

Flimsy twig platform. The Mourning Dove builds a very flimsy platform of twigs in a tree or shrub, occasionally on the ground or on a building ledge, usually below 40 feet. You can sometimes see the eggs through the bottom.

Scrape on the ground. The Killdeer lays in a shallow scrape in soil or gravel, unlined or lined with pebbles, grass, twigs or debris, in the open where it can see around. This is the nest people find on driveways, gravel roofs and construction sites.

Cavities. Eastern Bluebird, Black-capped Chickadee, House Wren and Tree Swallow all use natural hollows, old woodpecker holes or nest boxes. Among ducks, Wood Duck, Hooded Merganser, Common Merganser, Bufflehead and Common Goldeneye are cavity nesters, and Audubon records Wood Duck nest sites in tree cavities up to 65 feet above ground.

Stick platform in a tree. A large stick structure high in a tree is a raptor, a crow or a heron rather than a songbird. Barred Owls do not build: Audubon describes them nesting in a large natural hollow, a broken-off snag, or an old nest of a hawk, crow or squirrel.

Size is easier to use here than on the bird itself, because a nest holds still and you can compare it with your hand.

A nest that would sit in a coffee cup belongs to a small songbird such as a chickadee, a wren, a goldfinch or a hummingbird. A nest the size of a cereal bowl belongs to a robin, a cardinal, a jay or a mockingbird. A nest larger than a bicycle helmet, built of sticks and set high in a tree or on a pole, belongs to a crow, a raptor or, near water, an Osprey or a heron.

Clutch size, if eggs are present, narrows it further. NestWatch gives 6-8 eggs for House Wren and Black-capped Chickadee, 4-5 for Eastern Bluebird and Blue Jay, 4 for American Robin and Northern Cardinal, and exactly 2 for Mourning Dove.

Count from a distance, and only once. If an adult starts calling in alarm, feigning a broken wing or diving at you, you are close enough to cause harm; step away.

NestWatch's instruction is explicit: do not take nests from the wild, and it is always best to leave them where they are, even if they appear unused. It ties this to protection under the Migratory Bird Treaty Act, which covers native North American birds.

Many species reuse a site or a structure, and an empty-looking nest in June may be between broods rather than finished for the year. Barn Swallows and House Finches in particular raise more than one brood a season.

If a nest is genuinely in a dangerous place, such as inside a working machine or on a doorway you cannot avoid using, call your state wildlife agency and ask before touching it. They will tell you what is permitted where you live.

To record it, photograph the nest from directly above and from the side, include something for scale, and take a wider frame showing the surroundings. Photograph it and the app identifies it, and the nest stays where the birds built it.

Most nests you will find fall into five types. The Woodland Trust's UK guide names the first four; NestWatch and the Colorado State Publications Library add the others.

• Cup: the most common garden type, from wide and shallow to deep and neat. Robins, cardinals, finches and thrushes.
• Platform: large and flat, on the ground or high in a tree. Doves build flimsy ones; birds of prey such as Ospreys build big ones.
• Cavity: inside a tree hole, a woodpecker hole or a nest box, which gives protection from weather and predators. Woodpeckers, bluebirds, chickadees and Tree Swallows.
• Scrape or ground nest: a shallow hollow or a nest built directly on the ground, often camouflaged. Killdeer is the familiar North American example.
• Hanging or suspended: a pouch or basket hung from a branch. NestWatch gives Red-eyed Vireos, which suspend their nest between forked branches, and Baltimore Orioles, which weave a hanging pouch.

Two special cases are worth knowing. Barn Swallows and Cliff Swallows build nests of mud against buildings and bridges, and Burrowing Owls, a threatened species in Colorado, nest underground in abandoned prairie dog holes.

In the temperate Northern Hemisphere, most birds lay in spring and early summer, timed so the chicks hatch when food is most plentiful. NestWatch explains that most birds read day length to tell the season, and begin breeding once daylight passes a critical level.

The exact month depends on the species and the place:

• UK: the Woodland Trust gives March to September for most birds, with some starting as early as February, and treats March to August as the main breeding season.
• Colorado: songbirds typically begin in April; raptors start earlier because their incubation is longer; Lesser Goldfinches breed later, when thistles bloom.
• Across North America: NestWatch notes that resident birds can nest early, a chickadee earlier in spring than a warbler; that American Goldfinches nest in late summer although they are year-round residents; and that southern populations nest earlier than northern ones.

So a nest with eggs in March points toward a resident bird or a raptor, and one with eggs in August could be a goldfinch or a later brood.

A nest is only in use for eggs and chicks. NestWatch lists the idea that birds use nests all year as a myth; they are not roosting in them in winter. Its species table lets you estimate how long a nest stays active, because a female lays at most one egg a day and then incubation and the nestling period follow. Average figures from NestWatch:

• Northern Cardinal: 4 eggs, 12 days incubation, 10 days in the nest.
• American Robin: 4 eggs, 13 days incubation, 15 days in the nest.
• Mourning Dove: 2 eggs, 15 days incubation, 14 days in the nest.
• House Wren: 6-8 eggs, 14 days incubation, 15 days in the nest.
• Eastern Bluebird: 4-5 eggs, 14 days incubation, 17 days in the nest.
• Barn Swallow: 4-5 eggs, 13 days incubation, 21 days in the nest.
• Red-tailed Hawk: 2-3 eggs, 30 days incubation, 30 days in the nest.
• Great Horned Owl: 2-3 eggs, 31 days incubation, 34 days in the nest.
• Bald Eagle: 2 eggs, 41 days incubation, 75 days in the nest.

Add laying, incubation and the nestling period and most of the songbirds above keep a nest busy for about a month. NestWatch says most songbirds leave after 2 or 3 weeks as nestlings, while raptors may stay 8 to 10 weeks. Many species nest more than once: the American Robin can make up to 4 or 5 nests in one breeding season. Wait until the season is over before assuming a nest is finished.

No tool identifies every nest from a photo, because nests vary so much within a species. NestWatch's own list of identification aids is:

• Merlin Bird ID, a free app, for identifying the adult that visits the nest.
• All About Birds, an online bird guide.
• NestWatch's Common Nesting Birds guide, which can be sorted by region.
• Books: the Peterson "Field Guide to North American Bird Nests", and "Nests, Eggs, and Nestlings of North American Birds" by Baicich and Harrison.

The fastest confirmation is usually the adult bird. Watch from a distance with binoculars, photograph the bird when it comes back, and BirdLens suggests the species. A named adult plus the nest shape almost always settles it.

In Britain the common garden builders are different species. The Woodland Trust's guide picks out the Song Thrush, whose cup has a distinctive lining of dried mud; the Dunnock, whose eggs are bright blue with no spots; the Goldfinch, whose neat deep cup is bound with spider silk and packed with thistle down; and the Woodpigeon, whose flat stick platform has visible gaps. Under the Wildlife and Countryside Act 1981 (England, Scotland and Wales) and the Wildlife (Northern Ireland) Order 1985, it is an offence to disturb nesting birds or to take, damage or destroy an active nest, and the Trust advises not cutting hedges from roughly March to August.

One thing that helps: pick up loose fishing line and string near water and in the yard. The Colorado State Publications Library notes that birds weave fishing line into nests, where it is a hazard to the chicks.