Bird Egg Identification: Color, Size and Clutch Size

An egg on its own is weak evidence. NestWatch, the Cornell Lab's nest-monitoring project, asks observers to record the nest's composition and structure, the materials, the shape, the location and height, the surrounding habitat, and the number of eggs, along with their size, shape, color and markings, and it warns that egg appearance varies within a single clutch.

Its strongest advice is to identify the adult instead. Different species favor specific substrates for their nests, so location narrows the list, and range and season narrow it further, because migratory birds nest only in their breeding range. Watch from a distance with binoculars rather than approaching.

In practice: a blue egg in a mud-lined cup in a shrub and a blue egg in a grass cup inside a box are two different birds, and it is the nest and the parent, not the shell, that tell you which.

• Plain blue, unmarked: American Robin, described by Audubon as pale blue or robin's-egg blue, usually 4 eggs, sometimes 3-7. Eastern Bluebird eggs are pale blue and unmarked too, sometimes white, in clutches of 4-5, and they are smaller.
• Plain white: Mourning Dove, exactly 2. Clutch size in this species is unusually consistent.
• Whitish to pale blue or greenish white with brown, purple and gray marks: Northern Cardinal, 3-4 eggs, sometimes 2-5.
• Pale blue with black and lavender dots concentrated at the larger end: House Finch, 4-5 eggs, sometimes 2-6.
• White spotted with brown: Barn Swallow, 4-5 eggs, sometimes 6, rarely 7.
• Bluish white to pale gray with brown and black markings concentrated at the larger end: Baltimore Oriole, 4-5 eggs, sometimes 3-6.
• Buff, blotched black and brown: Killdeer, usually 4, sometimes 3-5, laid in a scrape on open ground.

Markings concentrated at the blunt end are common and are not a species mark on their own; the combination of ground color, marking color and clutch size is what narrows it.

NestWatch publishes typical clutch sizes and incubation and nestling periods for more than 70 species common in North America. A selection:

• American Robin: 4 eggs, 13 days incubation, 15 days in the nest.
• Eastern Bluebird: 4-5 eggs, 14 days, 17 days.
• House Finch: 4-5 eggs, 13 days, 15 days.
• Northern Cardinal: 4 eggs, 12 days, 10 days.
• Mourning Dove: 2 eggs, 15 days, 14 days.
• House Wren: 6-8 eggs, 14 days, 15 days.
• Black-capped Chickadee: 6-8 eggs, 13 days, 14 days.
• Tree Swallow: 4-6 eggs, 15 days, 20 days.
• Barn Swallow: 4-5 eggs, 13 days, 21 days.
• American Goldfinch: 5 eggs, 13 days, 14 days.
• Blue Jay: 4-5 eggs, 17 days, 19 days.
• Northern Mockingbird: 3-5 eggs, 13 days, 13 days.

Count, do not estimate. Six to eight eggs in a small cavity points to a wren or a chickadee; exactly two on a flimsy twig platform points to a dove.

A robin's nest is a cup of grasses, twigs and debris worked into a solid mud foundation and lined with fine grass, on a horizontal branch usually 5-25 feet up, or on a ledge of a house, barn or bridge. A cardinal's is a cup of twigs, weeds, bark strips and rootlets, well hidden in dense shrubs or vines 3-10 feet up. A Mourning Dove's is a very flimsy platform of twigs, usually below 40 feet. A Baltimore Oriole's is a hanging pouch woven of plant fibers, bark strips, grass, yarn and string, attached at the rim to a slender drooping branch 20-30 feet up. A Barn Swallow's is a cup of mud and dried grass lined with feathers, in an open building, under eaves or under a bridge.

Eastern Bluebirds and many chickadees, wrens and swallows use cavities, so an egg in a box or a woodpecker hole belongs to a short list. Killdeer lay in a shallow scrape on bare soil or gravel, unlined or lined with pebbles, and the adults perform a broken-wing display to lure you away.

If an adult starts calling, feigning injury or diving at you, you are too close. Back away and watch from further off.

NestWatch states plainly that you should not take nests from the wild and that it is always best to leave them where they are, even when they look unused, and links this to protection under the Migratory Bird Treaty Act. The Act protects native North American birds and their parts, and the USFWS Feather Atlas notes there is no exemption based on how something was obtained.

That covers eggs and eggshells as well as feathers. A broken shell under a tree is usually a sign that a chick hatched and the parent carried the shell away, not that something went wrong.

Photograph it in place instead. Put a coin or ruler in the frame for scale, take the nest and its setting in one wider shot, and step back. Photograph it and the app identifies it, and nothing is disturbed.

NestWatch's rule for size is simple: egg size follows the size of the parent, so small eggs belong to small birds. Shape carries information too. Pointed eggs are an adaptation to nesting on cliffs or on the ground, as in the Killdeer and seabirds.

Perky-Pet suggests using coins for scale: a robin's egg is about as large as a quarter, and a hummingbird's is no bigger than a coffee bean. It also lists texture as a clue, from smooth and glossy to matte, pebbly or ridged.

Color and markings are the weakest evidence. NestWatch warns that they vary even within one clutch and should be treated as supporting evidence that is rarely definitive.

If one egg in a nest is clearly different from the rest, it may not belong to the nest's owner. The Brown-headed Cowbird lays its eggs in other birds' nests. Audubon describes its eggs as whitish with brown and gray spots concentrated at the larger end, and says a female lays nearly one egg a day for several weeks, up to 40 in a season and exceptionally 70 or more. Cowbirds are known to have laid eggs in the nests of more than 220 species, and more than 140 of those have raised young cowbirds.

So a single speckled egg among plain blue ones, or one egg noticeably larger than its clutch-mates, is worth a photo. Leave the nest as it is; the nest-disturbance rules below still apply.

Two tiny white eggs in a very small cup are a hummingbird's. Audubon gives the Ruby-throated Hummingbird a clutch of 2 white eggs, incubated by the female alone for 12-14 days. The nest is a compact cup of grasses, plant fibers and spider webs lined with plant down, camouflaged outside with lichens and dead leaves, usually 10-20 feet up on a branch, though anywhere from 5 to 50 feet.

Mourning Doves also lay exactly two white eggs, but much larger ones on a flimsy platform of twigs, so size and nest together separate them instantly.

NestWatch's own advice is to identify the parent bird, and it lists the tools for that:

• Merlin Bird ID: the Cornell Lab's free app, which identifies birds by photo, by sound or by answering questions.
• All About Birds: the Cornell Lab's online guide, browsable by shape or narrowed to birds in your location.
• NestWatch Common Nesting Birds guide: sortable by region and by where the nest is built.
• Books: the Peterson Field Guide to North American Bird Nests, and Nests, Eggs, and Nestlings of North American Birds.

For the egg itself, photograph the clutch in the nest with a coin beside it, then a wider shot of the nest and its setting, and step away. BirdLens gives you the likely species from the photo; the adult bird coming back to the nest is what confirms it. More on choosing a book is on the bird identification book page.