Why Are Robin Eggs Blue? The Science of Egg Color

What makes the eggshell blue?

Bird eggshells get their colors from just two main pigments:

Both pigments are linked to heme, the iron-containing part of hemoglobin in red blood cells. Biliverdin is produced when heme is broken down, and it is the same green pigment that can make a healing bruise look greenish.

In robins, the shell gland, part of the female’s oviduct, deposits biliverdin as the eggshell is completed, in the final hours before laying. Because the pigment is built into the shell, robin eggs are blue all the way through, not just on the surface. Robin eggs have very little protoporphyrin, so they are plain blue with no spots.

Why did robins evolve blue eggs?

There is no single agreed answer, and several explanations may work together. These are the leading ideas.

Protection from sunlight

American robins nest in open cups, where eggs can be exposed to strong light and heat. One idea is that a blue-green shell is a compromise: dark enough to block some harmful ultraviolet light from reaching the developing embryo, but light enough not to overheat in the sun the way a very dark egg would. Experiments on eggshells have supported the idea that shell color affects how much light and heat pass through, though how much this matters for robins is still debated.

A signal of the female’s health

Biliverdin is an antioxidant, and making it may cost the female some resources. The “sexually selected eggshell color” idea, proposed in the early 2000s, suggests that a healthy female in good condition can afford to put more pigment into her eggs, producing a brighter blue. A male that sees bright blue eggs might then invest more effort in feeding the chicks.

Researchers have tested this idea in robins and other blue-egg birds, and the results have been mixed. Bluer eggs sometimes go with healthier females or chicks, but clear evidence that males change their behavior based on egg color is limited.

Spotting a cowbird’s egg

The brown-headed cowbird is a brood parasite that lays its eggs in other birds’ nests. Cowbird eggs are whitish with brown speckles, very different from a robin’s plain blue.

American robins are among the birds that most reliably reject cowbird eggs. Experiments in which researchers added model eggs of different colors to robin nests showed that robins tend to throw out eggs that do not match their own blue, especially beige or speckled ones. A distinctive egg color makes it easier for robins to recognize intruders, which helps explain why cowbirds rarely succeed in robin nests.

Inherited from ancestors

Blue eggs are common among thrushes, the family that includes the robin, as well as in many other birds. Part of the answer is simply that robins inherited the trait from their ancestors, and natural selection has maintained it.

An older idea, that blue eggs are camouflaged, is not well supported: a clutch of bright blue eggs is easy to see in a nest. Robins mainly protect their eggs by hiding the nest and by the female sitting on them almost constantly.

Robin eggs and clutch size

The American robin lays 3–5 eggs per clutch, most often 4. Each egg is about 3 cm (1.2 in) long and weighs about 6 g (0.2 oz). The female lays one egg per day, usually in the early morning, and starts incubating once the clutch is nearly complete, so the chicks hatch within a day or two of each other.

Incubation by the female alone lasts about 12–14 days. The chicks leave the nest about two weeks after hatching. Pairs often raise two or three broods from spring to midsummer, laying a fresh clutch in a new nest each time. Shells found on the ground in spring are usually from eggs that have hatched, which parents often carry away from the nest.

Do eggs change color during incubation?

Freshly laid robin eggs are at their brightest. During incubation, the shell can become slightly duller or stained by contact with the nest and the parent, and very old museum eggs can fade with long exposure to light. The pigment itself, though, is part of the shell and does not wash off.

Robin’s egg blue

The color of robin eggs is so recognizable that it became a color name in English. “Robin’s egg blue” describes a light, slightly greenish blue, and similar shades are used in fashion, interior design and branding. Other birds lay similar shades: eastern bluebirds and gray catbirds in North America, and dunnocks and song thrushes in Europe, all use biliverdin too.

In the United States, American robins, their nests and eggs are protected by the Migratory Bird Treaty Act, so wild robin eggs should only be admired, not collected. To learn what robins do once the breeding season is over, see where robins go in winter, or explore other animals that lay eggs.