All About Blue Eggs: The Reason for Their Color

Spring is filled with lovely colors from the season's blooms and the soft, beautiful blues eggs from robins and other types of birds, such as snowy egrets and even some breeds of chickens. But why do some birds lay blue eggs, what is it about these eggs that differs from white or brown eggs, and is it safe or beneficial to eat blue chicken eggs? It turns out there are good reasons why some eggs are blue.

Why Eggs Are Blue?

The bile pigment biliverdin is responsible for blue tones in bird eggs, including robin's eggs. The color of an eggshell is determined by these pigments as they are deposited as the shell is formed in the shell gland. The shell gland is the avian equivalent of a mammal's uterus and is near the end of the oviduct, just before the cloaca. The shell is formed just before the egg is laid.

Depending on the concentration of the bile pigment, the coloration can range from bright, bold blue or blue-green to pale ice blue and every shade in between. Smaller eggs and those laid first in a brood are usually more intensely colored than larger eggs or those laid later in the nesting cycle.

Robin's eggs are not colored for camouflage as are certain breeds of hens that are genetically wired to lay creamy-brown chicken eggs meant to blend into the wild, for example. In many cases, color helps camouflage an eggshell from hungry predators that are happy to raid nests for an easy meal. This is especially true for eggs with neutral tones or those that have markings that help them blend into nesting material. Many birds that lay eggs in the open, such as in a scrape nest or directly on the ground in open areas, have such camouflaged eggs. The birds with those camouflaged eggs will often decorate their nests as well, to provide an additional layer of protection and concealment.

Birds Known for Their Blue Eggs

Robin's eggs aren't the only blue wild bird eggs. Many other species, including some chickens, also lay eggs in various shades of blue, teal, turquoise, and blue-green, including:

• Araucanas, ameraucanas, Easter egger, legbar, and copper maran chicken breeds
• Black tinamous
• Blue jays
• Common mynas
• Dunnocks
• Eastern bluebirds
• European robins
• European starlings
• Gray catbirds
• House finches
• Linnets
• Red-winged blackbirds
• Snowy egrets
• Song thrushes

Regardless of the species, however, the same principles that explain why robin's eggs are blue explain the bluish coloration of all different wild bird eggs. Birders who understand egg coloration can more easily identify wild bird eggs and appreciate all the special qualities of each nest they see.

Facts About Shell Color

The blue color of a bird's egg can also protect it from harmful sunlight. The radiation and heat from the sun can easily damage delicate eggs, both by influencing chick development and impacting the incubation period. A very dark blue egg, for example, will have better protection against harmful ultraviolet (UV) radiation, but it can also heat up too quickly in direct sunlight and the extreme temperature could kill an unhatched chick. A lighter blue egg, however, is less subject to the stress of overheating, but more at risk for developmental problems caused by UV radiation.

Blue Eggs and Other Clues

In addition to giving a clue about a bird's nesting habits with regards to the sunlight that reaches a nest, the colors of eggshells can indicate:

• Health of parent and chicks: Brighter, more colorful blue eggs (within the proper colors for the species) are typically a sign of good health and a nutritious diet for the mother bird. Those hatchlings are more likely to be strong and healthy as well.
• Nest care habits: While not a universal characteristic, it is typical that heavily camouflaged eggs are often part of nests that are often left untended as parent birds forage. Plain, or more conspicuous blue eggs, on the other hand, may be guarded more carefully by watchful parents and have less need for camouflage.
• Mismatched siblings: Dramatically different eggs in the same nest can indicate a brood parasite. These birds, such as brown-headed cowbirds and common cuckoos, lay their eggs in the nests of other birds, and let other species raise them as "foster" chicks. An egg that is a vastly different size, shape, or color in the brood may be from a brood parasite. If there's a blue egg in a nest of birds that lay non-blue eggs—or vice versa—you might anticipate some trouble in the nest.