Finding a small, vibrantly colored blue egg can be a surprising discovery. This blue coloration is a natural phenomenon found in various bird species.
Birds That Lay Blue Eggs
Many bird species produce eggs with a distinct blue hue. The American Robin (Turdus migratorius) is known for its iconic “robin’s egg blue” coloration. These uniform, bright turquoise-blue eggs typically measure 2.8 to 3.0 centimeters and are found in cup-shaped nests of mud and grass, often in trees or shrubs across North America.
European Starlings (Sturnus vulgaris) lay pale blue to bluish-white, unmarked, and slightly glossy eggs. These eggs, smaller than a robin’s, measure about 2.9 to 3.1 centimeters long and 2.0 to 2.1 centimeters wide. They are laid in cavities like tree hollows or nest boxes, common in urban and suburban environments throughout North America and Europe. House Finches (Haemorhous mexicanus), common across North America, lay pale blue or bluish-green eggs with sparse black or purplish-brown speckles, often at the larger end. Their relatively small eggs, measuring 1.6 to 2.1 centimeters long, are laid in cup-shaped nests found in various locations, from dense shrubs to building ledges.
Eastern Bluebirds (Sialia sialis) produce light, sky-blue eggs, sometimes appearing almost white, and typically unspotted. These eggs are approximately 1.8 to 2.4 centimeters long. They build nests in natural cavities, old woodpecker holes, or nest boxes, and are common in open woodlands, orchards, and suburban areas in eastern North America. Several thrush species, including the Hermit Thrush (Catharus guttatus) and Wood Thrush (Hylocichla mustelina), lay blue eggs. Hermit Thrush eggs are pale greenish-blue, sometimes with dark markings, and measure 2.0 to 2.5 centimeters long. Wood Thrush eggs are a slightly darker, uniform blue, without markings, and measure 2.3 to 2.8 centimeters long. These thrushes typically nest in the understory of forests across North America.
The Science Behind the Blue
The vivid blue coloration in some bird eggs originates from biliverdin, a pigment. This compound forms during the breakdown of hemoglobin and is deposited onto the eggshell as it forms within the bird’s oviduct, imparting the blue color. The intensity of the blue varies with the amount of biliverdin deposited.
Blue pigmentation may offer several evolutionary benefits. One hypothesis suggests biliverdin acts as an antioxidant, protecting the egg from harmful free radicals or pathogens. Another idea proposes the blue color signals the female bird’s health and genetic quality to potential mates, indicating her ability to invest resources into egg production. Research also suggests blue eggs may play a role in thermoregulation, reflecting or absorbing sunlight to maintain a stable temperature for the developing embryo.
Observing Blue Eggs
Encountering a bird’s nest with blue eggs is exciting, but observe them responsibly. Maintain a respectful distance to avoid disturbing parent birds, which could lead them to abandon the nest. Approaching too closely or handling eggs can introduce human scents or disrupt the nesting environment, posing a risk to developing chicks.
Federal and local laws, such as the Migratory Bird Treaty Act of 1918 (MBTA) in the United States, protect wild birds, their nests, and their eggs. It is illegal to collect or possess wild bird eggs without proper permits. To identify the species, observe the nest from afar, noting its structure, location (e.g., in a tree, shrub, or cavity), and surrounding habitat. These details, combined with the egg’s shade of blue and any markings, provide clues for identification without harming the birds.