Bird eggs come in a huge variety of colors and patterns, ranging from pure white to mottled brown. The color blue is a specific and somewhat rare hue in the avian world, often catching the eye and sparking curiosity. Finding a little blue egg usually points toward a small number of specific bird species. Identifying the bird that laid it involves looking closely at the egg’s exact shade, size, and the location of the nest.
The Primary Answer: Eastern Bluebirds
The most likely answer to what bird lays little blue eggs is the Eastern Bluebird, a small North American songbird. These birds are known for their bright plumage and their habit of nesting in cavities, such as old woodpecker holes or nest boxes. The female Eastern Bluebird typically lays a clutch of three to seven eggs, which are a very pale, light blue color.
The eggs are small and oval-shaped, measuring approximately 0.83 inches long and 0.63 inches wide. Their delicate, unmarked shell is slightly smaller and generally paler than the more famous blue egg laid by the American Robin. The bluebird’s choice of a secure, dark cavity for nesting means the eggs do not need the camouflage that eggs in open cup nests require. The female constructs the neat nest almost entirely using fine grasses and pine needles.
While the eggs are almost always pale blue, a small percentage of Eastern Bluebird females will consistently lay white or even pink eggs. This color variation highlights a slight genetic difference among individual females. The female incubates the eggs for about 13 to 16 days before the hatchlings emerge.
Why Eggs Are Blue: The Role of Pigmentation
The blue color in a bird’s eggshell is an integral part of the shell structure created during its formation, not a surface application. This coloration is due to a single pigment called biliverdin, which is deposited onto the calcium carbonate shell inside the female’s shell gland. Biliverdin is a byproduct of the breakdown of hemoglobin.
The amount and timing of biliverdin deposition determine the intensity of the blue or blue-green color. Biliverdin is a known antioxidant, and its presence in the eggshell suggests a potential link to the female’s overall health. Studies on Eastern Bluebirds have indicated that females in better body condition tend to lay more intensely colored eggs.
This blue coloration may serve as a signal to the male bird, indicating the female’s health and ability to invest resources in reproduction. The male may then adjust his paternal care efforts based on the perceived quality of the clutch. Biliverdin also combines with a second pigment, protoporphyrin, which creates the brownish-red hues and speckles seen on other eggs.
Common Mix-Ups: Other Birds That Lay Blue or Blue-Green Eggs
While the Eastern Bluebird is a strong candidate for the “little blue egg,” it is often confused with other species that also lay blue or blue-tinged eggs. The most common mix-up is with the American Robin, whose eggs are a vibrant, distinctive sky-blue, often described as “robin’s egg blue.” However, the American Robin’s egg is significantly larger than the bluebird’s and is laid in an open, cup-shaped nest, typically located in a tree or shrub.
Another common backyard bird that lays blue-green eggs is the European Starling. Starling eggs are pale blue-green, but they are generally larger than bluebird eggs and are found in untidy nests deep inside cavities. The Gray Catbird is another species that lays eggs that can appear blue, but their eggs are typically a darker, greenish-blue shade and are found in bulky nests in dense shrubs or thickets.
The key to distinguishing the bluebird’s egg from these others is its size and the nesting environment. Bluebird eggs are among the palest and smallest of the common blue eggs, and they are almost exclusively found inside a protected, enclosed space like a wooden nest box or tree hollow. A small, pale blue egg found inside a clean, grassy nest in a cavity strongly suggests the presence of an Eastern Bluebird.