While the American Robin is the most famous example, many smaller species also produce eggs with this vibrant hue. Identifying the source of a small, blue egg can be difficult because numerous birds, particularly cavity-nesting songbirds, utilize this coloration. The shade of blue, the presence of speckles, and the precise size of the egg are all important clues for distinguishing between species. The overall context of the nest and the surrounding habitat are equally important for proper identification.
The Biological Reason for Blue Eggshells
The striking blue color in certain bird eggs is due to a single pigment called biliverdin, which is a byproduct of hemoglobin breakdown in the body. This pigment is deposited onto the eggshell’s calcium carbonate structure during the final stages of egg formation within the female’s shell gland. The concentration of biliverdin determines the depth and intensity of the blue or greenish-blue coloration observed.
The evolution of blue eggs is a topic of scientific study, with several hypotheses suggesting its adaptive purpose. One theory is that blue pigmentation may signal the female’s health and fitness to her mate, as producing biliverdin is energetically costly and potentially indicates a strong immune system. Another prominent idea suggests that the blue pigment helps protect the developing embryo from harmful ultraviolet radiation, particularly in open-cup nests exposed to direct sunlight. For birds that nest in dark cavities, the pale blue or white color may help the parent locate the eggs against the dark background to prevent accidental breakage.
Identifying Common Species with Small Blue Eggs
Several common North American birds lay eggs that fit the description of small and blue, often measuring less than one inch in length. The Eastern Bluebird lays eggs that are typically a light, unmarked powder blue, though they can occasionally be white. These eggs are smooth and slightly glossy, measuring about 0.83 inches long and 0.63 inches wide. Western and Mountain Bluebirds also lay eggs that are a similar light blue to blue-green color.
The European Starling, an introduced but widespread bird, also lays small, pale blue eggs that can sometimes appear greenish-blue. Starling eggs are unmarked and slightly glossy, typically measuring around 1.1 inches in length and 0.8 inches in width, making them slightly larger than bluebird eggs. House Finches, common in urban and suburban areas, produce eggs that are pale bluish-green, often with light lavender or black speckles. These eggs are quite small, measuring roughly 0.5 inches in width.
The House Wren lays eggs that can sometimes be mistaken for blue eggs, though they are usually white, pink-white, or grayish. Crucially, House Wren eggs are densely speckled with reddish-brown or cinnamon-brown dots, which distinguishes them from the unmarked eggs of the bluebird and starling. These tiny eggs are oval-round, measuring about 0.64 inches long and 0.5 inches wide. Regional presence is also a factor, as the likelihood of finding a specific species’ egg depends on its geographic range.
Differentiating Eggs by Nest and Habitat
The location and construction of the nest are often the most reliable way to differentiate between small blue eggs. Eastern Bluebirds are cavity-nesters, placing their nests inside natural tree hollows or human-provided nest boxes. Their nest is a neat, cup-shaped structure woven almost entirely from fine grasses or pine needles. Finding an unmarked blue egg in a tidy, grass-lined nest within a cavity strongly suggests a bluebird.
European Starlings also utilize cavities, but their nest is typically a bulky, slovenly mass that fills the space with coarse grass, twigs, and sometimes trash, with a depression for the eggs. Starlings are often found in urban areas, competing for nest sites in holes in buildings or large tree cavities. The slightly larger size of the starling egg also provides a key differentiator from the bluebird’s smaller clutch.
House Wrens are also cavity-nesters, but their nests are distinctively built with a large number of small sticks that fill the cavity, creating a platform for the final nest cup. The presence of dense speckling on the wren’s egg is the most significant clue, regardless of the nest’s location. House Finches build an open cup nest, often in trees, shrubs, or on building ledges, which immediately rules out the cavity-nesting bluebirds and starlings.