Bird eggs display a remarkable array of colors and patterns, ranging from simple white to intricate speckles and vibrant hues. While many bird species lay eggs that are white, brown, or speckled to blend with their surroundings, a select group produces strikingly blue eggs. This distinctive coloration prompts curiosity about which birds lay such eggs and the biological mechanisms and evolutionary reasons behind this unique natural phenomenon.
Common Birds Laying Blue Eggs
Numerous bird species across different habitats are known for laying blue eggs, with the shade and intensity of blue varying among them. The American Robin is perhaps one of the most recognizable, laying eggs that are famously a distinct light blue, often referred to as “robin’s egg blue.” These smooth, slightly glossy eggs typically consist of three to five eggs per clutch.
The bluebird family, including the Eastern, Western, and Mountain Bluebirds, also lays eggs that are predominantly light blue, often a paler shade than those of the American Robin. Eastern Bluebird eggs are smooth and glossy. Western Bluebirds lay pale blue and unmarked eggs in tree cavities or nest boxes. Mountain Bluebirds, found in western North America, also lay pale blue, unmarked eggs.
House Finches, common in gardens and urban areas, lay pale bluish-green eggs that often feature delicate black or lavender speckles. European Starlings, an introduced species in North America, lay unmarked eggs that range from pale bluish to pale greenish. Blue Jays also lay blue-green eggs, which often have brown spots.
The Chemistry of Blue Eggshells
The striking blue coloration of bird eggshells originates from a specific pigment called biliverdin. This pigment is deposited onto the eggshell during its formation within the female bird’s reproductive tract, specifically in the shell gland. Biliverdin is one of only two primary pigments responsible for all bird egg coloration; the other is protoporphyrin, which produces reddish-brown hues and speckles.
Biliverdin is a byproduct of hemoglobin breakdown, an iron-containing protein. As the egg travels through the oviduct, the biliverdin pigment is released into the shell-forming process, permeating the calcium carbonate structure of the eggshell. The concentration and distribution of this pigment determine the intensity and shade of the blue, with higher concentrations generally leading to a more vibrant blue.
The Purpose of Blue Eggs
The evolutionary reasons behind blue egg coloration are complex and are the subject of ongoing scientific study, with several hypotheses proposed. One prominent theory suggests that the blue color, derived from biliverdin, can signal the female bird’s health and genetic quality to her mate. Producing biliverdin is thought to be energetically costly, and therefore, a female in good physical condition may be able to deposit more pigment, resulting in bluer eggs.
This signal may encourage the male to invest more in parental care. Studies have indicated that males might provide more care to clutches with more intensely colored blue eggs. Another hypothesis relates to protection from environmental factors, such as ultraviolet (UV) radiation. Blue pigments may act as a form of “sunscreen,” helping to filter harmful UV light that could damage the developing embryo.
However, intensely colored eggs can also absorb more heat, posing a risk of overheating in direct sunlight. The intensity of blue in an eggshell might represent a balance between UV protection and avoiding excessive heat absorption, depending on the nesting environment. Blue coloration may also offer a form of camouflage in certain light conditions, making the eggs harder for predators to spot from above.