Bird eggs display a remarkable array of colors and patterns. Among these, blue eggs stand out, presenting a visually striking phenomenon in the natural world. This distinctive coloration prompts questions about which birds lay them, the biological processes behind their hue, and the potential evolutionary advantages they offer.
Birds That Lay Blue Eggs
Many familiar bird species lay blue or bluish-green eggs. The American Robin is perhaps the most well-known, laying eggs of a distinctive light blue, often referred to as “robin’s egg blue.” These smooth, slightly glossy eggs typically measure around 1.1 inches in length and are laid in clutches of three to five.
Other thrushes in North America, such as the Wood Thrush, Hermit Thrush, and Veery, also produce blue eggs, some with small dark spots. The Eastern Bluebird is another species laying unmarked blue eggs. European Starlings lay pale bluish to pale greenish eggs that are smooth and unmarked, measuring approximately 1.1 inches long. Additionally, species like the Gray Catbird, House Finch, and Red-winged Blackbird lay blue eggs, sometimes with brown speckles. Some larger birds, such as Great Blue Herons, also lay blue eggs.
The Science Behind Blue Eggshells
The blue color in bird eggshells originates from a specific pigment called biliverdin. This pigment results from heme catabolism, the breakdown of hemoglobin, the protein found in red blood cells. Biliverdin is responsible for blue and green hues in eggshells, while another pigment, protoporphyrin IX, produces reds and browns.
During the final stages of egg formation within the female bird’s reproductive tract, specifically in the shell gland, biliverdin is deposited onto the calcium carbonate base of the eggshell. This deposition occurs throughout the shell layers, meaning a blue egg is blue all the way through, not just on the surface. Studies suggest biliverdin is primarily produced directly within the shell gland itself, rather than being transported from other organs.
Why Birds Lay Blue Eggs
The evolutionary reasons behind blue egg coloration are complex, with several theories proposed to explain its adaptive significance. One prominent hypothesis suggests that blue eggs may act as a natural “sunscreen” for the developing embryo. The blue pigment can help to balance the need to protect the embryo from harmful ultraviolet (UV) radiation while also preventing the egg from overheating. Darker colors absorb more heat, so blue may offer a compromise in sun-exposed nests.
Another theory suggests that the vibrancy of blue eggs can signal the female’s health and genetic quality to her mate. Biliverdin, the blue pigment, possesses antioxidant properties, and producing intensely blue eggs might indicate a female’s robust physiological condition. This signal could encourage the male to invest more in parental care, such as feeding the nestlings more frequently. Additionally, for some species, blue coloration might offer a form of camouflage, particularly in nests situated among green vegetation, helping the eggs blend into their surroundings.