Birds lay eggs in a variety of colors, including blue. But why do some bird species lay blue eggs? The color of a bird’s eggs is determined by pigments deposited in the eggshell as it develops in the mother’s oviduct. There are two main pigments that contribute to egg color: protoporphyrin, which produces brown colors, and biliverdin, which produces blue-green colors.
The role of biliverdin in blue egg color
Biliverdin is the pigment responsible for blue-green egg color in birds. It is produced when the heme molecule in blood breaks down. Heme contains iron and gives blood its red color. As red blood cells reach the end of their life cycle, the iron is removed from heme, leaving behind the blue-green biliverdin molecule. Birds that lay blue or blue-green eggs deposit this biliverdin into the developing eggshell.
Research has shown that the oviducts of blue egg-laying birds express genes that code for proteins that transport biliverdin and deposit it into the eggshell. The more biliverdin that is deposited, the more intense the blue-green color. This genetic difference explains why some birds lay brown or white eggs while others lay blue.
Birds that lay blue eggs
Some examples of birds that typically lay blue or blue-green eggs include:
- Robins
- Thrushes
- Wrens
- Blue tits
- Finches
- Swallows
- Kingfishers
- Jays
- Starlings
There are also some birds that lay eggs in other shades of blue. For example, cuckoos and some species of duck lay pale blue or whitish eggs. Some birds, like village weaverbirds and red bishops, even lay eggs that vary from pale blue to bright blue within the same species. The exact shade depends on the amount of biliverdin deposited in each egg’s shell.
Why lay blue eggs?
Scientists are not entirely certain why some birds lay blue eggs while others lay brown or white. There are a few hypotheses about the potential advantages of blue egg coloration:
Camouflage
Blue may help camouflage eggs in certain nesting environments. Blue eggs blend in among blue sky and water backgrounds in the open nests of some species like blackbirds.
Structural strength
Pigments like biliverdin may help strengthen the eggshell structure. Some studies show that blue eggs are stronger than expected based on shell thickness and density.
Mimicry
Blue egg color in parasitic cuckoo birds may mimic that of their hosts. This trickery can help cuckoos sneak their eggs into host nests without detection.
Recognition
For colonially nesting birds, blue color variation between eggs of different females may help parents recognize their own eggs.
UV reflectance
Some research indicates blue-green biliverdin may aid egg recognition because it reflects ultraviolet light. This UV reflectance may allow parent birds to more easily distinguish their eggs.
Are blue eggs different inside?
While blue eggs have different external coloration, the interior contents are the same as white or brown eggs. Egg yolk and white color is primarily determined by carotenoid pigments deposited from the hen’s diet. Shell color does not affect the nutritional value, taste, quality, or composition of the egg contents.
Blue eggs and chicken breeds
Most chicken eggs are shades of brown. The white Leghorn chicken lays white eggs. But some breeds, like Ameraucanas, Araucanas, and Cream Legbars lay blue or greenish-blue eggs. As with other blue egg layers, these chickens likely have genetic differences that result in biliverdin deposition during eggshell formation.
Ameraucana
Ameraucanas lay light blue or olive eggs. They have muffs and a beard characteristic of cold-hardy breeds.
Origin | United States |
---|---|
Egg color | Blue |
Egg size | Large |
Temperament | Active, can be flighty |
Weight | Hen: 5 – 6.5 lbs Rooster: 6.5 – 8 lbs |
Araucana
Araucanas come from Chile and lay blue-tinted eggs. They are rumpless, meaning they lack a tailbone.
Origin | Chile |
---|---|
Egg color | Blue |
Egg size | Small-Medium |
Temperament | Active, can be flighty |
Weight | Hen: 4 – 5 lbs Rooster: 5.5 – 6.5 lbs |
Cream Legbar
Cream Legbars from Britain lay blue eggs. They have a distinct crest and barred plumage pattern.
Origin | United Kingdom |
---|---|
Egg color | Blue |
Egg size | Medium-Large |
Temperament | Active, flighty |
Weight | Hen: 4 – 5 lbs Rooster: 6 – 7 lbs |
Conclusion
In summary, blue egg color in birds is produced by deposition of the bile pigment biliverdin. Some common blue egg layers include robins, thrushes, wrens, and tits. While the evolutionary reason is still debated, hypotheses for blue egg advantages include camouflage, structural strength, mimicry, and egg recognition. The blue egg trait is also found in some chicken breeds like Ameraucanas, Araucanas, and Cream Legbars. So the next time you see a bird’s nest of bright blue eggs, you’ll know it’s caused by the pigment biliverdin as part of the incredible diversity of avian eggshell colors.