Blue Cuckoo Eggs: How Females Have Learned To Disguise Their Offspring

Female cuckoos are known for laying their eggs in the nests of other birds and leaving their young to be raised by others. Until now, however, researchers were unsure how females have managed to lay such a variety of different egg colors to match those of their hosts. 

In the latest study, a team of researchers from the Norwegian University of Science and Technology (NTNU) examined several different varieties of cuckoo (Cuculus canorus) eggs from Europe and Asia. They found that egg color is largely dependent on the genetic material that offspring inherit from their mother (mitochondrial DNA). However, this has only been proven in relation to the production of blue-colored eggs.

"We've been able to show for the first time that the blue egg color is inherited via the female cuckoo only," Frode Fossøy, of the study researchers from the Department of Biology at NTNU, said in a news release. "The father has no effect on the color of his daughter's eggs."

A cuckoo's tendency to drop their young into the responsibility of others is known as brood parasitism. When the baby cuckoo hatches it usually kicks other chicks out of its surrogate's nest as a means of eliminating any other competition for the parents' attention. That's why potential host birds have adapted different traits to prevent being tricked by the parasite cuckoos: They immediately rid their nests of any eggs that don't look like their own. 

In turn, female cuckoos have learned to lay eggs that look identical to that of their hosts - both in color and shape. Therefore, different cuckoos can lay eggs of many different colors and patterns, ranging from blue to brown, green or grey, with different combinations of spots and patterns. (Scroll to read more...)

There are several evolutionary advantages to having only females carry the genetic trait for egg color, researchers say. For instance, a male cuckoo can mate with several female birds that lay eggs of different colors, and if a male with genes for one color mated with a female with a different egg color, they could inevitably produce eggs that don't match any of their hosts'. This is why egg color is a trait inherited solely from the mother. 

However,  researchers have only been able to confirm the trait for blue eggs is only inherited from female birds, so this may not be the case for other colors. Therefore they suggest there is much more research to be done.

Their study was recently published in the journal Nature Communications. 

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