A Cuckoo Bird’s Egg Trick

By Julia Newman ’19

Cuckoo (Cuculus canorus) egg laid in Dunnock's (Prunella modularis) nest, Europe
A poorly-disguised cuckoo egg sits in a Dunnock bird’s nest.

Robins lay blue eggs, sparrows lay speckled eggs, and cuckoos – well, it depends. Cuckoo birds have the ability to lay different colored eggs over time depending on the other common birds in their environment. For example, if robins are abundant in an ecosystem, the cuckoo birds eventually evolve to lay blue eggs.  However, according to studies done at the Norwegian University of Science and Technology, a male bird’s genes has no impact on egg color. Much like the presence of X-linked chromosomes in humans, the genes for egg color in birds appear to be Z-linked.  However in birds, it is the females rather than the males that display these traits more often, unlike humans.

But why lay different colored eggs? In order to spend less energy in taking care of eggs and raising chicks, cuckoo birds lay their eggs in the nests of other species of birds. This has proven troublesome in recent years, as birds are evolving to better recognize these foreign eggs.



  1. Dodgson, How cuckoos lay deceptive blue eggs: it’s in their genes. Live Science (2015).
  2. Image retrieved from: https://suttonnature.wordpress.com/2014/03/



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