Uncovering The Genomic Secrets Of Color Variants In Female Cuckoo Birds

Why do female common cuckoos have one of two different colors, whilst the males are all the same color?

© by GrrlScientist for Forbes | LinkTr.ee

Common cuckoo (Cuculus canorus) in flight. This is an adult male: Grey morph adult females have a pinkish-buff or buff background to the barring and the sides of the neck, and sometimes small rufous spots on the median and greater coverts and the outer webs of the secondary feathers. (Credit: Chris Romeiks / GFDL 1.2)

Sexual dimorphism — the visible differences between males and females of the species — can easily be seen in a wide variety of animals — even in humans. Although plenty of research has focused on understanding the evolutionary foundations underlying these distinct traits, there is another type of dimorphism that has received far less attention: intrasexual polymorphisms — also known as sex-limited polymorphisms — where one sex shows greater variation in a particular trait than the other.

The common cuckoo, Cuculus canorus, is one such species. Common cuckoos are obligate brood parasites that lay their eggs into other species’ nests, thereby forcing the reluctant songbird hosts to care for a cuckoo baby. Interestingly, cuckoo birds show a plumage color polymorphism that is limited to females: Adult males are gray whereas females are either gray or reddish-brown (rufous). They are sparrowhawk mimics: a casual human or avian observer might mistake a common cuckoo for a Eurasian sparrowhawk, Accipiter nisus, a small raptor that commonly preys on the cuckoos’ songbird foster parents. But why would a cuckoo mimic a sparrowhawk?