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?

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.