An adult reed warbler feeds a common cuckoo chick,
not recognizing the baby bird as a parasite
In the world of birds, cuckoos are pretty unpopular. Maybe it’s something about how they lay their eggs in others’ nests so that their chicks will steal food and attention from the natural-born chicks. This can kick off an evolutionary arms race that researchers are already familiar with: the cuckoo eggs evolve to look more and more like the host eggs, and the hosts evolve to get better and better at recognizing the foreign eggs.
Now researchers have discovered another cuckoo-versus-host evolutionary race running in parallel—and it has led to the evolution of two different forms of the same species of female cuckoo.