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.
A bird called a reed warbler often falls victim to the common cuckoo, but it doesn’t just sit back and take it: These birds will mob female cuckoos that stray too close to their nests, driving away the cuckoos and their eggs. To combat this, some female cuckoos, called gray morphs, have evolved to look like predatory sparrowhawks, which scares the warblers and prevents their attacks. But if the warblers observe their neighbors mobbing a hawk-like gray morph, they will learn to see through the cuckoo’s disguise and attack it.
This social learning helps the warblers combat a threat, but it also leaves an opening for a different form of cuckoo, the reddish-brown “rufous morph,” to slip in under the warblers’ beaks. In new research published in Science, researchers found that having two forms of female cuckoo was evolutionarily advantageous for both. When one form of female cuckoo dominated, the host birds learned to recognize and attack that form, leaving the second form free to thrive and begin to dominate in turn.
Image courtesy of Per Harald Olsen / Wikimedia Commons