Some battles aren’t worth fighting. The rewards of victory are too small or the costs of combat are too high. Good generals know this, and so does evolution. The natural world is full of intense arms races between predators and prey, hosts and parasites. If one side evolves a small advantage, the other counters it with an adaptation of their own, and both species are locked in an ever-escalating stalemate. But sometimes, these arms races never take off. The  costs of engagement just aren’t worth it.

Cuckoos and their hosts are usually excellent examples of evolutionary arms races. Over time, the cuckoo eggs evolve to look like the eggs of their hosts. In turn, the hosts evolve a sharper eye to