How do European cuckoos and their British host birds coevolve?

European cuckoos (Cuculus canorus) and their British host birds—such as meadow pipits, reed warblers, pied wagtails, and dunnocks—engage in a dynamic process known as coevolutionary alternation.12

As brood parasites, cuckoos have evolved to lay eggs that mimic the appearance of their hosts' eggs to avoid rejection. Because cuckoo populations are polymorphic regarding egg color, different cuckoo "gentes" (clans) specialize in parasitizing specific host species.13

This relationship drives a continuous evolutionary arms race:
* Host Defenses: Over generations, host species develop the ability to discriminate between their own eggs and foreign ones, ejecting those that do not match.12
* Cuckoo Counter-adaptations: As hosts improve their defenses, natural selection favors cuckoos that produce better-mimicking eggs or shift to new, less-defensive host species.12
* Relaxed Selection: When cuckoos abandon a host species, the evolutionary pressure on that host to reject foreign eggs is relaxed, and they may eventually lose their defensive traits, making them vulnerable to future cuckoo attacks.12

This cycle ensures that cuckoos and their hosts remain in a state of perpetual, shifting adaptation.12