Cuckoo finches are bad news to other birds. They are brood parasites, meaning they lay eggs in other species's nests to trick their victims into raising the finches' children as their own. This kicked off a supercharged evolutionary arms race.
A frequent target of the cuckoo finch is the tawny-flanked prinia. To defend against these parasitic eggs, the prinias have evolved the ability to lay eggs with complex spotted patterns that will vary substantially from one female to the next, making it much more difficult for a finch to successfully imitate its intended victim's eggs. What's interesting is that this alone apparently isn't enough for the finch eggs to escape detection for very long, as the prinia eggs keep changing and adapting in other ways. Cambridge researcher Martin Stevens explains:
"For example, 30 years ago, cuckoo finches predominantly laid eggs that look red to our eyes, but now lay mainly blue ones. Prinias in turn now more commonly lay olive-colored eggs, perhaps to escape their pursuing parasite."
Stevens and his colleague Claire Spottiswoode noticed the change in color when comparing prinia eggs in the wild in Zambia and some 40-year-old specimens from a nearby museum. The change happened in what Stevens describes as "a mere blink of an evolutionary eye."
This arms race is likely a function of both species being able to lay eggs with a wide variety of appearances - once the prinia is able to lay an egg pattern that the finch temporarily can't duplicate or the finch manages to replicate the prinia eggs, natural selection takes hold phenomenally quickly and favors this new type of egg over all others. As Spottiswoode observes, "Just as humans need to invent new drugs to defeat evolving bacteria and viruses, so host defenses undergo rapid changes to evade cuckoos."