Would you raise the bird that murdered your children? - Steve Rothstein

This is a little bee-eater’s nest. But this is not a little bee-eater chick. It’s a newly hatched honeyguide—and it’s much more lethal. When its mother placed it here, she punctured all the other eggs in the nest. However, one little bee-eater chick survived the attack and is now hatching.

Although the honeyguide nestling is still completely blind, it instinctively stabs the little bee-eater chick with its sharp, hooked beak. And over the following weeks, the host parents devotedly care for the hatchling that murdered their offspring. This is but one example of brood parasitism, an evolutionary strategy in which one animal tricks another into rearing its young. It’s especially well-known among birds.

By depositing their eggs into a stranger’s nest, brood parasites are able to shift the major costs of parenting onto others. Brood parasite chicks usually hatch early, then monopolize their host parents’ attention. Some stab their fellow nestlings to death, while others shove the remaining occupants out of the nest. Meanwhile, others are less harmful to their hosts.

Not all brood parasites kill all of their host’s offspring outright. Brown-headed Cowbirds usually outcompete them by begging for food louder, more frequently, and with a wider mouth. Among the most benign, black-headed ducks lay their eggs in other nests to be incubated. However, a few hours after hatching, they simply saunter off.

But in the most egregious cases of brood parasitism, why don’t host parents take a stand? In fact, hosts will often drive adult brood parasites away from their nests, and many take their defenses further. But whether a host can recognize and reject parasitic eggs and nestlings seems to depend on a few factors.

Eastern phoebes will accept a Brown-headed Cowbird’s speckled egg into their nest, though theirs are pure white. The gray catbird, on the other hand, is an expert at rooting out the very same parasites. It memorizes what the first egg in its nest looks like, which is usually its own, and tosses any aberrations.

This retaliatory adaptation can fuel an evolutionary arms race where brood parasites evolve eggs that closely mimic their host’s. Interestingly, birds that do reject parasitic eggs are usually clueless when it comes to parasitic chicks. Reed warblers are good at ejecting poorly matching cuckoo eggs. But if one hatches in their nest, they’ll care for it even after it’s grown six times their size.

Though chick rejection is a rarer phenomenon, there are some noteworthy examples. While incubating its eggs, the Australian superb fairy-wren sings to them, imparting a unique note that its chicks use as a kind of password. When a cuckoo is in the wren’s nest, it hatches first and pushes the others out.

But, perhaps because it hatched sooner, the cuckoo chick wasn't able to learn the password, and so it doesn’t croon the right begging call. At this point, the adults usually abandon their nest and start another. Altogether, host species show a remarkable variety of responses.

This seems to partially be a result of how long brood parasitism has been in their environment, and thus how much time they've had to evolve suitable adaptations. In fact, studies have shown that those hosts that reject parasitic eggs less frequently can visually distinguish between their eggs and a brood parasite’s. They simply lack a response to the visual information.

This is probably because, before brood parasitism appeared, responding would have likely had no adaptive value. And even when hosts do recognize a parasite, getting rid of it may not be the best option. The host, especially if it’s small, might not be able to kill the parasite—or could risk breaking its own eggs in the process.

Unless the brood parasite kills all of the host’s young, it may be best to simply foster the imposter. Brood parasitism tends to evoke horror and disdain. But why should it be thought of as any more objectionable than predator-prey relationships? And is it ever productive to impose human morals onto other animals?

Or does it end up saying more about us than it does them? Whichever way you swing it, brood parasitism is yet another example of the fascinating turns evolution has taken.