(Intro)

Raising kids is a big job, and one that costs a lot of energy. But what if you could just drop your kids off at someone else's place, and let them have the responsibility? Well, there are some animals that do just that. They are called brood parasites, and they lay their eggs in other animal's nest and let them do all the hard work. 

There's a kind of evolutionary arms race between these hosts and parasites. Hosts try to remove parasite eggs from their nests, but the parasite's species often have ways to trick hosts into keeping them.

The cuckoo catfish, for example, is a brood parasite of cichlids, a family of diverse fish found around the world that rear their young in their mouths. The cuckoo catfish is what's known as an obligate parasite, meaning that they are no longer capable of raising their own young. When the female cichlid lays her eggs, the catfish quickly swims in, eats some of them, and deposits her own eggs. The cichlid then scoops up the catfish's eggs, along with her remaining brood, and takes care of all of them in her mouth until they hatch, but the cichlid's eggs won't be around for long. The young catfish hatch first and eat them. Then the mother cichlid takes care of the catfish's babies. She has no idea that they are not hers.

The cuckoo paper wasp does something similar, leaving its eggs in the nest of other species of paper wasp. Cuckoo wasps are also obligate parasites, so if the queen can't find a host nest she won't lay any eggs that season. If she is successful though, her larvae actually attract more attention from host workers than their own larvae. Scientists think the parasites somehow give the "we're hungry!" signal more often and more forcefully than the host larvae. That signal is very important for a young wasp. When they grow up the ones that are fed the most will be reproducers instead of workers.

You might have noticed that all the examples we have talked about so far have "cuckoo" in their names. That is because cuckoo birds, like the recurring character, Dino, over on SciShow Kids, are probably the most famous brood parasites. The European Common Cuckoo is especially well known for leaving its eggs in other bird's nests.

Cuckoo birds, like the cuckoo wasp and catfish, also can't take care of their own young. So their only hope of successful reproduction is to rely on their hosts. This sneaky bird looks a lot like a raptor, so it scares away the host birds long enough to drop an egg in their nest and leave. If the parasitic egg isn't detected and pushed out of the nest by the host bird, then once it hatches it'll push the host's eggs out of the nest. It'll even push out the host chicks, if they've already hatched! The adults don't seem to notice, or, if they do, there is nothing they can do about it. So few, if any, of the host chicks survive. Another type of bird, the parasitic honey-guide chick, will take this one step further. It'll actually stab the host chicks to death with its beak.

But the host parent bird can easily toss an egg out of its nest. So, why don't host birds get rid of the intruders? Well, sometimes they do. Pied wagtails and red-faced cisticolas are experts at spotting eggs that don't belong to them, and will throw them out of their nest. But some cuckoos have eggs that look like their host eggs, so the host bird might not realize that the invading egg is there. Some host birds have other options. Reed warbler communities try to stop brood parasites from invading in the first place by using alarm calls to warn their neighbors when cuckoos are nearby. Superb fairy-wrens use a much more complicated tactic. They teach their chicks a secret password while the chicks are still in their eggs. Each female has an unique call that they share with their eggs, and, when the chicks hatch, they use the same call. The cuckoo chicks that invade the wren nests can't replicate the call, so they don't get fed. 

But for the host bird, sometimes it's not even worth trying to get rid of the brood parasites. In some species of cuckoo and in the brown-headed cowbird, if the host bird removes the parasite's eggs, the invading parent bird will come back and destroy the host's eggs. So the host can raise the cuckoo's chicks and hope that at least one of its chicks makes into adulthood, or it can risk losing everything. Talk about high stakes parenting.

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