Listen to part of a lecture in a biology class. So last week we were talking about different examples of different types of parasitic behaviors. Most of those behaviors were pretty straightforward. So today I wanna talk about how they can be... how one species exhibits parasitic behavior that's a little more complex. The species I'm thinking of is a bird called the brown-headed cowbird. Now the cowbird is actually a brood parasite. Let me write that down. Okay, so brood parasitism is where a particular species of bird enters another’s nest. And they'll actually enter the nest of a different bird species, lay an egg there, and then leave the chick to be raised by the host bird instead of doing it themselves. The cowbird parasitizes over 100 different host species in this way. Black birds, warblers, all kinds of smaller birds. Because of this, maybe you've heard this before, it got this reputation as being lazy because it's assumed the cowbirds just lay an egg in a random nest and forget about it. Why would a host bird accept an alien egg and raise the cow birds chick? It does seem odd. Since there's an obvious cost to incubating and feeding the young of another species. Several hypotheses have attempted to explain this. One is that abandoning her nest to find another nesting site involves two largest sacrifice for the host bird. So it accepts the cowbird's egg, because that's better than not hatching the eggs she's already laid, right? Another idea is something called evolutionary lag. This means that the host birds, in evolutionary terms, these host species haven't had sufficient time to develop a defense against brood parasitism. But let them… given enough time, they will, presumably. But then a couple of researchers noticed that if they removed a cowbird egg from a warbler’s nest, then later, when they revisited the nest, the warbler eggs had been destroyed. So an idea occurred that the cowbirds, adult female cow birds, were actually watching, making sure that their chicks were being taken care of. But this really went against the accepted wisdom about cow birds. Remember their reputation? They're lazy birds, right? This sort of complex behavior had never been seen in a brood parasite. So the researchers set up an experiment. They set up a bunch of nest boxes for warblers, boxes that were constructed so that they be safe from non avian predators like snakes or raccoons, then after a cow bird laid an egg alongside the warbler eggs, the researchers removed the cowbird egg from each nest box. After removing the eggs. For some of the nests, they restricted access. They made sure that the small host bird could enter the nest, but the larger cowbird couldn't. So for some of the nests, cow birds could reenter, and for others, they couldn't. And sure enough, the nests with smaller openings remained undisturbed. While most of the others were destroyed, all the warbler eggs were destroyed. So the researchers were right. The cow birds were coming back to check on their eggs, and if one had disappeared, they noticed and retaliated报复. So it turns out that warblers do better auspicious by accepting the cowbird egg. Because if they ejected it from the nest, all of the warblers’ eggs could be lost as well. So the hosts are encouraged to accept the parasitism, which is really… that study really documented behaviors that no one else had thought to look for. And it turns out that it's not only the adult females that researchers have taken note of . The cowbird chicks compete with a host birds chicks for food, right? Most brood parasites would just push their rivals out of the nest. But normally, the young cowbird doesn't even try doing this. I mean they probably could, because cowbird eggs tend to hatch earlier, so the cowbird chick is larger than its nest mates. But most cow birds allow themselves to be raised alongside the hosts’ young. You see, by ignoring the other chicks, the young cowbird gets more food, more chicks in the nest means more food is brought back to the nest. a lot of it goes straight to the bigger, more aggressive cowbird.