Cowbirds (in the New World) and cuckoos (in the Old World) are completely unrelated birds which have evolved the same bizarre breeding tactic, called "brood parasitism." These birds have figured out that it takes a lot of time and energy to build a nest, incubate your eggs, and feed the chicks. So why go to all the trouble?
Cowbirds and cuckoos have discovered that the nesting and brooding instinct of other bird species is so strong, they will brood and feed anything in their nest, regardless of whether or not it belongs to them. So much easier than raising your own chicks! As a bonus (to the cowbirds and cuckoos), their chicks almost always result in the death of the other chicks in the nest, thus reducing the competition.
Gruesome though it is, I have to admire the way in which these birds have figured out how to game the system. Even though their eggs are typically larger than the others in the nest, and their chicks are usually far bigger than the other chicks, the hapless parent birds don't notice - or can't stop themselves, depending on how you look at it.
Brood parasitism is one of the few avenues available to the cowbird. This species gets its name because it follows a herd of grazing animals (typically cattle) across the plains. They feed on all the insects kicked up by the grazers. If the cowbird falls behind the herd, it will starve to death. So it can't afford to stop and nest its young. Parasitizing the nest of another bird species allows it to drop and run, so to speak.
On the other hand, Old World cuckoos have no such excuse. I guess they're just lazy. (Kidding!) The cuckoo's tactics are even more forceful than those of the cowbird. If a cuckoo hen finds that her target nest has hatched already, she will eat all the chicks, thus forcing the host bird to lay more. The cuckoo chick is similarly murderous, and will push out all the other eggs. If they hatch before the cuckoo chick can dispose of them, it will push the chick out of the nest to a certain death on the ground below.
Cuckoo chicks are several times larger than their host birds. And they have a hunger to match! A cuckoo chick will work its adoptive parents half to death providing enough food for it day after day until it finally fledges.
In the Gaelic language, the cuckoo is called "gowk," after its alarm call. Gowk Stones stand all across Scotland, random giant boulders left behind after the last ice age. The naming convention is obvious - the stones are reminiscent of the cuckoo's inexplicably large egg sitting in the nest.
The cuckoo is also considered a trickster (obvious conclusion there), and the inference is that idiots can be tricked into trying to crack open the Gowk Stones like large eggs.
Cuckoos have a long history in Gaelic tradition, where they often collude with the devil. It's easy to see where that comes from, too!
Photo credit: Flickr/Cloud Vixen