Imagine if you found a way to kill the baby inside a woman’s body and replace it with a fetus of your own. The very idea is absolutely disgusting and parasitic, just sick enough for a bird to think of it as totally acceptable.
There are several species of birds which do this, including cowbirds, cuckoos and the black-headed duck. These birds have all been known to shamelessly get rid of eggs in a foreign nest and lay several of their own. They aren’t like the blue jay, who simply eats fertilized eggs from time to time to quell a craving.
The parent birds who rear these parasitic young are actually not bright enough to tell the difference between say, a little baby sparrow and a huge baby cowbird. Birds are much more intelligent than they’re given credit, but this tidbit does not work in their favor.
Here is an informative video if you want to be amazed at how truly evil a wittle baby birdie can be.