Cliff Swallows are colonial -- that is, they nest in colonies, sometimes numbering in the thousands of birds. In many ways the members of a colony appear to display remarkable social cohesiveness. They work together to mob predators and will even learn from each other where the good food sources are. But if you look closely at a Cliff Swallow colony, you'll see that this seemingly cooperative community also harbors its share of dastardly misbehavior.
In every colony there are a few swallows (you might call them bad eggs) that parasitize their neighbors. They do this not by sucking other swallows' blood or stealing food, but by putting their eggs in nests other than their own. Sometimes the sneaky swallow will even toss out one of the nest owner's eggs before laying her egg in its place! This behavior is known as brood parasitism. The extra eggs go undetected, and the surrogate parents end up doing the work of raising the slacker's young.
These parasitic egg-laying visits are clandestine and quick, but some Cliff Swallows have been spotted launching an even faster, more remarkable sneak attack: carrying eggs in their very small beaks (adapted for catching tiny insects on the wing) and quickly dropping them into a neighbor's nest. Incoming!
Click here to read more about the Cliff Swallow.