This is Scientific American's 60-second Science. I'm Jason Goldman.Brown-headed cowbirds are obligate brood parasites.That means they lay their eggs in the nests of other birds, leaving all the hard work of incubating the eggs and feeding the chicks to somebody else.Luckily, many of those other species have evolved some antiparasitic defenses.Yellow warblers, for example, produce a certain type of call to warn other warblers about nearby cowbirds. Researchers call it a "seet" call."We were working on the yellow warbler.And whenever we were doing seet call playbacks to the yellow warblers, the red-winged blackbirds kept showing up.That was truly a discovery, this aha moment that allowed us to explore this question even further."Ecologist Mark Hauber from the University of Illinois at Urbana-Champaign, who ran the study.The yellow warblers' seet call is called a "referential alarm call," because it is only produced for a single threat: the brown-headed cowbird."And in response to the seet call, the female yellow warbler, instead of attacking or mobbing the cowbird,she sneaks back to her nest and sits on it tightly so as to prevent the cowbird from being able to lay a parasitic egg."Red-winged blackbirds, which sometimes nest near warblers, do not produce referential alarm calls.