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This is scientific Americans' 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 anti-paracitic defenses.

Yellow warblers, for example, produce a certain type of call to warn other Warblers about nearby cowbirds.

Researchers call it a seat call.

We were working on the Yellow Warbler and whenever we were doing seed call

playbacks to the yellow warblers the Red Wing Blackbirds kept showing up.

That was truly a discovery you know sort of this aha moment that allowed us to explore this question even further.

Ecologist Mark Halber from the University of Illinois at Urbana-Champaign, who ran the study.

The Yellow Warbler's seat 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 seat 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 nests near warblers, do not produce referential alarm calls.

But it turns out that they can eavesdrop on the yellow warblers calls, and they respond to

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