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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.
这里是科学美国人――60秒科学。我是杰森・古德曼。
棕头牛鹂属于专性巢寄生。
意思是,它们将卵产在其它鸟类的巢里,抒孵卵和喂养雏鸟的所有辛苦工作都留给别人。

比如,黄莺会发出某种叫声,提醒同类周围有棕头牛鹂。研究人员称之为“see”叫声。
“我们一直在研究黄莺。
只要我们给黄莺播放seet叫声,红翅黑鹂就会出现。
这真的是个重大发现,是让我们得以进一步探索这个问题的好时机。”
负责这项研究的伊利诺伊大学香槟分校的生态学家马克・豪伯说到。
黄莺的seet叫声被称为“指示警报叫声”,因为它只针对一种威胁:棕头牛鹂。
“听到seet叫声后,雌黄莺没有攻击或围攻棕头牛鹂,
而是偷偷回到巢中,紧紧地坐在上面,防止牛鹂在此产寄生卵。”
红翅黑鹂有时在黄莺附近筑巢,它们没有发出指未警报叫声。
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