Host provisioning behavior favors mimetic begging calls in a brood-parasitic cowbird

The vocalizations of some young brood-parasitic birds closely resemble those of their host’s young. Such similarities might arise because hosts bestow the greatest parental care in response to their own species’ call type. We used a playback experiment to assess the effectiveness of the nestling call structures of two brood parasites, the specialist screaming cowbird (Molothrus rufoaxillaris) and the generalist shiny cowbird (M. bonariensis), in stimulating parental provisioning in a shared host, the baywing (Agelaioides badius). Screaming cowbird begging calls closely resemble those of baywing young and thus should best exploit any bias for species-specific cues. Shiny cowbird calls, in contrast, are unlike baywings but can instead exploit non-specific sensory biases for long call duration and syllable repetition. We found playback of screaming cowbird’s mimetic calls elicited increases in feeding rates equivalent to those of playback of the host’s own young, while shiny cowbird calls failed to increase provisioning rates above those of no-broadcast control sessions. These results indicate that baywings discriminate between nestling call structures in favour of their own species calls when adjusting parental investment, and support the view that selection for optimal host provisioning can favour vocal mimicry by parasitic offspring.

部分幼年巢寄生鸟类（brood-parasitic birds）的鸣叫声与它们宿主的幼鸟鸣叫声极为相似。这类相似性的产生可能源于宿主会针对自身物种的鸣叫声类型，给予最为充足的亲代抚育。我们采用回放实验（playback experiment），评估了两种巢寄生鸟类——专性寄生的啸声牛鹂（screaming cowbird，*Molothrus rufoaxillaris*）与广宿主寄生的辉牛鹂（shiny cowbird，*M. bonariensis*）的雏鸟鸣声结构，在刺激共同宿主栗翅拟鹂（baywing，*Agelaioides badius*）产生亲代投食育雏行为方面的有效性。啸声牛鹂的乞食鸣叫声与栗翅拟鹂幼鸟的鸣叫声高度相似，因此能够最充分地利用宿主对物种特异性信号的偏好。相较之下，辉牛鹂的鸣叫声与栗翅拟鹂的鸣叫声差异显著，但可利用宿主对长鸣唱时长与音节重复的非特异性感官偏好。我们发现，播放啸声牛鹂的拟态鸣叫声所引发的投喂速率提升，与播放宿主自身幼鸟鸣叫声的效果相当；而辉牛鹂的鸣叫声则未能使育雏投喂速率高于无播放的对照组。上述结果表明，栗翅拟鹂在调整亲代投入时，会对雏鸟鸣声结构进行甄别，并优先选择自身物种的鸣叫声；同时也支持了这一学术观点：旨在促使宿主提供最优育雏投喂的选择压力，会倾向于青睐寄生后代的鸣唱拟态行为。