March 17, 2015
Thanks to Kate Bemis for this excellent guest student blog post. Now we understand the meaning of Chipping Sparrow trills in a bit more detail!
Imagine that you are a male chipping sparrow and have just landed a prime territory that will allow you to attract a good mate and be successful during the breeding season. Now, you just have to protect it from potential competitors. Who you would prefer as a neighbor: a “great” male chipping sparrow who could out-compete you for your territory or mates, or a “sub-par” male who is not as good as you? A “sub-par” neighbor obviously would be less of a threat, so you would prefer him as a neighbor. But how do you judge quality and influence the kind of neighbors that hold territories near you?
Researchers at the University of Massachusetts found that male chipping sparrows recognize when it is in their benefit to form alliances with neighbors to protect territories. By eavesdropping on and assessing the relative song performance of their neighbors, male chipping sparrows decide whether to form alliances with neighbors. How do they actually do this? The most prominent feature of chipping sparrow songs are trills in which notes are repeated in quick succession. Faster trills are more difficult to produce, so people – and birds, as it turns out – can quantify variations in trill speed to identify “higher quality” individuals. The researchers mapped territories of male chipping sparrows and used song playback to simulate territorial intrusion while observing how males responded. Male chipping sparrows responded more vigorously when the trill of a simulated intruder was faster, i.e., more difficult to produce, and when there was greater disparity between their singing ability and that of the playback individual.
In 48 playback trials, nine coalitions formed in which neighboring males left their own territories, trespassed into a neighbor’s territory, and directed responses to the simulated intruder (the speaker that the was the source of the song). In all cases, coalitions formed when a bird’s own singing ability (trill rate) exceeded that of the residents in its alliance. In other words, individuals only helped out birds they deemed “less good” than themselves. In most of the coalitions, the trill rates of simulated intruders were higher than those of residents. Thus, birds were most likely to help neighbors when they perceived the intruder was of “higher quality” than their current neighbor. This suggests that individual chipping sparrows evaluate each potential interaction with their neighbors and modify their behavior in a way to maximize the benefits of their specific situation. Like people, birds can be more devious than you might expect!
Goodwin S. E. and J. Podos. 2014. Team of rivals: Alliance formation in territorial songbirds is predicted by vocal signal structure. Biology Letters. 10: 20131083. http://dx.doi.org/10.1098/rsbl.2013.1083