Male Sparrows Form 'Team Of Rivals' With Neighboring Birds To Protect Territory From Aggressors
Male sparrows form alliances with weaker sparrows to prevent rivals from moving into their territory, according to a study published in the current issue of Biology Letters. This phenomenon is known as the "dear enemy effect," and finds neighboring territorial animals animals tolerating one another--or, in this rare case, even working together. The researchers referred to the male sparrows' behavior as a "team of rivals" approach.
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In the study, led by Sarah Goodwin and Jeffrey Podos of the University of Massachusetts Amherst, the researchers placed a loudspeaker and a taxidermic sparrow model in the territory of a male chipping sparrow (Spizella passerina), and played male birdsong to simulate a rival sparrow. The researchers did 48 playback trials, half of which made of fast trills and half of which were slow.
The sparrows responded aggressively to the fast trills, spending more time near the loudspeaker and attacking the taxidermic model; the faster trills thus seemed to make sparrows think their territory was more threatened. As she went through the experiment, Goodwin started to notice something weird happening about 20 percent of the time: a male sparrow attacking the taxidermic model would be joined be a neighboring male, who would aid in the attack. Not only did the second male sparrow attacked the taxidermic model, but the first male sparrow was totally cool with it. Goodwin said that this forming of "territorial defense coalitions" has been "very rarely documented in birds."
The researchers also found that neighboring sparrow consistently swept in to help the other sparrow when the other sparrow had a slower trill rate than the neighboring sparrow did (and was thus considered "less manly" by the neighboring sparrow). The neighboring sparrow also consistently aided the other sparrow if the other was considered less manly than the taxidermic model.
"We interpret this to mean that the ally not only prefers having a lousy neighbor, but also specifically does not want that lousy neighbor replaced by a more serious competitor," said Podos. "What's really neat is that there are specific vocal relationships that predict when and with whom the birds will form these coalitions."
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