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Posted by Chris on 12:55:58 12/17/08
After the two recent sightings of lone yellow-headed blackbirds hanging out with and interacting with brown-headed cowbirds I became curious about what exactly is going on in these interactions (particularly the apparent solicitations of grooming). The cowbirds are brood parasites, so it is natural to think that maybe the ones soliciting the grooming were reared in YHBB nests. Cowbirds, however, do parasitize red-winged blackbird nests but apparently do not parasitize YHBB nests (although it is always possible that the research may be lagging behind changes in population dynamics).
I contacted the brown-headed cowbird behavior experts that I know (from the West and King lab at Indiana) and was told that the headdown display that the cowbirds do during the winter is poorly understood. It may not be about grooming at all, even if it does occasionally produce some preening. One intuitive possibility is that it is used to establish dominance (and there is a little research evidence to support this), although it is not an aggressive behavior. Andrew P. King thinks that it probably has some function related to producing group cohesion, however, none of his students have been able to successfully study this behavior yet (although many have been interested).
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