Mid afternoon today (March 25) in downtown Norwich, two female Brown-headed Cowbirds joined a flock of about 16 House Sparrows in a backyard tangle of forsythia and other vegetation. No male cowbirds were seen. Both female cowbirds, which were some feet apart, appeared to be inviting the sparrows to preen. In this behavioral display, a perched cowbird lowers her head with bill down by the breast so that the back of her neck (nape) becomes about the uppermost point on her body. This posture invites another bird, in this case House Sparrows, to preen the cowbird. Because of the density of the vegetation I could not see a lot of what was going on. Some of the time, the House Sparrows seemed to be ignoring the cowbird displays. However, at one point a female House Sparrow was pecking near the head of one of the displaying cowbirds.

Cowbirds, as brood parasites, lay their eggs in the nests of a variety of other bird species which then potentially become foster parents for the cowbird young, often to the detriment of the nesting success of the hosts. The preening invitation, when directed at another species, has been often interpreted as encouraging a potential host to accept a cowbird as not threatening, thus aiding the cowbird in approaching a nest of that potential host. Cowbirds most commonly lay their eggs in nests of host species which build open cup nests rather than in cavities such as commonly used by nesting House Sparrows. Thus House Sparrows don't seem to be a promising host for rearing cowbirds.

I don't know where my backyard-visiting House Sparrows might nest, but it is not close by. House Sparrows have in recent weeks been attracted here daily by my bird feeding, whereas cowbirds have been seen only rarely. The two today were the first female cowbirds I've detected here this spring.

George Clark
Norwich, VT





 


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