Cowbird/host interactions in a southeastern old-field: a recent contact area?

Abstract

Although reports of Brown-headed Cowbird (Molothrus ater) parasitism have increased in the Southeast over the past 30 yr, the cowbird's status, distribution, and host relations within this region remain largely unknown. We collected information on cowbird abundance and cowbird/host interactions (parasitism frequency and intensity, hatching success of cowbirds, and host anti-parasite strategies) in an old field in the South Carolina coastal plain from March through August 1997 and 1998. We located 346 nests, representing 16 species, within the study area (Dill Sanctuary, James Island, Charleston Co., South Carolina). Female cowbird abundance (1.2% of all songbird territories) was low within the avian community; however, cowbird eggs were discovered in nests of 12 of 16 species found nesting within the study area. Although cowbird young were successfully fledged from 67% of host species' nests, five species exhibited an anti-parasite strategy (nest abandonment, cowbird egg burial, or egg ejection). The presence of these behaviors is possibly the result of gene flow into the population from areas of longer co-existence with the parasite. However, we present an alternative theory that southeastern, early-successional hosts may have retained rejection behaviors from ancestors previously exposed to cowbird parasitism.