Responses of nesting song sparrows (Melospiza melodia) and red-winged blackbirds (Agelaius phoeniceus) to models of parasitic cowbirds and nonthreatening towhees
Strausberger, B.M.; Horning, M.E.
Bird Behavior 12(3-4): 71-78
Many studies have compared nest defense elicited by models of brood parasites and innocuous species placed near potential host nests. Because the majority of costs associated with brood parasitism are incurred in the nest, it may be more appropriate to conduct such experiments at nests unless hosts respond to all intruders at their nests as enemies. We found that nesting species distinguish among threatening female brown-headed cowbirds (Molothrus ater) that frequently parasitize them, male cowbirds generally thought not to participate in parasitism, and innocuous eastern towhees (Pipilo erythrophthalmus). Taxidermic mounts of each were placed at the nests of two common host species: song sparrow (Melospiza melodia) and red-winged blackbird (Agelaius phoeniceus). Both host species reacted more aggressively toward female cowbird models than towhee models, whereas red-winged blackbirds also reacted more aggressively toward male cowbird models than toward towhees. These finding:; suggest that host species can discriminate between potentially threatening and nonthreatening intruders (both species and sex specific) at the nest; however, this discrimination may be imperfect as was indicated by red-winged blackbird's aggressive response toward male cowbirds. Alternatively, male cowbirds may be perceived as more of a threat to red-winged blackbirds than female towhees.