Brood parasites manipulate their hosts: experimental evidence for the farming hypothesis

Parasites show a wide variety of strategies to maximize the successful transmission of themselves and/or their offspring, by exploiting hosts. One such strategy occurs when parasites manipulate host behaviour in a way that increases their probability of transmission to an uninfected host. Here, we examine whether brood parasitic brown-headed cowbirds, Molothrus ater, attack and cause nest failure in late-stage, and hence, inappropriate host nests, which theory suggests they may do to parasitize the replacement nests at an opportune time, effectively manipulating their host's reproductive behaviour and improving their own transmission. Critical to this 'farming' hypothesis, cowbirds must be attuned to the reproductive stage of their host and act accordingly by destroying nonparasitized clutches they find late in the nesting cycle. We conducted a series of experimental manipulations in which we presented captive cowbirds with nests simulating early and late stages. We found that cowbirds caused significantly greater destruction in the late-stage nests. Moreover, our results suggest that cowbirds are capable of using both direct assessment and absolute egg number to assess which clutches to destroy. Corroborating our findings in the laboratory, 10 years of field data show that cowbirds significantly increase the intensity of their attacks (i.e. the proportion of the clutch destroyed) on nonparasitized host nests as the nesting cycle progresses; however, we found no such trend for parasitized host nests. These results indicate that cowbirds evaluate the reproductive stage of their hosts using multiple mechanisms and use this information to vary the intensity of their attacks.