Shiny cowbirds synchronize parasitism with host laying and puncture host eggs according to host characteristics

Obligate avian brood parasites lay their eggs in nests of other species, which thereafter provide all parental care. Brood parasites synchronize parasitism with host laying and remove or puncture host eggs to increase their reproductive success, but the benefits of these behaviours may depend on hosts characteristics such as body size. We experimentally analysed the effects of synchronization between parasitism and host laying and reduction in number of host eggs on hatching success and chick survival of shiny cowbirds, Molothrus bonariensis (50 g), in two common hosts that differ in body mass: chalk-browed mockingbirds, Mimus saturninus (75 g), and house wrens, Troglodytes aedon (13 g). We found no effect of synchronization of parasitism or of the number of host eggs removed on parasite hatching success in either host. However, survival of cowbird chicks in mockingbird nests was lower when cowbird chicks hatched after host chicks and when there was no removal of host eggs. In contrast, in wren nests, there was no effect of hatching asynchrony or egg removal on cowbird survival, but asymptotic weight was higher in nests without egg removal. In natural nests, the proportion of cowbird eggs laid during host laying was higher and the number of host eggs punctured per parasitic event was greater for mockingbirds than for wrens. These differences between hosts in the extent of synchronization between parasitism and host laying and the intensity of egg punctures suggest that shiny cowbirds may adaptively adjust these behaviours to host characteristics.