Host–parasite contact and sensitivity to parasitism predict clutch abandonment in cowbird hosts

Abandoning a brood to maximize lifetime reproductive success may result from the current costs affecting future reproductive opportunities. In certain contexts, clutch abandonment can be an evolved breeding strategy rather than a generalized response to stressors such as inclement weather and predation. Obligate brood parasitism, a reproductive strategy in which a parasitic species relies solely on other species to raise its young, imposes fitness costs to hosts and could serve as a trigger for clutch abandonment. This cost, and the resulting clutch abandonment strategy, may vary according to contact with the parasite, sensitivity to parasitism and the value of the current reproductive effort (i.e. brood value). We conducted a phylogenetically controlled meta-analysis covering 85 host species of the three generalist cowbird species (Molothrus sp.) in which we examined the following effects on the abandonment (nest desertion and egg burial) of parasitized versus unparasitized clutches: habitat at a small scale (higher host–parasite encounter frequency in open versus forested habitats) and at a large scale (longer interaction between parasites and hosts in prairie versus nonprairie regions), brood value (relative value of a clutch) and host species sensitivity to brood parasitism (relative body mass). Parasitism increased clutch abandonment overall. This increase was strongest in open nonforested habitats, with smaller, more sensitive hosts being more likely to abandon their clutch. Brood value and occurrence in prairie regions did not affect clutch abandonment, indicating that recent, more fine-scale host–parasite interactions were more important than coevolutionary history. Therefore, the abandonment of a brood can be used in diverse parental care strategies, including antiparasitic defences of brood-parasitic hosts.