Factors affecting frontline host defences against avian brood parasitism: a meta-analysis

Obligate avian brood parasitism is a reproductive strategy in which a bird lays its egg in the nest of other bird (host), thereby forcing the host to raise the parasitic offspring as its own. This behaviour triggers a coevolutionary arms race, driving the evolution of host defences and parasite counter defences. Hosts can defend against brood parasitism at all stages of the breeding cycle, with frontline defences (those aimed at preventing a parasite from laying its egg) representing the earliest and critical stage of defence. Despite their importance, the main factors driving frontline (prelaying) antiparasitic behaviours remain unclear. Here, a meta-analytical approach was used to quantify the influence of various factors on host aggression, as an important and widespread frontline defence, towards adult brood parasites. In general, host aggression is assessed in field experiments by presenting brood parasite models near nests and observing the host response. Results show that host aggression towards brood parasites was influenced by the geographical overlap between host and parasite, as well as the distance of the experimental model from the host nest. In particular, aggression was higher in host populations living in sympatry with the parasite. In addition, hosts showed greater aggression towards parasite models placed closer to their nests in field experiments. Other factors, including differences in host–parasite body size, nest type (enclosed or open) and exposure time to the experimental model, did not affect host aggression. Our findings indicate that host aggression towards brood parasite models is primarily influenced by the sympatry–allopatry distribution between brood parasites and hosts. Furthermore, our study highlights the key methodological aspects that should be considered in future field experiments involving artificial brood parasite models. • Hosts living in sympatry with brood parasites exhibit high aggression levels. • Host aggression increases with proximity of the brood parasite model to nest. • Origin of parasite, nest shape and body size do not affect host response.