Nest architecture influences host use by avian brood parasites and is shaped by coevolutionary dynamics

Brood (social) parasites and their hosts exhibit a wide range of adaptations and counter-adaptations as part of their ongoing coevolutionary arms races. Obligate avian brood parasites are expected to use potential host species with more easily accessible nests, whilst potential hosts are expected to evade parasitism by building more concealed nests that are also difficult for parasites to enter and lay eggs in. We used phylogenetically informed comparative analyses and a global database of the world’s brood parasites, their host species, and the design of avian host and non-host nests (~6200 species) to examine first, whether parasites preferentially target host species building open nests and, second, whether host species targeted by specialist parasites build enclosed nests. As predicted, the nests of host species tend to be more accessible to brood parasites than the nests of non-host species, and host species targeted by specialist brood parasites build more concealed nests than host species targeted by more generalist brood parasites. Furthermore, evolutionary transition analyses demonstrate that host species building closed nests evolve to either remain nesting in closed structures and become non-hosts or evolve to build open nests and remain hosts. We conclude that nest architecture, and the accessibility of nests for parasitism, represent a critical and previously under-appreciated stage of the ongoing coevolutionary arms race between avian brood parasites and their hosts.