Coevolution with hosts underpins speciation in brood parasitic cuckoos

Coevolution is considered to be a likely driver of speciation, but evidence linking macroevolutionary patterns to microevolutionary processes is scarce. We report that coevolution with hosts drives divergence in bronze-cuckoos. Bronze-cuckoo hosts reject cuckoo nestlings, selecting for mimicry of host nestlings by cuckoos. This has generated a diversity of bronze-cuckoo nestling morphologies matching those of their respective hosts across their geographic range and promotes diversification in sympatry; little bronze-cuckoos that exploit multiple host species in sympatry show evidence of genetic differentiation and corresponding divergence in nestling morphology. This process is reflected in macroevolutionary patterns: rates of speciation are faster in virulent cuckoos than in more benign species and simulation models indicate greater support for a sympatric mode of speciation in bronze-cuckoos than in sister taxa.