Brood parasites like cuckoos lay their eggs in the nests of other birds – a costly interaction that has led to the evolution of host defenses, followed by reciprocal counter-adaptations in parasites. This rapidly evolving arms race has resulted in noticeable changes in just a few decades. According to new findings published in Proceedings of the Royal Society B, parasitic cuckoos have faster rates of plumage and egg evolution than non-parasitic cuckoos. But while the coevolution of parasites and hosts is linked to the diversity of phenotypes (or observable characteristics), it doesn’t increase speciation.
In birds, brood parasitism evolved at least seven times independently, with some imposing higher costs than others. The chicks of Vidua finches and whydahs can be raised alongside host chicks, but most parasitic cuckoos and honeyguides are highly virulent: Their chicks kill or outcompete the offspring of the host. In a series of tightly coupled interactions over time, after a host figures out a better way to detect and reject parasitic eggs and chicks, the parasites start producing eggs and chicks to better mimic the appearance of host offspring. Some cuckoos are even known to have yellow legs, a yellow eye ring, and barred chest plumage to mimic hawks and intimidate their hosts.
Coevolution has often been thought of as an engine of biological diversity, leading to increased rates of speciation and the generation of phenotypic diversity. Rapid genetic divergence may occur when, for example, a population of a generalist species begins to specialize on exploiting one particular host. But is that actually the case?
Using computer models, Australian National University’s Iliana Medina and colleagues compared the rates of speciation and phenotypic evolution of three brood parasite lineages with that of closely related, non-parasitic lineages. They compared parasitic cuckoos with non-parasitic ones, honeyguides with woodpeckers, and Vidua finches with Estrildid finches and African weavers.
The team found little consistent evidence that lineages of brood parasites have higher speciation or extinction rates than non-parasitic species. But they did find evidence that the evolution of parasitic behavior affects the rates of evolution in morphological traits that may facilitate parasitism: Egg size and plumage color and patterning evolved up to nine times faster in parasitic cuckoos than in non-parasitic species. Additionally, different cuckoos that parasitize similar hosts show higher rates of phenotypic evolution – which might be linked to competition for hosts.