Hepatic cuckoos and the evolutionary arms race

The plaintive cuckoo is a fairly common breeding bird in Hong Kong, where it typically parasitises the broods of prinias and tailorbirds. Even so, the bird which appeared in Kowloontsai Park recently was the first 'hepatic morph' female I have come across. That is, the bird was rufous brown, with similar barring to the more common grey form. The term 'hepatic' means 'liver-coloured' and is apparently only used for aberrant female cuckoos. Intriguingly, it applies to a whole range of cuckoos, including the Eurasian or Common Cuckoo, Oriental Cuckoo, Himalayan Cuckoo and Lesser Cuckoo (genus cuculus) as well as the Plaintive, Grey-bellied, Rusty-breasted, Brush and Banded Bay Cuckoos (genus cacomantis). In all these species, females have two colour forms, a common grey 'morph' and a rarer, rufous-brown 'hepatic' one. But why?
Brood parasites such as cuckoos are in a perpetual evolutionary arms race with their host species. The typical grey morphs look superficially like hawks (accipiters such as sparrowhawks and goshawks), which are fearsome predators of small birds, and this discourages the hosts from defending themselves or their nests. But some hosts learn to distinguish between true hawks and cuckoos, which can then be attacked or mobbed with impunity. This is where the 'hepatic' morphs come in: they resemble neither the typical cuckoo nor the hawk (in fact the hepatic Plaintive Cuckoos look more like kestrels, which may be another form of mimicry). This is advantageous only for females, because only females need to visit the nest of the host species, and they can swoop down on a nest without being recognized (perhaps even being mistaken for a kestrel).
Recent genetic work has found that the colour morphs are determined by the W chromosome, which is unique to female birds. Basically, a genetic mutation has been selected but is only advantageous when it applies to a minority of females, since it is then difficult for hosts to learn to recognize the parasitic cuckoo (just as it took several years' birdwatching for me to encounter a hepatic female, and even then it took me several seconds to realize what it was). If all females were brown, they would lose the original resemblance to hawks.
Now for the really interesting part. Genetic analysis shows that the 'hepatic' mutation first developed in a common ancestor, and has been around since long before the speciation of the current species. This explains why the 'hepatic' forms are shared among so many cuckoo species, even across different genera.

The genetic research by Merondun and colleagues is published in Science Advances (2024):
Evolution and genetic architecture of sex-limited polymorphism in cuckoos, https://www.science.org/doi/10.1126/sciadv.adl5255

Posted on September 19, 2024 11:32 AM by stephenmatthews