Diario de stephenmatthews

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

In last week's post I looked at the curious case of the 'hepatic' forms of female cuckoos. The Long-tailed Shrike poses a similar puzzle: a minority of shrikes of this species have a distinct dark form, known as the 'dusky' morph. With today's observation from the Ngong Chung peninsula on Po Toi, I calculate that 90 of 900 observations of Long-tailed Shrikes in Hong Kong on iNaturalist are of the 'dusky' morph (there are various intermediate forms or intergrades, so I considered the round dark face mask as the decisive criterion). Records from eBird show a similar rate, close to 10%.
The dark morph appears to be especially common in Hong Kong and Guangdong province. It is often described as melanistic, but melanism alone should not affect 10% of the species. Moreover, the dusky morph is not simply a darker form of the usual plumage. In particular, the black facial mask takes the form of a round disc rather than a horizontal band. This point is noted in the eBird description ("melanistic form has more extensive dark mask and is darker overall") and shown in a fine photo by Derek Hon: https://www.inaturalist.org/observations/67244093
A fuller explanation may involve aggressive mimicry, whereby a predator takes on the appearance of a harmless species.
When I came across this shrike today, I at first thought we were looking at a Masked Laughingthrush. The size and shape were right, and the black facial disk suggested this species. When my companion with superior eyesight and equipment recognised that it was a shrike, I realised that we were looking at a 'dusky' Long-tailed Shrike. In fact, a few of the 'dusky' shrikes observed on iNaturalist were initially identified as Masked Laughingthrushes. If the disguise is good enough to fool birders, it is good enough to fool prey species into thinking they are facing a relatively harmless laughingthrush, rather than a predatory shrike. Compare this view of the shrike, https://www.inaturalist.org/observations/36365381 with this laughingthrush: https://www.inaturalist.org/observations/221759253
With this form of mimicry, the dusky shrike is effectively a 'wolf in sheep's clothing'. This will give a selective advantage to 'dusky' shrikes, but only as long as the disguise remains relatively rare, since otherwise prey species will come to recognise dusky shrikes as dangerous. This would be an instance of frequency-dependent selection.
We then have an explanation for the 'dusky' morph making up 10% of Long-tailed Shrikes in Hong Kong and Guangdong, where the Masked Laughingthrush is a common resident. If this explanation is correct, the frequency of the dusky morph should be lower where Masked Laughingthrushes (or similar black-faced laughingthrush species) are absent or rarer than they are here.
Aggressive mimicry of this kind is not uncommon in birds of prey. An example is the Grey Goshawk of Australia. The white morph of the Grey Goshawk mimics a harmless white cockatoo, and indeed uses cockatoo flocks as cover when hunting. I believe I have witnessed this, and Australian birders such as these have seen it on multiple occasions:
http://bioacoustics.cse.unsw.edu.au/birding-aus/2013-01/msg00283.html
https://donaldknowler.com/goshawks-get-cocky/
Thanks to Ton Yeh and Kalvin Chan for discussion of this puzzle (which is not to imply that they agree with any of the above).