29 de septiembre de 2024

The 'dusky' Long-tailed Shrike: a wolf in sheep's clothing?

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).

Publicado el 29 de septiembre de 2024 por stephenmatthews stephenmatthews | 2 observaciones | 0 comentarios | Deja un comentario

19 de septiembre de 2024

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

Publicado el 19 de septiembre de 2024 por stephenmatthews stephenmatthews | 1 observación | 0 comentarios | Deja un comentario

17 de julio de 2024

A woodpecker paradise, and 1000 birds

A little-known fact about Prague is that the city, like much of the Czech Republic, is a paradise for woodpeckers (and for their afficionados). Even city parks such as the Petřín Hill and Královská Obora parks are rich in mixed woodlands which attract at least six species of woodpecker, as well as their congeners such as Nuthatches and Treecreepers. The Middle Spotted Woodpecker is a central European species which requires mature deciduous forest. In the Královská Obora park today, a photogenic male perched for some time on a the trunk of an oak tree.
According to the eBird database, this woodpecker was the 1000th species I have recorded in a total of 1900 checklists. This is not exactly a list of species I have seen, since it includes some 'heard only' species such as owls, and excludes various birds which I saw before the eBird era and did not record accurately enough to enter in the database retrospectively. But when I began using eBird to record sightings in 2017, 1000 birds seemed like a reasonable goal. At the time, it was agreed that there were around 10000 bird species in the world, so that 1000 would be a representative sample of 10%.
Now for the interesting part. Despite ongoing extinctions, the number of known bird species is rapidly rising. Thanks largely to genetic sequencing, it has become clear that birds which look alike do not necessarily belong to the same species, and vice versa. 'Cryptic species' appear similar but do not interbreed because they are separated by behaviour and vocalizations as well as geography. The Chinese Blackbird, for example, looks much like the Eurasian Blackbird but differs in its vocalizations and behaviour. The Lesser Sand Plover no longer exists, but turns out to have been an amalgam of two similar-looking species that are not particularly closely related, having diverged around 2 million years ago. Even the familiar Eurasian Wren is likely to be 'split' into several species.
As a consequence of these 'splits', a recent estimate suggests that there may be around 18,000 bird species -- almost twice as many as was believed less than ten years ago. This is a valuable lesson in what Konrad Lorenz called scientific humility: all knowledge is provisional, and students of the natural world should not be too confident about what we think we 'know'.

Publicado el 17 de julio de 2024 por stephenmatthews stephenmatthews | 6 observaciones | 0 comentarios | Deja un comentario

11 de abril de 2024

Kingfishers at Po Toi

With its sheltered bays and creek, the island of Po Toi attracts various kingfishers. Two Common Kingfishers were present at the 'creek' today, even if it was reduced to more of a muddy puddle. The creek is one of the few places in Hong Kong where Ruddy Kingfishers are occasionally seen on passage.
A surprise was in store today for visiting UK birder Nick Brown, who had not even got off the ferry when he spotted a kingfisher on the rocks which did not look at all like the British one. He soon identified it as a Collared Kingfisher, causing quite a stir among local birders who were able to view it from the ferry pier and even from the departing ferry.
The Collared Kingfisher is a common species in its core range, as in Malaysia and Singapore, but a rare vagrant in Hong Kong: David Diskin's authoritative database lists only five previous records between 1990 and 2019. It also occurs as a vagrant in Taiwan and the Ryukyus, presumably arriving from the Philippines.

Publicado el 11 de abril de 2024 por stephenmatthews stephenmatthews | 2 observaciones | 0 comentarios | Deja un comentario

12 de febrero de 2024

Endemic birds thriving in Otago

From previous visits to Aotearoa New Zealand, I was expecting endemic land birds to be few and far between, with introduced British and Australian species dominating the landscape. So it was a pleasant surprise to find the tables turned in favour of the natives in the Otago Lakes District. The first bird I encountered on venturing outside was a Tui, perched at the top of a conifer. Then on entering a patch of manuka/kanuka scrub, an unfamiliar bird popped up. It proved to be a South Island Tomtit, actually one of the Australasian robins belonging to the genus petroica. The orange chest markings indicated a male, springing into action to investigate or challenge the intruder.
After sitting down to investigate some bird calls emanating from the scrub, I was soon surrounded by curious passerines investigating the intrusion into their territory. First to arrive were South Island Fantails, displaying at point-blank range by fanning their tails. They were soon joined by a chattering group of Pipipi or Brown Creeper, a relative of the endemic Whitehead and Yellowhead which inhabits scrub across the South Island. Also in the scrub were a Grey Gerygone, the most widespread of the endemic land birds, and a Silvereye, a species of whiteye from Australia, known to the Māori as tauhou 'the stranger'. A Swamp Harrier, now the commonest raptor in New Zealand, was hunting over the pine forest. This is also an immigrant from Australia, having established itself only after the extinction of the endemic Eyles's Harrier.
The resilience of the native passerines owes much to the use of trapping to control introduced predators such as stoats and rats. This is a step towards the goal of making the islands predator-free by 2050.

Publicado el 12 de febrero de 2024 por stephenmatthews stephenmatthews | 8 observaciones | 0 comentarios | Deja un comentario

13 de enero de 2024

An uncommon gull at San Tin

Large flocks of Black-headed Gulls winter in Deep Bay and forage around the fishponds of San Tin, occasionally accompanied by other gulls. In January 2024 they have been joined by an immature Common Gull, which in Hong Kong is enough of a rarity to attract a crowd of birdwatchers and photographers. The main subspecies breeding in northeast Asia, known as the Kamchatka Gull, is rather larger that the European Common Gull. A single Whiskered Tern, the only tern species to be seen locally in winter, was fishing over the fishponds, along with a Pied Kingfisher. Also present were wintering ducks including Pintail, Shoveler and Garganey, Common Snipe, and Green, Wood and Common Sandpipers.
The winter of 2023-24 may be the last at San Tin as we (and the wintering birds) know it: a substantial chunk of the wetland area is to be the site of a San Tin Technopole, part of the Northern Metropolis development. Substantial loss of habitat and disturbance due to construction are to be expected, though a wetland park is included in the proposal. The gulls, at least, should be sufficiently adaptable to continue using the site.

Publicado el 13 de enero de 2024 por stephenmatthews stephenmatthews | 8 observaciones | 0 comentarios | Deja un comentario

04 de enero de 2024

The curious tailorbirds

Tailorbirds are named for their nests which are built inside a cradle 'stitched' together from large leaves. Hong Kong has two "tailorbirds", though phylogenetic research reveals that they are not closely related.
The Common Tailorbird is a resident species, common in urban as well as country parks. It will often "scold" intruders with its insistent call, which has the effect of warning all the other wildlife in the area.  Our other tailorbird is the Mountain Tailorbird, one of several forest species which have recolonized Hong Kong as the forest cover has increased and matured. According to Birds of the World, it favours "bushy thickets, bamboos and hanging tangles within broadleaf evergreen forest, especially along watercourses." This is exactly where it can be found wintering along streams on the campus of the Chinese University campus, as well as at Lung Fu Shan. As its name suggests, it prefers mountain habitats, although these sites are only 100-200 meters above sea level.
As my "record shots" attest, this is quite a secretive species, typically located by its distinctive piercing whistle or chattering call. However, both tailorbirds are curious creatures which will often approach an observer, offering a brief opportunity for a photo. After satisfying their curiosity they will disappear into the forest or return to their foraging.

Publicado el 04 de enero de 2024 por stephenmatthews stephenmatthews | 2 observaciones | 0 comentarios | Deja un comentario

16 de septiembre de 2023

Return of the 'Swintail' snipe

Following record September rainfall in Hong Kong, sports fields have been flooded, forming ideal habitat for passing snipe, and just at the right time of year too: groups of snipe pass through on migration in April and September, having been observed at Kowloontsai Park on April 24, 2020, September 6, 2020, and from (at least) September 8 to 10, 2023. On these occasions there were respectively 2, 4 and 6 birds present. Since snipe are known for their 'site fidelity', revisiting the same spots each year, it seems likely that the same family group is involved.
Now for the difficult part: which species of snipe are these? Local expert John Allcock has identified them as either Pin-tailed or Swinhoe's Snipe, both of which have shorter bills and subtly different markings than Common Snipe. But these two species are so similar that separating them requires either sound recordings or photos showing a spread tail in exquisite detail, failing which, they are known as 'Swintail' snipe. Since photos from Kowloontsai to date do not suffice to distinguish the two species, the puzzle looks set to remain for another year.

Publicado el 16 de septiembre de 2023 por stephenmatthews stephenmatthews | 1 observación | 0 comentarios | Deja un comentario

16 de julio de 2023

In Aoteroa: half a native avifauna

Aotearoa ("Land of the long white cloud") is the traditional Maori name for the North Island of what is now Aotearoa New Zealand. For the naturalist, it is a fascinating but frustrating place. Like Hawaii, it is fascinating because the isolation of the islands has given rise to a wealth of endemic species; and frustrating because so many of these are reduced to a marginal existence. Also like Hawaii, Aotearoa has gone through two waves of extinction: one following Polynesian settlement of the islands, and another following European colonization. The extinctions brought about by these two waves amount to around half the native land birds, from the flightless moas and the eagles which preyed upon them to the iconic huia and piopio. The ecological gaps have been filled by introduced species. Especially bizarre is the suite of British birds, half a world away from home -- Eurasian Starling and Blackbird, Song Thrush, Chaffinch, Goldfinch and Yellowhammer -- juxtaposed with Australian species like Eastern Rosella, Common Myna and Australian Magpie.
Of the surviving endemic species, a few are widely distributed around the main islands. These include the Grey Gerygone, New Zealand Fantail, Tui, New Zealand Scaup and Red-breasted (New Zealand) Dotterel. The New Zealand Pigeon and Bellbird have a patchy distribution on the mainland. The remaining endemic land birds have been extirpated from most of the mainland, surviving only in sanctuaries -- offshore islands like Tiritiri Matangi, or enclosed areas of the mainland from which predatory mammals have been eradicated, like Shakespear Regional Park. In creating these sanctuaries Aotearoa has acted more swiftly and decisively than Hawaii. The sanctuaries have proved so successful that surplus birds can be translocated to new sanctuaries. The success has prompted a bold "moonshot" project: to eradicate predators from the whole of the country by 2050. Local efforts towards this goal are already bearing fruit, with native birdsong returning to more and more areas of the country.
Aotearoa has another claim to ornithological fame -- as the seabird capital of the world. Thousands of shearwaters congregate in the fish-rich waters of the Hauraki Gulf north of Auckland, and can be viewed from ferries or from the shore, including Auckland's North Shore and the Whangaparaoa peninsula. Although few breeding colonies of shearwaters and petrels remain on the main North and South islands, offshore island colonies have benefited from eradication of rats and other predators.

Publicado el 16 de julio de 2023 por stephenmatthews stephenmatthews | 28 observaciones | 0 comentarios | Deja un comentario

07 de junio de 2023

The Pheasant-tailed Jacana

With their long feet and toes, Jacanas are built to walk on floating vegetation and have quite specific habitat requirements, ideally ponds with water lilies or similar plants on which to walk. Formerly there were many such ponds in the New Territories and Pheasant-tailed Jacanas (水雉 'water pheasant', Hydrophasianus chirurgus) bred around Mai Po until the 1970s. Since then, Jacanas have occurred mostly on passage, typically stopping over in wetlands such as Mai Po and Long Valley in May.
In order to tempt the birds back to breed, an ideal Jacana habitat has been created at the Lok Ma Chau wetlands. Visible from the Lok Ma Chau MTR platform, the lily pond is part of a mitigation project to compensate for the loss of wetland to make room for the new rail connection. But the habitat is small, and the Jacanas first have to find it. This year a bird has instead taken up residence in the unlikely setting of the Cyberport on Hong Kong Island, which has an artificial lake at its centre. The two levels of the lake are separated by a concrete structure, effectively forming a catwalk on which the Jacana has been strutting, to the delight of the assembled photographers.
Let us hope the Jacanas eventually return to breed in more suitable habitat. Like Greater Painted-snipes, female Pheasant-tailed Jacanas are polyandrous, keeping a harem of males to whom they delegate the childcare.

Publicado el 07 de junio de 2023 por stephenmatthews stephenmatthews | 2 observaciones | 0 comentarios | Deja un comentario