| Chatham Islands shag | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Suliformes |
| Family: | Phalacrocoracidae |
| Genus: | Leucocarbo |
| Species: | L. onslowi |
| Binomial name | |
| Leucocarbo onslowi (Forbes, 1893) | |
| Synonyms | |
Phalacrocorax onslowi | |
TheChatham Islands shag (Leucocarbo onslowi), also known as theChatham shag, is a species ofbird in the cormorant and shag family,Phalacrocoracidae. It isendemic to theChatham Islands of New Zealand. For a long time the species was placed in the genusPhalacrocorax; today it is mostly placed with the other blue-eyed shags of New Zealand and Antarctica in the genusLeucocarbo. Its closest relative is theOtago shag ofSouth Island.
This is a large, pied shag with a long crest. It is highly coastal, rarely foraging far from shore, although sometimes feeds in the large andbrackishTe Whanga Lagoon. It forages for fish and cephalopods and roosts on rocky shores. The species iscolonial, breeding in small colonies around the Chatham Islands. Little is known about its breeding behaviour. The species is threatened byintroduced predators, disturbance at its breeding colonies and habitat loss.
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| Phylogeny of the Blue-eyed shags of North, South and the Chatham Islands, showing the position of the Chatham Islands shag, based on Rawlenceet al, 2017 |
Henry Forbes described the Chatham Islands shag asPhalacrocorax onslowi in 1893, although it had been mentioned at theRoyal Society Te Apārangi in 1871 byWilliam Travers (New Zealand politician), sharing field notes from his sonHenry Travers (naturalist).[2] The specific name commemoratesWilliam Onslow, 4th Earl of Onslow, who was theGovernor of New Zealand between 1889 and 1892.[3]
The question of which genus to place the species in has, like the general question of the taxonomy of the cormorants and shags on the whole, been a long-standing mystery. It was long retained inPhalacrocorax along with the rest of the family when the family was treated as monogeneric. It was also placed in the genusEuleucocarbo along with the other "blue-eyed shags" found in New Zealand. The species has sometimes been considered a subspecies of an enlargedNew Zealand king shag species (Leucocarbo carunculatus).[4] Most recently a 2014 study of themitochondrial andnuclear DNA of the family placed it within the New Zealand blue-eyed shags and the other Antarctic blue-eyed shags in the genusLeucocarbo.[5] This arrangement has also been adopted by theInternational Ornithologists' Union.[6] The same study found that its closest relative was the species formerly known as Stewart shag.[5] Since 2016, the Stewart shag is now treated as two species, theOtago andFoveaux shags. The same study that split these two species also found that the Chatham Islands shag is closely related to the Otago shag and that this pair are in turn a sister clade to the Foveaux shag.[7] These three species are in turn related to the New Zealand king shag and anextinctLeucocarbo species, the Kohatu shag, from the far north of New Zealand.[8]
"Chatham Islands shag" has been designated as the official common name for the species by the IOC.[6] The species is also known as the "Chatham Island shag".[9]
An adult Chatham Islands shag is 63 cm (25 in) in length and weighs around 1,790–2,400 g (63–85 oz).[4] It is a large pied shag with a white throat, breast, belly and patches on thescapulars of the wings, and black or black-brown elsewhere. When breeding the head and hind-neck have a blue metallic sheen, the wings andmantle have a greenish sheen and the rest of the black areas have a purplish-blue sheen. The head has a prominent crest and there are whitefiloplumes on the neck. Adults have a redgular sac and orange-red caruncles on the face and the gape of the mouth. In spite of the common name of the genus the iris of the eye is brown, and the eye has a violet eye-ring. The bill is dark grey, with a bright red mouth, and the legs and feet are dull pink. Non-breeding birds look similar to breeding birds but are duller, have no crest or filoplumes, and duller bare parts. The caruncles are yellow, the eye-ring blue, the mouth orange and the bill is light-grey.[10]
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The species isendemic to theChatham Islands, an archipelago around 700 km (430 mi) to the east of New Zealand. In the Chathams it is an inshore species, rarely going more than a few kilometres away from shore, and it is more commonly found in sheltered water such as bays and inlets.[4] It also forages in the largebrackishTe Whanga Lagoon.[9] It breeds incolonies on coastal areas and around Te Whanga Lagoon, either on flat areas, slopes or wide cliff ledges. There are currently thirteen colonies of this species, found on the mainChatham Island as well asPitt Island,Star Keys,Rabbit Island and North East Reef. These colonies are located close to the high-tide mark and can have as many as 300 nests in them.[11] It roosts on rocks close to shore or at sea,[4] although it may have different roosting sites for day and night. The species is non-migratory and has never been recorded away from the Chatham Islands,[10] although the species does feed and roost on islands in the Chathams away from their breeding colonies, for exampleMangere.[12]
Like all cormorants, the Chatham Islands shag is apursuit-diver, propelling itself underwater with its feet and returning to the surface to consume small prey caught in its bill.[13] The specific diet of the Chatham Islands shag is poorly known,[4] but is dominated by fish, includingopalfish, flatfish andbullies, as well as cephalopods such as octopuses, squid and cuttlefish. It generally feeds alone,[9] but will form small feeding flocks at times.[4]
Little is known about the breeding biology. Males attract a mate with an advertising display in which it stands with its tail cocked up and throws its neck and head back towards its tail, up to 13 times, while making a gargling noise. Mated pairs recognise each other at the nest with gaping and head-lowering displays. During these the male makes a barking noise, but the female is almost silent, making only a soft puffing noise.[10]
Breeding is seasonal, and the timing of laying can vary from colony to colony. Most Chatham Islands shags lay between October and December, but the colonies on Te Whanga Lagoon breed three months earlier than other colonies.[9] The nest is made oficeplant, grasses and other plants. Small nesting territories are defended from others of the species, and birds nesting in the centre of the colony have to run the gauntlet of biting to reach their nests as they pass other territories.[10] Two to four eggs are laid;[9] the eggs measure 59 to 63 mm (2.3–2.5 in) × 37 to 40 mm (1.5–1.6 in) and are matt pale blue with a chalky covering.[10] The incubation time is not known but is thought to be around 30 days.[9]
The Chatham Islands shag is threatened by a range of human activities and threats. The species is sensitive to disturbance at its breeding colonies, as nests are trampled by stampeding adults and nests are opportunistically raided bysilver gulls after the eggs are unattended. At worst, entire colonies have been abandoned due to disturbance by people or stock. Colonies on the main islands are also vulnerable tointroduced predators such asweka,feral cats andpossums. It is consideredcritically endangered by theIUCN.[1]
