OBSERVATIONS island survivor Text & photographs peter ryan

The Tristan Thrush or Starchy Turdus eremita is the only land bird that survived human colonisation of the main island of Tristan da Cunha. As a result, it is the only land bird at the Tristan archipelago that is not listed as threatened. Peter Ryan considers the traits that allowed this supreme generalist to persist where other land birds went extinct.

ristan da Cunha, home to the islands: Blue Petrel Halobaena caerulea, same time. However, early visitors to world’s most isolated human MacGillivray’s Prion Pachyptila macgil- Tristan described the islands as teem- community, lies almost half livrayi and Grey-backed Storm Petrel ing with wildlife and by 1790 teams of wayT between Cape Town and Buenos Garrodia nereis. sealers spent up to nine months ashore, Aires. Access is only by ship and typi- The Tristan archipelago was discov- killing seals and penguins for oil and cally takes five to seven days from the ered by the Portuguese in 1506, but few skins, while whalers operated offshore. Cape. Although often referred to as the early mariners landed on the islands Goats and pigs were released on Tristan ‘lonely island’, Tristan is an archipelago due to the lack of safe anchorages. This and Inaccessible to provide food in of three islands. The main island of spared the islands from the wave of ex- the event of a shipwreck, and house Tristan, where some 250 people live, tinctions that occurred at islands such mice were introduced accidentally at is an active volcano, rising more than as Mauritius and St Helena shortly af- Tristan’s main island and Gough Island 2000 metres above the Atlantic Ocean. ter they were discovered at about the during this era. > It last erupted in 1961, resulting in the entire community being exiled to the UK for two years. Tristan lies 30 kilometres north of two smaller, older volcanic islands, Nightin- gale and Inaccessible, which are unin- habited. Gough Island, 380 kilometres farther south, is the only other island in the vicinity and it is sometimes included in the Tristan group, although it is dis- tinctly cooler. Gough shares no land birds with Tristan and is home to three seabirds not found at the more northern right Nightingale Island is the oldest island in the archipelago. It no longer resembles a volcanic peak, unlike the much younger main island of Tristan, visible in the distance. opposite The Tristan Thrush was placed in its own genus, Nesocichla, but genetic evidence shows it evolved fairly recently from South American Turdus ancestors.

july/august 2019 tristan thrush 51 recognised as a distinct subspecies. Compared to the thrushes on the two uninhabited islands, they are unusual in not foraging along the shoreline, possibly because of the large numbers of rats in this habitat. Even today, after more than 40 years of protection, most are confined to elevations above 300 metres, although a few pairs have colo- nised the gullies behind the settlement plain and in recent years the occasional bird has even visited the settlement.

o really appreciate Starchies, you need to visit Nightingale or Inaccessible island. Shortly after youT step ashore, a thrush will arrive to announce your presence with a queru- lous ‘tseeep’ call. This serves to gather the troops and within minutes you will be surrounded by a beady-eyed posse, eager to test whether you or any of your possessions are edible. Rounded objects are particularly attractive – per- haps because they resemble birds’ eggs. inadvertently disturbed a Common Starchies can break open eggs as large as some 2400 kilometres to the north. Such familiarity did not always end Pots, cans and mist-net poles receive Noddy Anous stolidus from its nest on those of Atlantic Yellow-nosed Albatrosses The settlers converted the few coastal well for the Starchies. When Henry vicious pecks, time and time again, and the ground under an apple copse (the (above), and lap up the contents with their lowlands into pastures and croplands Moseley, ornithologist on the Chal- if you go barefoot around camp you first such nest I have seen on the island specially adapted tongues. and introduced a host of animals, in- lenger expedition, visited Inaccessible can expect to receive a similar attack – they usually nest in trees or on cliff cluding cattle, sheep, dogs, cats and Island in 1873, he reported how the on your toes. ledges). Before I could react, a thrush south poultry. The endemic flightless Tristan Starchies were easily killed by hitting Like other thrushes, Starchies are darted in and broke open the egg with africa Moorhen Gallinula nesiotis and the them with a stick, despite the fact that omnivores, eating fruits of the berry a single blow. Cape Town local population of Tristan Bunting the Stoltenhoff brothers, two Germans bush Empetrum rubrum and fowl berry The thrushes are also not averse to Inaccessible Island Nesospiza acunhae probably went ex- marooned on the island, had killed Nertera species, as well as insects and venturing into burrows in search of Tristan da Cunha tinct before black rats reached Tristan them for food in this way for two years. other invertebrates such as introduced food. Surface-nesting albatrosses and Nightingale Island from a wreck in 1882. Quite what He did note, however, that the Starchies earthworms. They use their strong legs penguins incubate continuously and Gough Island SOUTH ATLANTIC OCEAN caused these birds’ demise remains un- were ‘not quite so tame in Tristan Is- to pull apart vegetation and flick over then brood their chicks for a few weeks clear, although hunting, predation by land’, presumably because they had clods of earth with their bills, even until they are large enough to look after introduced animals and even poultry been subjected to similar hunting for picking up larger items to expose any themselves. By comparison, burrowing After dragging a petrel from its burrow diseases may all have played a role. more than 50 years. prey hidden underneath. But they are petrels sometimes abandon their eggs either through the burrow entrance or Hunting might seem an unlike- Indeed, the thrush was thought to best known for their predatory habits for a few hours and usually leave their an excavated hole, the thrush uses its Starchies only lay two eggs on Tristan and ly threat, but as is the case on many be extinct on Tristan when the Quest on bird eggs and chicks. chicks alone within days of hatching superior weight to hold it down and Inaccessible, but half of the clutches on oceanic islands, the birds on Tristan expedition visited in 1922, and the Their tongues are fringed to aid lap- because their nests are sheltered from then pecks at the poor petrel’s head Nightingale contain three eggs (above). have little fear of people and the Tristan Reverend Rogers, who arranged the ping up egg contents and they are able predators and extremes of wind, rain until the skin is torn away. It then eats Breeding is well synchronised, with the first Thrush is the most bold in this regard. collection of the first two skins of the to break into the eggs of virtually all the and heat. However, if a Starchy can the eyes and works its way down the juveniles appearing in November (top). The origin of Starchy, the Tristan name Inaccessible Island Rail Atlantisia rog- bird species that breed on the islands. reach the nest chamber, the petrel eggs neck to the breast. This leaves a carcass for the thrush, is not known, but it suits ersi also failed to record it in the 1920s. Piercing the egg of an albatross might and small chicks are at risk of being that is quite distinct from those killed Tristan was first settled in 1811, but its cocky nature. Yngvar Hagen, orni- However, Hagen found Starchies in take the attention of several Starchies eaten. I’ve seen Starchies killing pet- by Brown Skuas Catharacta antarctica, the current community dates back to thologist on the Norwegian Scientific small numbers on the island’s coastal for a few minutes, but the eggs of small- rel chicks weighing 75 grams, close to which usually swallow storm petrels 1816, when the British sent a garrison Expedition to Tristan in 1937/38, aptly slopes and higher plateau or ‘base’ er birds are dispatched in seconds. This their own weight. whole. Oddly, this behaviour has only to Tristan to prevent the French from described the birds’ behaviour as ‘enor- in 1937. The thrushes on Tristan are makes it hazardous to disturb incu- Even adult storm petrels are at risk. been observed on the eastern plateau using the islands as a base to liberate mously inquisitive’ and noted that ‘they smaller and paler than the birds on bating birds from their eggs. During I first recorded Starchies killing storm of Inaccessible Island and prior to Napoleon from exile on St Helena, immediately fly to meet every new thing’. Nightingale and Inaccessible and are a recent visit to Inaccessible Island, I petrels on Inaccessible Island in 1989. 2004 only Fregetta storm petrels were >

52 african birdlife july/august 2019 tristan thrush 53 In Tristan’s depauperate land bird community, the thrush occupies a range of niches that on the main- land would be filled by a host of different species. Its ability to innovate allows it to exploit a wide range of feeding oppor- tunities, which doubtless helped it to persist on the main island

petrel is incapacitated. The struggle soon attracts a crowd of thrush on- lookers, triggering a series of confron- tations. Once again, Hagen relates this perfectly when describing Starchies competing for the bodies of birds he skinned: ‘they fought each other with all the signs of jealousy, the strongest indefatigably driving off all competi- tors, satisfied with having no less than cliff dorse a whole penguin or albatross-carcass they glean from the low shore or from quite alone’. During the ensuing mêlée, stranded kelp on the high shore. They some petrels escape, fluttering away to also scavenge goose barnacles from hopefully elude any patrolling skuas debris that washes ashore and one was and reach the safety of the sea. seen to eat a number of by-the-wind The Starchies’ inquisitive nature con- sailors Velella velella, in what is seem- founds estimates of their population ingly only the second record of a pas- size, because it’s hard to know the ra- serine eating a jellyfish. dius over which birds are attracted to Passerines seldom feed in the in- investigate people. To overcome this tertidal zone because they lack salt challenge, in 2018 Ben Dilley and I glands, the specialised supra-orbital individually colour-banded Starchies glands that most birds use to offload along the west coast of Inaccessible Is- excess sodium and chloride ions. As a land. We concentrated our effort at a result, Starchies cannot readily excrete small bay, which we visited regularly the high salt concentrations found in to re-sight marked birds. This showed marine invertebrates. At Inaccessi- that while some birds were largely resi- ble Island, they obviate this problem dent, others came and went, and a sur- by regularly drinking from the many prisingly large proportion were seen freshwater springs that emerge near only once. This suggests that the popu- the coast, but this is not an option at cliff dorse lation on Inaccessible Island is appreci- Nightingale Island, where there is little, would be filled by a host of different above One of our colour-ringed thrushes above and top Starchies lack the tools to killed. Since then, however, White- ably larger than the current estimate of if any, fresh water. It would be interest- species. Its ability to innovate allows it eating a large isopod on the shore at kill storm petrels quickly and they resort to faced Storm Petrels Pelagodroma mari- 850 pairs. ing to compare the renal structure of to exploit a wide range of feeding op- Inaccessible Island. repeatedly pecking the petrels' heads with na have been killed and in recent years The study also revealed that some Starchies feeding in different habitats portunities, which doubtless helped it their powerful bills. Some storm petrels Starchy-killed storm petrels have also thrushes – probably mostly immature on the two islands. to persist on the main island. However, top Starchies are always on the lookout for escape if their captor is distracted by other been found on Nightingale Island. birds – specialise in feeding along the In Tristan’s depauperate land bird I can’t help but feel that its pugnacious, food. The main challenge at a campsite is thrushes trying to steal their prey, but most It is a gruesome way to die and it shoreline. They eat mainly amphipods community, the thrush occupies a never-say-die attitude to life also played to keep them off your food before you have eventually succumb. takes several minutes before a storm and isopods, small crustaceans that range of niches that on the mainland a key role in its survival.  chance to eat it.

54 african birdlife july/august 2019 tristan thrush 55