I’ve spent a good chunk of the last 10 years working on seabird islands. During the breeding season, birds look for the safest spot to raise their young, and this often means they find themselves on coastal cliffs, offshore islands, and remote parts of the world. This natural order of things is thrown on its head when novel predators like mice, rats, or cats are introduced. The birds, having evolved over millions of years in the absence of ground-based predators on breeding islands, are prime targets because they have no behavioural defences. In some cases, like I wrote in my last installment, the birds are, in a word, screwed.
The main island of Tristan da Cunha had rats and mice introduced in the 1880s, about 75 years after the island was settled permanently, and had cats from the early 1800s until about 40 years ago. Today, Tristan is an island without birdsong.
When the UK established a garrison on Tristan in 1816, Captain Dugald Carmichael, Fellow of the Linnean Society, accompanied the voyage, and recounted his journey on December 16, 1817, recorded in the Transactions of the Linnean Society as “Some Account of the Island of Tristan cla Cunha and of its Natural Productions”. It offers us a unique perspective on what this island was like almost 200 years ago before rats, mice, cats (now absent), dogs, livestock, and people arrived on the most remote inhabited island in the world.
“Further on, the ground becomes more firm, but is perforated in all directions by the various species of Petrel, which resort in myriads to the island during the season of incubation, and burrow in the earth”
Here, Carmichael is referring to the habit of many seabirds to nest in burrows underground. These are chiefly petrels and shearwaters, which abound on other islands in the Tristan group, like Nightingale, Inaccessible, and Gough. But today, there are scant few burrowing petrels, and those that remain are on steep cliffs, offshore sea stacks, or suffer low reproductive success and survival thanks to the rats and mice.
“As we walked down the mountain on our return, we passed among flocks of albatrosses engaged in the process of incubation, or tending their young. There are four species of them that breed on the island, none of which hatches m0re than one egg at a time ; the Diomedea spadicea, exulans, chlororynchos, and fuliginosa”
In modern parlance (and with the benefit of a better understanding of albatross plumage changes and taxonomy), these are three species: D. exulans and D. spadicea are Tristan Albatross (D. dabbenena, male and female respectively), D. chlororhynchos is the Atlantic Yellow-nosed Albatross (Thalassarche chlororhynchos) that I’ve written about before, and the last is now Phoebetria fusca, the Sooty Albatross. There are few places now where the albatross could be said to form flocks, and Tristan Albatross has been extirpated from the island entirely. Sooty Albatross breed on steep cliffs, and the density of Atlantic Yellow-nosed Albatross nests is surely lower than that even 40 years ago.
Just last week, we went with staff from the Tristan Conservation Department to the Atlantic Yellow-nosed Albatross study colony at the top of Hottentot Gulch, about 800m above the settlement. After a 3-hour climb, we found ourselves at the edge of this “colony”. About 10 person-hours later, we had found a grand total of 30 nests, and covered an area I would estimate to be around 30 hectares. Clearly many fewer than in Carmichael’s time.
This is to say nothing of the landbirds. There is one native landbird remaining, the Tristan Thrush (Nesocichla eremita). A finch (or more properly, a tanager), and moorhen (Gallinula nesiotis) are gone, and the Gough Moorhen (Gallinula cormeri) was introduced in the 1950s, but exists mainly above the settlement plain. The result is that around town, the sounds of birds singing, calling, courting, and otherwise announcing their presence is absent.
So how did this come to be?
Like many other remote islands, there is a history of harvesting birds on Tristan, though today this is no longer permitted on the main island (there is a small subsistence harvest of shearwater chicks on Nightingale). Cats on South Africa’s Marion Island consumed more than 400,000 birds a year when they were present (from the early 1950s until officially eradicated in the early 1990s); cats were present on Tristan for much longer (early 1800s through the 1970s), and likely exacted a massive toll on the breeding seabirds.
But disentangling the role harvesting played from the catastrophic effects of introduced predators is practically impossible. Regardless of their relative importance, the result is the same – when we’re out hiking and we see a slope, a hill, a cliff, a plain and think “there MUST be birds there!” we are nearly always wrong.
What we do know, though, is that the conservation ethic on the island is as high as it has ever been, and the folks we work with in the Conservation Department are dedicated, resourceful, and fantastic to work with. With about 3 weeks left for me on Tristan, I’m already looking forward to coming back.
The Lab and Field 