Falklands Conservation

The Twins.

The Twins, located about 3km west of the north west point of Carcass Island, have been a Falklands Conservation Nature Reserve since 1984 when they were leased from the Royal Society for Nature Conservation. The freehold was acquired in 1993. They comprise two islands: South Twin and North Twin and are legally designated as a Wild Animal and Bird Sanctuary.

The Twins are good examples of low lying, tussac-covered islands that have not been grazed by herbivores or invaded by alien predatory mammals. Their status as Nature Reserves owned by Falklands Conservation will ensure that they remain in their present natural condition.

South Twin
South Twin is about lkm long and about 250m across at its widest point. Around the northern point a wide fringe of shelf rock is exposed at low tide. The Island is generally very low lying, the highest point being only 8m (25 feet) above sea level. There are sand dunes behind a noticeable white sand beach on the northeastern side and some low rocky points. A large accumulation of dead, rotting kelp was found to the north of the southeastern point along a narrow boulder beach.

North Twin
North Twin (8ha) is about 600m long and generally less than 100m in width. There is a wide rocky reef at the southwestern point which is exposed at low tide. A narrow reef extends along the western coast with a boulder beach on which dead kelp and a few bleached, drifted tree trunks have become stranded. The Island barely reaches 5m (about 16 feet) at a few small rocky points.

Plants
There are extensive beds of giant kelp in the surrounding waters, particularly to the east and north. Tussac Grass is dominant on both Islands, covering most of the ground down to high water mark and clothing the tops of even narrow rocky points. On South Twin, much of the Tussac is dense, up to 2.5m tall and the bogs are surrounded by deep Tussac leaf litter.

There was little variety in the plants, only seven species being identified. Four of these are native species commonly found on small outer islands: Tussac Grass, the large sedge known as Swordgrass, Wild Celery and Antarctic Starwort. The Swordgrass occurred in small patches at damp places on the outer fringe of the dense Tussac but seemed to be shorter than plants seen elsewhere. The Celery plants were more widely spread about the Tussac fringe and were particularly noticeable among the low sand dunes. Only one small patch of Starwort was found.

Marram Grass was noticed among the sand dunes on the eastern coast. This plant was introduced to the Falklands in about 1924 to stabilise drifting sand, then threatening to engulf the lighthouse on Cape Pembroke near Stanley (Hubbard 1937). Marram has since been recorded on other coastal dunes and may have reached The Twins from West Point Island to the SSW.

Groundsel seedlings were found in flattened disturbed sand and peat near entrances to Magellanic Penguin burrows. A few small plants of Lesser Swine-cress were found in a crack on a low coastal crag. This peppery-tasting member of the cabbage family is a native of temperate South America that has become widely naturalised in Europe. On North Twin, the only plant species found was Tussac Grass. It was healthy but tended to be shorter and more open than on South Twin, probably because Sea Lions hauled out and rested on the Island.

Birds
A total of 24 species was recorded, 20 on South Twin and 19 on North. Both Islands appeared to have good numbers of Tussacbirds and Cobb's Wrens around all the coasts, suggesting the absence of rats and mice. The Grass Wren and Falkland Thrush were also recorded on both Islands. A pair of Black-throated Finches was seen on South Twin and a pair of Black-chinned Siskins with a fledged juvenile, illustrating the ability of this usually woodland species of southern South America to utilise the Falklands substitute, mature Tussac Grass. Also on South Twin, over 50 Southern Giant Petrels were resting on a sand beach, perhaps prospecting for nesting sites.

On North Twin, the remains of two other petrels were found; the tail of a Grey backed Storm-Petrel and wings of Diving Petrels. These may have been discarded by feeding Johnny Rooks but elsewhere, both these small petrels are taken at night by Short-eared Owls. The presence of fresh remains suggests quite strongly that both petrels are nesting in the underlying peat and fibrous parts of mature Tussac.

Magellanic Penguin burrows were found on both Islands, though few adults were seen. There were pairs of Crested Ducks and Flightless Steamer Ducks in the coastal waters and many pairs of Kelp Geese on the shores. About ten Snowy Sheathbills were seen near Sea Lions on the rocks of both Islands.

Falklands Flightless Steamer Ducks (Photo: R. White)

Several adult Black-crowned Night Herons were seen in rock pools and at least 16 immature birds were gathered around a large accumulation of drifted kelp on the eastern side of South Twin. A noisy pair of Crested Caracaras was seen there, probably with a hidden nest. A few Dolphin Gulls and more Kelp Gulls were seen on the coasts of both Islands and it is possible that there is a small colony on South Twin.

On both Islands we found Johnny Rook (Striated Caracara) nests with clutches of three eggs but surprisingly each nest appeared to have three fully adult birds in attendance. On North Twin a third adult arrived and landed by one pair standing near the nest while the mate was incubating. In both cases, the three birds showed no sign of antagonism towards each other, in complete contrast to the behavior of a pair when an 'immature' bird approaches close to an occupied nest. The adult usually attacks the immature and chases it far away. The breeding behavior of these most unusual birds of prey deserves much closer attention. There were at least two, possibly three, pairs of Johnny Rooks breeding on each Island.

Mammals
On South Twin, one bull and three cow Sea Lions were found in a small bay while on North Twin a fortnight later, two bulls and at least 13 immatures were seen. More animals were almost certainly present, but remained hidden in the Tussac. A pair of Elephant Seals was found on the shore of North Twin but no pups were seen.

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