Manx Shearwater

CHARACTERISTICS

Manx ShearwaterPhoto David Boyle 2006
The Manx Shearwater is a member of the Procellariiformes, the order of birds which includes the albatrosses, the Fulmar and the storm-petrels. It is beautifully adapted to leaving at sea, with long, narrow wings, and its feet placed far back on its body for efficient swimming. Unfortunately this makes life on land difficult; it cannot walk properly, but shuffles along on its belly, making it easy prey for predators such as gulls, hence the carcasses. In order to minimize this danger, Manx Shearwaters nest in burrows and only come to, or leave, the island in the dark, however, visitors to Skomer can see a bird, sometimes two, and in late summer a chick, in a burrow, pictures being provided on a monitor in the interpretative centre at the Farm from a nearby micro-camera. Manx Shearwaters are extremely noisy at their colonies after dark, the darker the night, the more tumultuous the sound, as birds fly in whilst others emerge from the burrows.

DISTRIBUTION

There are an estimated 120,000 breeding pairs on Skomer and a further 45,000 pairs on Skokholm, making the two islands the largest known concentration of this species in the world. Elsewhere the Manx Shearwater nests in a small number of island colonies from the Western Islands, Iceland, in the Faroes, in northern and western Britain and Ireland, Brittany, to the Azores, Madeira and the Canaries.

The Manx Shearwater is a recent colonist of islands in the western Atlantic. Ringing recoveries reveal that birds from Skokholm and Skomer colonised Middle Lawn Island off Newfoundland in the 1970’s, while the may now also nest on other small islands south to Rhode Island.

Although nocturnal at the colonies Manx Shearwaters can often be seen off the west side of Skomer during the day especially during inclement weather, while in the evening large numbers come into St. Bride’s Bay and Broad Sound and can be viewed from vantage points like Wooltack Point. An ideal way to see Manx Shearwaters at close quarters is by taking one of the mid-summer evening boat cruises – the Seabird Spectaculars- from Martins Haven. During this one can often come close to rafting birds, while others are flying past. Each evening, and again in early mornings, vast passages of birds moving between the islands and distant feeding grounds can be observed from Strumble Head and St. David’s Head, with smaller numbers off St. Govan’s Head.

BREEDING

Once they have reached breeding age, Manx Shearwaters return in March, when they clean out their burrows and find their mates, most choosing the same partner as the year before. After mating, the female leaves the island for about a fortnight to form the single, large egg, 15% of her body weight. She returns to the Bay of Biscay which she passed through on her northwards migration, back from South America, where she can feed on the plentiful stocks of sardines. The male, meanwhile, stays nearby, visiting the burrow each night, perhaps to ensure that no one else takes it over. When the female returns, usually by the middle of May she lays the egg and departs again to feed, leaving the male to take the first incubation stint. Thereafter the pair take it in turns to incubate the egg, each doing so from four to eight days at a time while the other goes off to feed.

Manx ShearwaterPhoto David Boyle 2006

Incubation stints get shorter towards hatching, which takes place after 51days. The chick remains in the burrow for a further 70 days during which time feeding visits become progressively more frequent until, when the chicks are large, the parent bird may bring food each night.

Towards the end of their time underground the chicks are deserted, most remaining on average a further eight days before leaving the island. At this stage they weigh about a third heavier than an adult, the large amount of stored fat being a necessity which helps them on their first long journey.

GREAT JOURNEYS

Manx Shearwater in flightPhoto Juan Brown 2006
Young Manx Shearwaters go to sea, at night, without their parents, and immediately head for the winter quarters off the coast of southern Brazil and Argentina. Ringing studies on Skokholm and Skomer show that some of the young make this 6000 – 7000 mile journey in less than a fortnight. Some Manx Shearwaters at the start of their journey are lured to the mainland, probably attracted by the bright lights, or are blown inland during stormy weather, to be reported in the main towns and villages of Pembrokeshire, and not infrequently further a field. Their speedy return to sea is essential if they are to begin their migration whilst still in good condition.

We do not know what the young birds do in the following summer – almost none of them land and there are roughly as many recoveries of ringed first-year birds in the western Atlantic as there are in European waters. The following year, when the young birds are two, some return to colonies, though only on a few dark nights around the new moons in mid-summer. Remarkably, they tend to make landfall close to the burrow in which they were born. Progressively more birds return at the ages of three and for, spending longer and longer visiting the island, searching for a mate and a burrow. Most Shearwaters do not breed until they are at least six years old.

AN UNUSUAL HAZARD

The most conspicuous cause of mortality is a disease called puffinosis which kills off many fledglings in some years. Apart from this, and a few breeding birds which fail to lay an egg, Manx Shearwaters have a high breeding success, about 90% of eggs hatch and about 90% of those that do so produce a fledgling which leaves the island; overall about 70 chicks fledge per 100 pairs. About one third of the fledged chicks survive from one year to the next. This means that, on average, each adult breeds for about 10 years. It must be stressed that this is an average figure; the oldest known Manx Shearwater (on the Nortrh Wales island ofo Bardsey) is more than 50 years old.

THE MANX PUFFIN

The scientific name of the Manx Shearwater Puffinus puffinus often causes surprise. In the Middle Ages the plump young Shearwaters, when taken for food, where known as ‘puffins’ or ‘puffings’ from their plump and fatty nature. In 1676 the bird was first described from specimens collected on the Calf of Man and named the ‘Manx Puffin’, Shearwater not appearing in the name until about a century later, by which time puffinus had been incorporated into the scientific name for all time.