Most of the estimated world population of c.340,000 – 410,000
pairs of Manx Shearwaters breed in Britain and Ireland. Of the
British and Irish population, 36% breed on Rum (Lochaber), and 45%
in Pembrokeshire on the three adjacent islands of Skomer, Skokholm
and Middleholm.
Manx Shearwaters spend most of the year at sea returning to
land only to breed. They nest in burrows and under boulders, and
come ashore only under the hours darkness in order to evade
predators such as Great Skuas and Great Black-backed Gulls. They
breed exclusively on islands, usually free of rats that depredate
eggs, chicks and adults. Manx Shearwaters were believed to have
been exterminated from their eponymous colony on the Calf of Man by
the introduction of rats from a wrecked ship in the late 18th
Century. More recently, in 2000, rats and cats were responsible for
the extirpation of Manx Shearwaters from Canna (Lochaber). Those
few colonies that occur on islands with rats are generally small
and limited in distribution. The exception is on Rum, where the
largest single colony in the world appears to have evaded rats, or
at least limited their recent impact, by nesting on the slopes of
the island's mountains at altitudes of more than 450m; their
nocturnal calls led Vikings visiting the area in the 11th century
to believe that trolls inhabited Rum's mountains.
Manx shearwaters' nocturnal and subterranean habits have
caused problems for surveyors in the past. Hence, Operation
Seafarer and the SCR Census, estimates of 175,000-300,000 pairs and
250,000-300,000 pairs respectively were based solely on order of
magnitude estimates and should not be compared to those of Seabird
2000 to calculate trends. The results of Seabird 2000, which used a
method called tape playback to survey apparently occupied sites
(AOS), represent the first accurate baseline estimate of the number
of Manx Shearwaters breeding in Britain and Ireland. The tape
playback method involves playing calls of Manx Shearwaters to
elicit a response from adults hidden in burrows during the day
whilst incubating. Unfortunately not all shearwaters present at a
colony will respond to the taped calls, thus counts of responses
will underestimate the number of AOSs and have to be adjusted by a
response rate measured at the colony. Some colonies were also
surveyed by counting burrow entrances that had visible signs of
use, though this method is difficult or impossible to use in
colonies that are shared with other burrow nesters, i.e. Rabbits
and Atlantic Puffins, or where burrow entrances are obscured, i.e.
under boulders or in thick vegetation. Evidence of possible or
probable breeding obtained during Operation Seafarer and the SCR
Census and from other sources was used as a basis for selection of
sites to survey during Seabird 2000.
The main gaps in coverage in Britain were in the northern
Isles, where only relict populations remain, Bearasay off Lewis
(Western Isles), Eigg and Muck in the Small Isles (Lochaber) and
the Sanda Islands off Kintyre (Argyll & Bute). However, the
combined population of all these islands is thought to be no more
than 1,000 AOS. In Ireland, those islands with probably the largest
populations were surveyed as first priority. Nine sites were
omitted due to time constraints, but on only five of these had
breeding been previously confirmed or strongly suspected with
previous population estimates of up to 100 AOS on four and up to
1000AOS on the fifth (Inishtearaght, Co. Kerry). However, there may
be other significant colonies yet to be found in Ireland - a report
from a member of the public in 2000 led to the discovery of a
colony of 3,000 AOS on Cruagh off Connemara (Co. Galway).
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