Fróðskaparrit - 01.01.2001, Síða 131

ÚTBREIÐSLA OG TAL AV MÚSABRÓÐUR (TROGLODYTES) í FØROYUM
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comm.) and in several other places. On
Skúvoy, where Feilden (1872) at the end of
May 1872 inspected as many as 7 or 8 nests
with eggs or young at the small settlement,
the recent removal of cats may well be the
main cause for the increased birdlife
around the village (Fløjgaard, 1995). How-
ever, there are settlements where breeding
wrens are common despite many cats, for
instance at Kirkja on Fugloy and on Kunoy
(pers.obs.). In this context it can be noted
that rats (Rattus norvegicus) occur on most
of the larger island but are absent on the
smaller ones (Bloch, 1999). At present it is
not known whether the presence of rats has
any effects on wren densities. Furthermore,
some local informants claim that the num-
ber of breeding wrens in the settlements is
negatively correlated with the number of
house sparrows (Passer domesticus), a
species that has colonised effectively all
settlements during the past 50 years (Jensen
and Kampp, 1997). This hypothesis re-
mains to be tested.
The total Faroese breeding population of
wren, based on data collected throughout
the 1980s and 1990s, is estimated to be be-
tween about 600 and 850 pairs (Tab. 1).
This estimate is a conservative ”overall av-
erage”, and in view of the methodological
limitations (e.g. a difficult survey terrain in
combination with frequent bad weather),
that presently can not be corrected for, it
seems quite possible that the number of
pairs, especially in sea cliffs and remote
outfield areas could be at least a hundred
more. For the Faroes the estimate (Tab. 1)
equals about 4-6 pairs/10 km2. There are no
comparable estimates for neighbouring ar-
eas, but in Shetland (almost exactly of the
same size as the Faroes), where the wren
(Troglodytes t. zetlandicus) is common and
breeds in the same types of habitats, counts
of singing males on some of the smaller is-
lands and inland observations suggest that
the breeding density is about the same as in
the Faroes (Armstrong, 1952; Venables and
Venables, 1955). For instance, on Fair Isle
(8.5 km2) the local subspecies of wren
(Troglodytes t. fridariensis) was represent-
ed by 45-50 singing males (Williamson,
1965), which is about the same number as
on the slightly larger Mykines, perhaps the
best wren-island in the Faroes (Tab. 1).
Further south, on St. Kilda, the wren (Tro-
glodytes t, hirtensis) is abundant with about
230 pairs in the four islands (Hirta 638 ha,
Dun 32 ha, Soay 99 ha, and Borerey 77 ha),
and 116 on Hirta alone in 1957 (William-
son, 1958), and for Hirta about the same
numbers in 1960 and 1962 (Harris and
Murray, 1978). Thus, the wren population
on Hirta is four-fold that on Hestur (6.1
km2) and twice that of the slightly larger
Mykines. As to the wren in Iceland, Pe-
tersen (1998) has made an educated guess
at 500-2000 breeding pairs, corresponding
to 0.05-0.2 pairs/10 km2.
Wren populations in Scandinavia, on the
European continent, and in the British Isles
frequently exhibit marked fluctuations in
breeding numbers, especially in response
to winter weather conditions (e.g. Shar-
rock, 1976; Glutz et ai, 1985; Nilsson,
1986;Cramp, 1988; Marchant etal., 1990).
In Shetland fluctuations occur; for in-
stance, on Fair Isle in 1979 the number of
singing males, following the severe winter