Fróðskaparrit - 01.01.2001, Side 130

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BREEDING DISTRIBUTION AND NUMBERS
OF WREN (TROGLODYTES TROGLODYTES) IN THE FAROEISLANDS
siskin (Carduelis spinus), but surprisingly
few pairs of breeding wrens. This contrasts
with the Icelandic wren (Troglodytes t. is-
landicus), another endemic, sedentary is-
land subspecies, which has attained high
densities of breeding pairs in several of the
relatively recent plantations of coniferous
and some decidious trees, e.g. in Skorra-
dalur and Hallormsstaðarskógur (pers.
obs.). Elsewhere in Iceland the wren breeds
in many of the once much more extensive
native woods and shrubs of birch (Betula
pubescens), but also on cliffs and gullies as
well as in lava fields. The Faroes has not
had any native trees or higher shrubs since
the wren colonised the islands (i.e. in post-
glacial time), which leads one to speculate
whether the lack of response to the planta-
tions might have a genetic component to it.
Genetic aspects have been invoked in dis-
cussions of the success of biological inva-
sions (Williamson, 1996).
All the small islands (< 11 km2), except
Koltur for some unknown reason, exhibit
relatively high breeding numbers of wrens
(Tab. 1). This negative correlation between
numbers and island size has previously, on
the basis of the survey data from 1981,
been associated with the favourable shore-
line length to area ratio, and as the wrens
disperse to, and defend winter-territories
along the shores, small islands can hold rel-
atively larger numbers. Furthermore, on a
small island the distance to the shore is in-
variably short and there are fewer inland,
mountainous areas unsuited for breeding
than on larger islands (Bengtson and Bloch,
1983). The present study suggests alterna-
tive, non-exclusive explanations to the
small-island effect. The smaller islands
usually have an abundance of cliffs, gjáir
and generally craggy terrain, and of breed-
ing seabirds (e.g. Mykines, Skúvoy, and
Fugloy). The puffin-wren association (see
e.g. Williamson and Boyd, 1963) is con-
spicuous also in the Faroes, notable exam-
ples are Lamba on Mykines, Urðin Mikla
on Fugloy, and Urðin on Nólsoy where
wren densities are particularly high; some-
times 5-10 singing males within a couple of
hundred meters. On Koltur seabirds are
abundant but wrens relatively few in num-
ber. Trøllhøvdi supports dense seabird
colonies and the wren breeds there, but in
unknown numbers (Bertelsen, 1965;
Bengtson and Bloch, 1983). Also, the hu-
man populations on these islands are small
and the single (two on Fugloy) settlement
consists of relatively few, often old-style,
sod-roofed houses and lots of stone walls.
Such settlements often hold several pairs of
wrens in contrast to the expanding commu-
nities of the larger islands, where modem
buildings and wire fences seem less attrac-
tive to the wrens. However, settlements of
the former kind also occur on the larger is-
lands (e.g. Gjógv, Elduvík, and Funningur
on Eysturoy, Bøur on Vágar, Skarvanes and
Húsavík on Sandoy, others on Kalsoy,
Kunoy etc.) and all these small settlements,
irrespective of island size, may or may not
have a good number of breeding wrens.
The presence of cats has for a long time
been considered a menace and threat to
wrens at the settlements (see Introduction),
for instance by Andersen (1898; 1905) with
regard to Nólsoy and Mykines and today in
Sumba on Suðuroy (Aksal Poulsen, pers.