Fróðskaparrit - 01.01.2001, Síða 132

136
BREEDING DISTRIBUTION AND NUMBERS
OF WREN (TROGLODYTES TROGLODYTES) IN THE FAROEISLANDS
1978/79, dropped by about 75% from a pre-
viously rather stable level at about 50
males, but since 1981 (10 pairs) the number
has been increasing (Dymond, 1991). For
St. Kilda, where many census-takings have
been attempted over the years, it has been
claimed that the number of pairs in the
”Village area” remained stable over a long
run of years and that this reflected the situ-
ation in the rest of the population (Fisher,
1947; 1948). This was later refuted by
Williamson (1958), who claimed a subse-
quent, decreasing trend in the "Village
area” and argued for fluctuations also in the
population breeding in the cliffs (about
85% of total population). Also from Ice-
land fluctuations have been reported (e.g.
Bengtson, 1970). As to the Faroes, the an-
nual fluctuations reported for the four study
plots (Tab. 2) represent the only informa-
tion available. The plots were selected for
special studies of colour-banded individu-
als, and thought to represent wren strong-
holds. Therefore the plots should, accord-
ing to the theory of ”ideal-free distribution”
(Fretwell and Lucas, 1970; Fretwell, 1972)
exhibit less variation than more marginal
areas with lower densities. The variation
(CV) for the four plots and the pooled num-
bers all fall in the range 15-28% (Tab. 2),
which is considerably less than the 63%
variation recorded in Sweden (Nilsson,
1986), and lower than the about 45% calcu-
lated from data from Iceland (Bengtson,
1970). It is not the purpose of this study to
discuss the population dynainics of the
wren in the Faroes but to note that in
favourable wren-areas in the Faroes the
magnitude of annual flucuations usually
appears to be in the order of 15-30%.
Whether this holds also for less favourable
areas and for the wrens breeding on the sea
cliffs is not known.
During the course of present study, i.e.
the past 20 yr, no single years stand out as
being particularly ”good” or ”poor” with
respect to the number of breeding wrens,
nor is there any obvious trends in numbers.
The previous total estimates at 250-500 and
500 pairs (Bloch and Sørensen, 1984;
Bloch et ai, 1996) most likely underesti-
mated the true numbers and should not be
taken as evidence for a recent increase. A
literature search for other evidence of fluc-
tuations or long-term changes produced lit-
tle of value. Most authors refer to the wren
as "common” or ”abundant” without giving
any numbers. Dare (1966), who made a
fortnightly visited to several islands (v/z.
Streymoy, Eysturoy, Vágar, and Nólsoy) in
Maj-June 1965, found the wren to be un-
common with < 10 records. Williamson
(1953) visited Streymoy, Nólsoy, Skúvoy,
and Stóra Dímun in late July and early Au-
gust 1947 and found the wren rnore abun-
dant than in any of the years 1942-1945, in
spite of exceptionally harsh weather condi-
tions in the early months of the year. On
Mykines in 1900 the wren occurred ”in no
small numbers” and in the following year it
was found ”in rather few numbers; fewer it
seems than in the previous year” (Ander-
sen, 1901; 1902); not stating whether it
refers to breeding birds or includes young
(in the Faroes, recruitment, and thus au-
tumn and early winter numbers, may vary
considerably between years according to
pers.obs.). Two published accounts, dating