Fróðskaparrit - 01.01.2004, Blaðsíða 250

248 A history of colonization and current status of the house sparrow
(.PASSER DOMESTICUS) IN THE FAROE ISLANDS
year of fírst colonization and settlement
area (Fig. 3) does not necessarily imply that
area per se is important but that it is oth-
er variables related to the Faroese human
habitations, e.g. there is a strong correlation
between area and numbers of houses and
inhabitants (unpubl.). Hence, a large settle-
ment provides more nest sites, shelter, and
food (warehouses, gardening etc) than a
small one. In addition, as argued above, the
larger settlements offer more opportunities
for human aided transport leading to rescue
effects (Brown and Kodric-Brown, 1977).
In spite of the rapid increase in numbers
in source areas such as Vágur and Tórshavn
it took the house sparrow more than 30
years, but probably less than 40, to spread
to 50% of the settlements that currently
have been colonized at least once (n=95 in
2002; i.e. 80.5% of the number available),
and possibly more than 50 years to reach
the 65% level (Fig. 2). Empirical studies
of the average velocity of the house spar-
row invasions into Siberia from Europe
and in North America have been estimated
to be 28 and 17 km/year, respectively; and
when aided by man between 48 and 120
km/year (Summers-Smith, 1956; Johnston
and Klitz, 1977). Theoretical studies, based
on diffusion equations and demographic
characteristics, suggest a velocity of 15-30
km/year (Okuba, 1988; Bosch etai, 1992).
Compared with these velocities the coloni-
zation of the Faroes was a slow process, at
least during its first 20-25 years for which
the most detailed data is available. For the
1960s and 1970s the actual numbers of set-
tlements colonized are probably higher than
those recorded here and thus the cumulative
curve steeper than depicted in Fig. 2. For
the period between 1981 and 2001, when a
large number of settlements probably were
colonized for the first time, the information
is inadequate and the shape of the cumula-
tive curve uncertain. It seems likely that the
house sparrow had a potential for a much
more rapid spread over the Faroes than has
been the case since geographical distances
are relatively short and propagule pressure
may be high as most pairs produce two, oc-
casionally three, broods per season (Jensen,
1988). Probably the local conditions and
their effects on local population dynamics
have a significant influence on the process
of spread.
Regarding population size and changes
in numbers of the house sparrow on the
Faroes information is meagre and rarely
quantitative. However, some authors and
interviews with local people accounted for
above testify to a rapid build-up of num-
bers of house sparrows in some areas soon
after immigration and in the 1960s the
species seems to have been quite numer-
ous in several places. For instance, in 1972
the house sparrow was reported to be very
common in the larger places and breed-
ing in most settlements (Flensted-Jensen,
1973). While still increasing its overall
range on the Faroes (see Fig. 3), it there-
after is said to have decreased in numbers
and disappeared from some settlements
(Salomonsen, 1982). Burton (1995: 296)
states that “a serious decline was reported
in the Faroe Islands in the early 1980s” and
Bloch (n.d., though issued in 1988 and re-
ferring to the 1980s, Bloch, pers. comm.)
claims that the house sparrow “is declining