Fróðskaparrit - 01.01.2004, Page 255

HVUSSU MÚSABRÓÐIR (TROGLODYTES TROGLODYTES) YVIRLIVIR,
SPJAÐIR SEG OG HVUSSU STAÐBUNDIN HANN ER í FØROYUM
253
Introduction
The wren (Troglodytes troglodytes) has
been remarkably successful in colonizing
even the most remote and isolated North
Atlantic islands, from the Scottish islands
to Iceland. Besides, in a number of cases
the island populations are recognized as
endemic subspecies (Fig. 1), collectively
referred to as ”island races” (Armstrong,
1955; Armstrong and Whitehouse, 1977).
In contrast to some northern, mainland pop-
ulations (e.g. Scandinavian) that are migra-
tory, but like most British and Continental
wrens the island populations are considered
non-migratory, though local and short-dis-
tance movements are commonplace (Arm-
strong, 1955; Hawthorn and Mead, 1975;
Armstrong and Whitehouse, 1977; Cramp,
1988). Hence, there is indirect evidence of
past long-distance water-crossings and ef-
fects of isolation and strong selective re-
gimes that have lead to relatively fast evo-
lutionary changes; indeed, the island races
of wren may be viewed as an early stage of
adaptive radiation (Williamson, 1981).
As evidenced (though circumstantially)
by the seasonal occurrence of wrens in non-
nesting areas or in numbers not account-
able for by the local populations, intra-is-
land as well as inter-island movements of
wrens occur on offshore Scottish islands,
the Faroes, and in Iceland (Williamson and
Boyd, 1960, 1963; Williamson, 1965; pers.
obs.). Presumably these movements are
associated with the rigorous environmen-
tal conditions prevailing in most of these
wind-swept, treeless islands, where the
primarily insectivorous wrens often have
to track other food resources and, for in-
stance, frequently feed on marine wrack
fauna in the intertidal zone. Precise data
on the occurrence of such movements, and
especially inler-island movements (i.e. the
crossing of water barriers), are scarce. Such
dispersal patterns have important social
and genetic implications for the structure
of local populations and differences within
and between the archipelagos. In Shetland,
mainland wrens (assumedly Scandinavian
and/or British that are morphologically dis-
tinguishable from the island subspecies)
are frequently recorded during migration
and inter-island movements occur as in-
ferred from observations of unusually large
numbers of wrens on small isolated islands
(Williamson, 1965; Cramp, 1988). In the
Faroes, assumedly Scandinavian wrens
have been caplured on three occasions (Sø-
rensen and Jensen, 1999). The existence
of inter-island movements in the Faroes is
suggested by the continuous presence of
small, extinction prone poulations on some
of the smallest islands (Bengtson, 2001). As
to Iceland no other than the Icelandic sub-
species (islandicus) has been recorded. In
this context it should be noted that a ringed
wren observed at Mývatn in NE Iceland
in 1963 was, since no wrens had been re-
ported ringed in Iceland in the years before,
by Armstrong and Whitehouse (1977:238;
also cited in Cramp, 1988:528) considered
a possible immigrant that might have been
ringed ”several hundred miles away across
a sea-passage”. However, this bird was
in all probability one of several Icelandic
breeding wrens that had been ringed (using
Icelandic rings but not yet reported to the
ringing ol'fice in Reykjavík at the time of