Fróðskaparrit - 01.01.2004, Side 262

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PHILOPATRY, DISPERSAL, AND SURVIVAL OF WREN
(TROGLODYTES TROGLODYTES) 1N THE FAROE ISLANDS
English and Dutch breeding studies have
reported a number of cases of male philo-
patry, but with low return rates (Kluijver
et al., 1940, Armstrong, 1955; Armstrong
and Whitehouse, 1977, Garson, cited by
Cramp, 1988), and an extensive German
ten-year study showed very few recoveries
and the wren was considered ”nicht beson-
deres ortstreu”, though one polygamous
male and 3 females showed faithfulness
to their breeding territories in consecutive
years (Dallmann, 1987). With regard to the
Faroes it would be most interesting to know
where the non-philopatric females choose
to breed and is the overall relatively high
breeding site fídelity in Faroese wrens as-
sociated with a reluctance to cross water
barriers?
Evolution of social systems and patterns
of dispersal is usually considered to be in-
ter-linked and male philopatry and female-
biased dispersal as observed in the Faroese
wren is consistent with a pattern commonly
found in monogamous birds with resource
defence (Greenwood, 1980). The island
races of wren are territorial through most
of the year and, in contrast to the frequently
polygamous mainland wrens, monogamy
seems to prevail (Armstrong and White-
house, 1977); though polygamy has been
recorded in the Faroes (Bengtson, 2001 and
unpubl.). Over the past decades numerous
descriptive and experimental studies have
put forward evolutionary explanations for
sex-biased dispersal in birds and mammals,
mostly invoking inbreeding avoidance and
competition (e.g. Pusey, 1987). Inbreeding
is generally assumed to have negative con-
sequences at indivual and population lev-
els (e.g. Thornhill, 1993; Keller, 1998) and
might be a factor of importance with regard
to the local, and often rather small wren is-
land populations. Flowever, inbreeding may
not always be detrimental (e.g. Jarvinen
and Varvio, 1985), for instance with regard
to lifetime fitness (e.g. Shields, 1982; Van
den Casteele et al., 2003) and may even fa-
vour maintenance of local adaptations and
coadapted gene complexes (Shields, 1987).
Furthermore, the effects of inbreeding may
be environmentally dependent (Pray et al.,
1994; Bijlsmaeíc//., 1999). In many studies
sex differences in competition (intersexual
and parent-offspring) are believed to be a
driving force (though difficult to distinguish
from the relative importance of inbreeding
avoidance) behind the evolution of philo-
patry and sex-biased dispersal. Analyses
of dispersal patterns are usually based on
asymmetries between males and females in
the costs and benefits of being faithful to a
site or seek new grounds.
The small sample size from the Faroes
is inadequate for any wider conclusions
to be drawn. Nevertheless, the observa-
tions clearly suggest that males are more
philopatric than females, which is consist-
ent with suggestions that in species where
males defend resources such as a territory
a female-biased dispersal prevails (Green-
wood, 1980). Male wrens occupy exclusive
territories within which they feed, display
and build several nests, and the number of
vacant nests seems to be a cue for female
matechoice(Garson, 1980; EvansandBurn,
1996). Hence, the significance of being
aquainted with suitable nest-sites, which is
a highly predictable resource from one year