The Greenland White-fronted Goose Anser albifrons flavirostris

eas (see Raveling 1978, Ganter & Cooke 1996)
based on their own internal condition and the
prevailing environmental conditions. This would
involve assessment by the females of their ability
to meet the nutritional demands of laying differing
numbers of eggs and incubating the clutch
given available stores and the supplement possible
from exogenous food sources. Evidence is accumulating
to suggest that exogenous sources
supply much, or perhaps all of the fat needed for
egg formation (Choinière & Gauthier 1995, Ganter
& Cooke 1996, Meijer & Drent 1999). Hence, access
to adequate food resources prior to first egg
date may have a considerable impact on the ability
of a bird to reproduce successfully.
We know little about female condition and its potential
to affect reproductive success (before, during
and after nesting), so the accumulation of
knowledge relating to this parameter remains a
priority. In particular, following body mass
changes in particular individuals during the period
from first arrival in west Greenland through
to the end of incubation would be highly desirable.
Tracking changes in body mass by capture
and the use of balances under nests offers the
opportunity to assess and contrast the potential
of different individuals to successfully invest
stores and reserves in reproductive attempts.
Similarly, it is of great interest to understand more
about how brood females recoup stores and reserves
exploited during the laying and incubation
period, a process about which we know nothing
at present. Clearly behavioural adaptations
(i.e. mechanisms resolving the conflict between
self-maintenance and investment in brood protection)
and dietary selection are both potentially
involved, but could differ between individuals.
Despite the recent expansion in total population
size, the absolute numbers of successfully breeding
pairs returning with young to the two major
wintering sites combined have been more or less
constant, suggesting some density-dependent
mechanism is operating on the breeding grounds
which restricts recruitment. Amongst known age
marked individuals at Wexford, the probability
of recruitment has declined over time and the
mean age of first breeding has increased from c.3
prior to 1988 to c.5 years of age in subsequent
years.
Although difficult to measure in an objective way,
the extent of breeding habitat available through-
56
out the summer range does not appear limiting.
Nevertheless, the extent of habitat available in
spring for pre-nesting feeding as well as later in
the summer, are likely to vary with weather conditions,
especially in the north. The Wexford geese
breed mainly in the north of the breeding range,
and the recent declines in fecundity of birds wintering
at that site seem likely to be the result of
conditions these birds encounter on their prebreeding
and nesting areas. Their migration to
west Greenland differs little in distance or route
from the Scottish wintering element of the population
that breed further south. They could experience
less access to energy-rich foods (such as
barley and potatoes) in western than in southern
Iceland, which could enable greater energy stores
to be accumulated by predominantly Scottish
birds staging in these Iceland lowlands (MS4,
MS19). However, if it is exogenous energy derived
on the breeding areas that represents a major determinant
of clutch size or quality, early arrival
to staging areas in southern Iceland (MS19) would
give these birds an advantage over geese breeding
further north. The latter would not only compete
with local breeders in the staging areas of
central west Greenland but also then migrate
northwards within Greenland with a high probability
of encountering severe weather conditions
on arrival at ultimate nesting grounds. As goose
densities have increased in recent years, it may
be that all potentially breeding White-fronted
Geese are encountering more competition for limited
resources in spring, but the birds still needing
to continue north face increased competition
from non-breeders. As there is no further habitat
to the north of the current range into which to
expand, it might therefore be expected that geese
breeding in the north of the range show greater
density-dependent effects on the summering areas
than those nesting further south. There is
some evidence that this is the case; on Islay, the
production of young per potentially breeding female
has not declined significantly, the decline in
successful breeding being compensated for to
some extent by increases in mean brood size in
very recent years. Increased mean brood size implies
(i) adequate stores to lay large clutches, (ii)
to incubate these successfully and (iii) to raise
goslings to fledging. If the breeding range of the
Islay-wintering birds has not changed, it is unlikely
that their increased brood sizes have been
brought about by change in habitat. Since their
breeding area (mainly 66-69ºN) is the area with
greatest density of colonising breeding Canada