The Greenland White-fronted Goose Anser albifrons flavirostris

LEVEL OF NUTRIENT STORES
Figure 9.1. Patterns of theoretical accumulation of stores
by adult female Greenland White-fronted Geese during
the first 5 months of the year. Trajectories represent
4 different individuals that differ in their rate of
accumulation of stores (because of feeding efficiency,
behavioural dominance, parasite load, etc.). If birds fail
to accumulate sufficient reserves to reach a particular
threshold at winter departure, they may fail to reach
Iceland (individual 'd'), or do so with insufficient stores
and too little time to accumulate stores to complete the
journey to the west coast of Greenland (individual 'c').
Even if accumulated stores are sufficient to support the
flight successfully to the breeding areas, the individual
'b' still cannot accumulate stores rapidly enough postarrival
to the level required to initiate a clutch (shown
by 'A' above), hence she incurs a fitness cost in terms
of failed breeding. There will be a range of breeding
options available to the female dependent upon the
level of energetic reserves at the commencement of
breeding. Hence, within the band “A” the timing and
extent of nutrient acquisition will affect reproductive
investment through factors such as manipulation of
first egg date or clutch size. It is also possible that birds
arriving in Greenland and acquiring sufficient nutrients
on the pre-nesting areas to invest in a clutch may
potentially affect the relative quality of her investment.
This has consequences for her reproductive output (in
terms of her egg size, clutch size, incubation constancy,
etc.). Seen in this way, small differences in feeding efficiency,
and hence accumulation of stores, can be seen
to have a cumulative effect during the five or so months
before first egg date. This is why the sward type a bird
feeds upon, or the feeding efficiency of an individual,
or the level occupied by the individual in a dominance
hierarchy may have such consequences in terms of fitness
measures. Note also that the critical periods of
store acquisition are those in Iceland and Greenland,
where rates of accumulation are most rapid and therefore
where small perturbations are likely to have most
effect. Note also however, that even during the slow
accumulation of stores on the wintering grounds, failure
to accumulate stores bears a future cost, insofar as
the short episodes of rapid store accumulation in Iceland
and Greenland do not permit individuals to 'catchup
lost ground' at these later stages in the spring period.
74
Winter accumulation of stores
a
b
c
d
Staging in
Iceland
Pre-nesting
feeding
Greenland
Mortality
JAN FEB MAR APR MAY
A
tion and depletion events has therefore the potential
to influence the fitness of individuals.
Let us assume that we can use body mass as currency
to reflect the 'adequacy' of stores accumulated
by an individual to complete migration to
Iceland and onwards to Greenland. In this way,
we can diagrammatically represent the mass trajectories
of individuals which exhibit different
rates of accumulation of such stores under the
same environmental conditions (Figure 9.1). Body
mass may in reality represent a proxy measure of
energy stores in the form of fat deposition, or
perhaps storage of a scarce resource, such as the
protein required to develop the musculature required
for flight. If such a representation reflects
reality, small differences in the rate of accumulation
of such stores can potentially have a cumulative
effect on individuals throughout the course
of the spring, with knock-on effects from each of
the resource 'hurdles' encountered.
The accumulation of stored mass is relatively slow
in winter, but failure to reach high enough thresholds
by departure from wintering areas leaves
insufficient opportunity to recoup stores in Iceland.
Hence, differences in individual quality (in
terms of ability to acquire such reserves at critical
periods) can affect the amount of mass accumulated
and the amount available for investment
in migration and ultimately reproduction.
This model is useful when compared with actual
data compiled in the previous chapters. If the cost
of egg production in Greenland White-fronted
Geese is calculated using the methods of Meijer
& Drent (1999), it is possible to estimate the mass
required to produce a clutch of 3 or 6 eggs and
then to incubate these (Table 9.1). Using the field
estimates of abdominal profiles as a crude index
of body mass throughout the period, it is possible
to construct a graph of changes in mass of
adult male and female geese up to the point of
laying. The investment by the female in clutch
production and incubation can then be assessed
relative to the body mass available at the point of
first egg production (Figure 9.2). From this, it can
be see that the costs of laying a clutch of 6 eggs
takes the 'median' level of female body mass well
below the lowest weights recorded throughout
the annual cycle. This level is presumably well
below lean body mass and hence represents starvation
levels, even before the costs of incubation
are considered. Although costs of self-maintenance
during incubation are offset by feeding