The Greenland White-fronted Goose Anser albifrons flavirostris

h -1 and none showed any sign of resting on the
sea en route.
A number of different approaches have been
made to estimate the energetic costs of flights for
birds of different sizes. The simplest approaches
have been those methods, which use crude estimations
of the energetic costs of flight. Based on
simple energetic formulae, it is possible to estimate
the costs of flight to Iceland from Wexford.
Given that many Wexford wintering birds stage
at Hvanneyri in western Iceland (MS27), the distance
between these two areas (1,500 km) was
used to calculate the minimum flight range necessary
for geese to reach their staging site. The
still-air flight-range estimation methods of
McNeil & Cadieux (1972), Summers & Waltner
(1978), Greenewalt (1975) and Davidson (1984)
were used to back-calculate the minimum fat
stores necessary to sustain flight over that distance.
These were used to generate a range of different
speeds for a female of departing lean body
mass 2.3 kg and a male of 2.6 kg (see Table 3.1).
Given the observed ground speeds of 50-90 km
h -1 observed amongst satellite tagged geese making
this journey, a range of values were obtained
for these observed speeds. These suggest that
males require between 145 and 292 g of fat and
females 130-272 g fat to fuel the passage from
Wexford to Hvanneyri.
A more sophisticated method is to attempt to
model the birds rate of use of fuel, based on the
mechanical work the organism must do, given its
morphology and the conditions of flight. This is
the aerodynamic approach of Pennycuick (1989),
which makes several assumptions about the
physiology of flight, but nevertheless provides the
best predictive model available at the present. The
most recent version of his software (Flight.bas
version 1999) was used which incorporates findings
from recent wind tunnel studies which suggest
that even for a large birds like a goose, the
coefficient of body drag (C db ) is lower than previously
thought (Pennycuick et al. 1996). In the
present analysis, the suggested lower value (C db
= 0.10) was used instead of 0.25 (see also Green &
Alerstam 2000). The results are shown in Figure
3.5, showing the range of flight range estimates
for a female of lean body mass 2.3 kg and male of
lean body mass 2.6 kg, the 1500 km flight necessitating
340g and 349g of fat for the male and female
respectively. Both calculations make the assumption
that the geese fly at maximum range
speed, which was 99 and 103 km/hr respectively.
Flight range burning fat only (km)
At lower speeds, for the same total energy expenditure,
the flight range estimate would be reduced
(Figure 3.6).
Despite considerable monitoring efforts, it is clear
that we still know very little about the prelude to
departure of the geese from their wintering
grounds in spring. All of the data presented above
derive from the main Irish wintering site, and
nothing is known from wintering sites elsewhere.
Satellite telemetry (MS20) showed that in a spring
when a relatively early departure occurred from
Wexford, at least one of the early departing birds
fitted with a transmitter left Wexford on 10 April
1997, but staged on the northern Ireland coast
until 16 April before departing for Iceland. Such
spring staging within Ireland may be simply due
to birds responding to initial cues, which suggested
that weather conditions were favourable
for migration, only to encounter unfavourable
conditions later on. Resightings of Wexford-wintering
birds seen soon after spring departure on
Flight range (km)
1800
1600
1400
1200
1000
0.2 0.25 0.3 0.35 0.4 0.45
Fat deposits at departure (kg)
Figure 3.5. Theoretical flight range estimates for male
and female Greenland White-fronted Geese of lean
body mass 2.6 and 2.3 kg respectively, given different
fat loads at point of departure based on Pennycuick
models (see text for details).
1700
1600
1500
1400
1300
1200
1100
Male lean mass 2.6 kg
Female lean mass 2.3 kg
1000
50 60 70 80 90 100 110
Flight speed (km/hr)
Figure 3.6. Effects of differences in flight speed on the
theoretical flight range of Greenland White-fronted
Geese.
31