The Greenland White-fronted Goose Anser albifrons flavirostris

Goose droppings/m 2
6
5
4
3
2
1
0
Phleum
Phleum
Phleum
and clover
Poa pratensis
and clover
Poa
pratensis
Figure 4.5. Dropping density (as an index of goose use
+ SE) of pure sown stands of different grass species in
seed trial plots in a single field at Hvanneyri from
spring 1997.
similar pattern can be seen based upon mean
dropping densities (as an alternative assessment
of goose use) in plots of sown single species
stands of different grass species on trial plots
within one field at Hvanneyri in 1997 (Figure 4.5).
Note that the young establishing Deschampsia
caespitosa formed a continuous open sward prior
to tussock development and for this reason was
therefore probably as attractive to geese as Poa.
The total goose use of any particular field can be
seen as the product of 4 elements over time:
1) food density (which in turn determines the
settlement density of geese)
2) food intake rate (which determines food depletion
rate)
3) the 'giving up' density of food (the threshold
at which the food resource is depleted to the
point where it is more profitable to forage elsewhere)
4) the rate of regrowth of forage plants (which
determines the time until the food resource
exceeds the 'giving up' density of food and
geese return for sequential harvesting
In the context of individual field units, containing
grasslands of different sward composition, the
settlement density represents the aggregative response
of geese as predators to their 'prey' (i.e.
grass blades, MS26). The length of stay of geese
in that field represents the interaction between
standing crop biomass and intake rates (i.e. the
rate of depletion of prey items down to a threshold
'giving-up' density). Finally, the length of absence
is defined by the regrowth rate and quality
of the prey to the point where this exceeds a profitability
threshold for the geese, at which time
Deschampsia
caespitosa
Festuca
rubra
Agrostis
tenuis
Festuca
pratensis
Alopecurus
pratensis
Deschampsia
beringensis
they will return in appropriate numbers to regraze
the accumulated green biomass regrown in their
absence.
The three major grass species differ in their quality,
biomass accumulation and growth pattern.
The Phleum is an ecotype introduced from Norway,
valued for its early season growth, which
commences before other grasses begin above
ground production. Even in the early stages of
growth, this species responds to defoliation by
geese by increasing leaf elongation and elevated
protein levels, a feature which together with its
growth form, makes it the most attractive forage
species for sequential harvesting by geese (MS15,
MS25, MS26). After reseeding of a field with
Phleum, leaf densities are high, but as the tussockforming
Deschampsia and the stoloniferous Poa
invade, densities of Phleum decline rapidly (Fox
1993). In the early stages of the spring staging
period, geese therefore assort themselves in response
to the shoot densities of this, the most
abundant green plant material available (MS26),
although geese select for the longest leaves
(MS25). As a consequence, highest densities of
geese tend to settle on Phleum fields and eat out
most leaves of suitable length, returning only
when regrowth has occurred above a threshold
bite size (Figure 4.6, MS15).
Poa grows at very low leaf densities and although
of moderate nutrient quality, its later and (in some
years) faster growth rate results in supporting
lower densities of grazing geese throughout the
staging period (unpublished data). In contrast to
Phleum, where the high quality (low fibre, high
protein) youngest leaf is always removed, leav-
Cumulative percentage bird use
100
90
80
Phleum fields
70
Field 29
60
Field 36
50
Field 58
40
Field 59
30
20
10
0
Field 48
15-Apr 20-Apr 25-Apr 30-Apr 05-May 10-May
Figure 4.6. Cumulative goose use of five hayfields
dominated by Phleum pratense at Hvanneyri in spring
1997. Note the rapid exploitation episodes, followed
by short periods with reduced or little exploitation.
39