Survival and dispersal in a new Barnacle Goose population 85
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 72–89
may have emigrated to the Barents Sea
population.
Discussion
Annual survival of Barnacle Geese breeding
in the Netherlands was initially very high,
with> 98% of geese surviving in the year
from mid-winter/spring 2004, and > 94% in
the year from 2005, the first two years of the
study. This is higher than has been reported
for most other non-hunted goose
populations (
e.g.
94.2–96.6% for adult
Barnacle Geese in Gotland, Sweden,
Larsson
et al
. 1998; and 92.7% for the
Svalbard Barnacle Goose population, Owen
& Black 1989), and is substantially higher
than reported for hunted species (78–88%
for Lesser Snow Geese
Anser c. caerulescens
in
1970–1987, Francis
et al
. 1992; 89–94% for
Lesser Snow Geese in 1990–1994, Cooke
et
al
. 2000; 72.3% for adult Icelandic Greylag
Geese
Anser anser
, Frederiksen
et al
. 2004;
and 81.4% for adult Icelandic Pink-footed
Geese
Anser brachyrhynchus
, Frederiksen
et al
.
2004). This high survival may be attributable
to a combination of the absence of
migration and a longer breeding season (van
der Jeugd
et al.
2009). However, survival
gradually declined and reached especially
low numbers during the last three years of
the study. The observed decrease in survival
can most probably be explained by the
introduction of a Barnacle Goose hunt
during the summer. Bag numbers in the
Delta area steadily increased from < 700
birds in 2006 (the first year of the summer
hunt) to > 5,000 in 2010 and 2011,
representing 25–30% of the total
population in the northern part of the Delta
recorded during summer (July) counts.
Relating the estimated mortality attributable
to hunting to annual survival rates reveals a
‘baseline’ survival of approximately 95%
that gradually drops to around 75% as
hunting mortality increases to 20%. In
Fryslân the drop in survival seems too large
in relation to the bag numbers, but this may
be due to a combination of a crude estimate
of the summer population size and the small
number of ringed birds reported dead.
A simple population model incorporating
constant fledgling production (0.6 young
per breeding pair, as observed in the Delta
area during 2004–2007; van der Jeugd 2012)
and the annual survival values from the
present study, indicated that the population
in the Delta area should be declining from
2009 onwards. This is not apparent from the
summer counts, however, which showed a
continuing substantial increase in numbers
after 2009 (Table 4). This discrepancy
illustrates the importance of continuing
demographic monitoring in harvested
populations, of validating monitoring using
counts, bag numbers and ring re-sighting
data, and the performance of analyses using
these data. Frederiksen
et al
. (2004)
performed such an analysis for two
harvested populations of geese in Scotland
and concluded that different data sources
yielded contrasting results, possibly leading
to an over-exploitation of one of the
populations concerned.
Natal dispersal was high, with 56% of
males attempting to breed > 10 km from
their natal site, and almost 38% > 100 km
from their natal site. In females, these
figures are 30% and 24% respectively. The
large amount of natal dispersal, over large
distances, from colonies in the Dutch
