Survival and dispersal in a new Barnacle Goose population 77
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 72–89
birds being identified was assumed to be
high, as most Barnacle Goose colonies,
brood-rearing areas and moult flocks are
screened for the presence of ringed birds by
observers. It was further assumed that
observations outside the Netherlands but
within the flyway between 1 April–15 May
were of birds on their way to breeding
grounds in northern Russia, and that
observations made between 16 May–15
August outside the Netherlands but within
the flyway were of birds that had attempted
to breed at the observation site. For
example, a bird that was observed at the
coast of Schleswig-Holstein, northern
Germany, on 14 May was assumed to be on
its spring migration to Russia, but a second
bird seen at the same location on 16 May
was assumed to be a local breeding bird.
There is a risk that the first bird also is a
local breeding bird, but it was classified as
migrating because there were no further
observations at the same locality. The risk of
misclassification is probably small because
sightings made from 16 May–15 August
overrule those made earlier in the year, as
being more likely to reflect the area used by
the bird during the breeding season.
For each individual bird, the mean
distance between the ringing site and the
breeding site was then calculated, using all
observations made in potential breeding
habitat based on the criteria mentioned
above. This resulted in a dispersal distance
being determined for 557 of the 804 ringed
birds included in the study.
Birds that were never observed within
these dates (1 April–15 August), but were
regularly seen outside these dates (
i.e
.
outside the breeding season),
and
that were
at least two years old at the time of these
observations (hence capable of breeding)
were assumed to have dispersed to
somewhere outside the Netherlands, most
probably to the Russian (sub-) Arctic (after
van der Jeugd 2001). This resulted in a
further 71 birds that could be classified as
having dispersed. Although it can be
assumed that these birds bred outside the
Netherlands, a precise dispersal distance
could not be calculated because their
breeding location was not known, but as
most of the birds probably dispersed to the
vast breeding areas of northern Russia, a
dispersal distance of 3,000 km was
substituted for these individuals.
For 176 of the ringed birds it was not
possible to infer anything about their
dispersal status because there were no
observations > 1 year after ringing; these
birds most probably had died.
The dispersal rate from the colony at Jan
Durkspolder in Fryslân was considered
likely to be much higher than in the Delta
area, because of the recent disturbances in
this colony by foxes (R. Kleefstra, pers.
comm.). Dispersal from this colony
therefore was analysed separately.
Results
Survival in the Delta area
Survival of birds ringed in the Delta area
was calculated from observations made of
live birds as well as on reports of dead,
mainly shot, birds. The final model, which
best fitted the data, indicated that there was
no effect of age class, age at ringing (group)
or sex on survival (S), but that the survival
rate varied with time (Table 2). Re-sightings
