102 Pitfalls of surveying breeding geese
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
study if we would have used the number of
territorial pairs as size of the local breeding
population. Sorge (unpublished) made an
intensive study of Greylag and Canada
Geese in parks in Munich and found
10–40% of the territorial pairs in both
species to be successful, falling to 0–14% in
years with population management. All
these studies show that nest success is highly
variable between sites and years, and that
our results fall around the average, neither
extremely high, nor extremely low.
Conclusions
The result of our study show that censuses
of breeding Greylag Geese and Canada
Geese which use assessment of territories
or counts of broods underestimates local
nesting abundance. Nest counts on the
other hand, provided the highest estimate
for the local breeding population, but may
not always be possible for practical reasons
or not desirable for reasons of disturbance
to other breeding birds. The underestimate
by using indirect methods is especially
pronounced when relying on counts of
broods only. Without intensive fieldwork or
marking of birds, the fate of individual
broods cannot be monitored and the
number of breeding birds is only a small
fraction of the apparent nesting population.
Assessment of territories is an alternative
for nest counts, provided that one is not too
conservative in separating breeding birds
and non-breeding birds. Moreover, territory
assessment should preferably be carried out
during multiple visits before the main part
of the population starts to incubate and
remains concealed in vegetation (
i.e.
early in
the breeding season).
Acknowledgements
We thank the staff from the Environmental,
Forestry and Hunting Service from the
municipality of Duisburg, as well as
Randolph Kricke, Elisa Kuprat, Tobias
Rautenberg, Julian Sattler, Ulf-Christian
Unterberg for surveying geese, and the many
field assistants who helped by participating
in the counts. Berend Voslamber (Sovon
Dutch Centre for Field Ornithology) shared
his experiences in counting Greylag Geese
and is thanked for discussing the results.
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