Pitfalls of surveying breeding geese 99
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
at a larger scale and is often not an option
for volunteer counters. Here, we discuss the
problems and advantages of each counting
method that does not rely on marking, and
compare guidelines from census manuals
and other methods that aim to assess
population size in breeding geese.
Nest counts
In the study area in Duisburg, nest counts
produced the highest assessments of the
size of the local breeding population. This
was likely the result of the physical
limitations imposed on the birds to breed on
an accessible and small breeding island.
Hence, nearly all potential nesting habitat
could be searched effectively for nests, and
nest identification was easy by visual
observation of the owner of the nest.
However, without a boat, and without
permission to access such islands (in case of
protected areas), a nest survey would already
be much more difficult, especially for
volunteer counters. Moreover, in marsh
areas with vast reed beds, nest surveys will
often be impossible and might pose an
important source of disturbance to other
reed-breeding birds (especially when done
repeatedly during one breeding season), as
has been shown in the Netherlands
(Schekkerman
et al.
2000; van der Jeugd
et al.
2006). Furthermore, dump clutching and an
unknown number of nests that are predated
before being found, may further confound
nest censuses and underestimate the
number of actively reproducing pairs. In our
study area, the nest site limitation resulted in
Canada Geese using nests of Greylag Geese
after hatching. Hence, identification of nests
by using downy feathers, as proposed by
Ferguson-Lees
et al.
(2011), would have been
difficult. The fact that multiple nest use
by two species with different breeding
phenology has occurred also implies that in
mixed breeding populations, repeated nest
counts should be carried out, including
marking of all nests found for both species.
Territorial pairs
When combining the results of all three
study sites, the assessment of territorial
birds revealed for Greylag Goose 60–
75% of the apparently nesting population,
and for Canada Goose 50–75%. Thus
assessment of territories is likely to
underestimate the size of the local breeding
population. Usually, a survey of territorial
pairs in March and April is recommended
(Gedeon
et al.
2004 (for Greylags); Südbeck
et al.
2005; van Dijk & Boele 2011).
However, a major source of bias in such a
survey is that all birds present in the
study area must be distinguished as breeders
or non-breeders. Single pairs showing
territorial behaviour or males with obvious
guarding behaviour around a nest site can
easily been assigned to the breeding
population. With single birds or small flocks
that do not show any territorial or alert
behaviour, this will be more difficult.
According to Voslamber
et al.
(2000), non-
breeding Greylag Geese often feed in flocks
on agricultural fields around the breeding
area, before leaving the area for moulting
sites from May onwards. They proposed to
take into account all birds around the direct
nesting site for an assessment of territorial
pairs, and exclude only those birds that are
obviously non-breeders, feeding in flocks on
agricultural fields at larger distances from
