100 Pitfalls of surveying breeding geese
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
the potential nesting site during daytime. It
is not clear if our study area supported any
non-breeding birds, as numbers observed
were always much lower than the number of
nesting pairs suggested. Indeed, if we had
considered all single birds at Lake
Uettelsheim on 4 April 2011 as breeders,
this would have resulted in 43 territorial
pairs (constituting 5 pairs with a brood, 16
pairs without a brood and 22 single birds
that were presumably guarding males),
in line with the number of active nests
(44). This implies that our approach to
distinguish breeders and non-breeders
was perhaps too conservative. Such a
conservative approach is also propagated
by several manuals (Gedeon
et al.
2004;
Südbeck
et al.
2005), that try to exclude as
many geese as possible in the assessment of
territories, by only taking into account
clearly distinguishable pairs and obvious
guarding males. This approach is likely
to underestimate the local breeding
population, as is shown in our study. On
the other hand, Sovon (2013a) proposes to
count all Greylag Geese present in the
breeding area and divide the maximum
number by 1.5, in areas where separation
of breeding and non-breeding geese is
impossible. Also this method would have
resulted in 43 territorial pairs at Lake
Uettelsheim in 2011, again matching with
the number of apparent nesting pairs.
Usually, it is proposed to survey
territories before breeding has started, often
by the end of February or in March for
Greylag Geese and in the end of March or
April for Canada Geese (Voslamber
et al.
2000; Südbeck
et al.
2005; Sovon 2013a,b).
This assumes, however, that breeding is
highly synchronised, which in our study and
in some of the studies elsewhere (
e.g.
Lensink 1998) was not the case. Moreover,
we observed that some geese in our study
area arrived late and immediately started to
nest. Hence, the timing of the survey should
be adapted to the local situation and
advancement of spring weather, preferably
with multiple counts carried out during a
short period before incubation starts,
e.g.
on
1 March, 16 March, 1 April and 16 April.
Guidelines like those given for the German
breeding bird atlas for Greylag Geese
(Gedeon
et al.
2004) and focusing on counts
in April (in order to avoid confusion with
migrants) are not recommended at most
breeding sites, as breeding usually starts
earlier than this and many birds will be
breeding concealed in vegetation by this
time and thus be overlooked during a survey.
Count of broods
When only successful breeders (
i.e.
the
number of broods) were counted, only
25–50% of the breeding population would
have been accounted for. However, this
figure varied considerably with counting
effort: more families were found when the
area was checked more frequently. Even with
a count frequency of once every two weeks,
proper monitoring of broods is prone
to largely underestimate the number of
successful pairs, especially when carried out
at larger breeding sites with many breeding
pairs. When only the maximum number of
broods seen at once was taken into account,
as few as 10–25% of the apparent nesting
pairs would have been recorded. This
large underestimate occurred both amongst
surveys of Greylag and Canada Geese.
