Pitfalls of surveying breeding geese 101
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
Moreover, a count of broods only
registers the successful element of the
population that is subject to annual variation
in nest or hatching success. Nevertheless,
many surveys assume the number of broods
to represent the breeding population (
e.g.
Geiter
et al.
2002; Gedeon
et al.
2004 for
Canada Geese). Brood surveys are often
favoured because they avoid the ambiguity
about assigning geese as “breeding” or
“non-breeding”. However, this method
ignores the fact that nest success often does
not exceed 50% (see next section), given
that many pairs attempt to breed but are
unsuccessful. Besides, only a proportion
of family parties are detected when
counting broods, especially when survey
intensity is low or where only one count is
carried out. In our study, where survey
effort was high and the fate of individual
broods could be followed, up to
c
. 50%
of the Greylag Geese breeding locally
were observed with broods. In Canada
Geese this was slightly lower (up to 40%).
Another source of bias is emigration or
immigration of broods from nearby nesting
sites (as we suspected at one of our study
sites). Broods might wander considerable
distances to suitable gosling-rearing areas,
a source of error likely to increase
through the course of a breeding season
(Schekkerman
et al.
2000). Without
individual marking of the adults, such
movements are not detectable and may
further confound assessment of local
breeding populations.
Comparison with other areas
In order to check whether the breeding
strategy and the results of our analysis
were not site-specific but could also be
applied to other regions, we compared
the reproductive output of breeding
geese in Duisburg with other studies of
(re-)introduced goose populations. For
this purpose the number of observed
broods was used as a proxy for nest
success, albeit it is likely that it is an
underestimation as we did not carry out nest
surveillance, as done in most of the studies
cited below.
In areas without any population
management, nest success was 63–69% in
Greylag Geese and 66–69% in Canada
Geese (Wright & Giles 1988; Buss 2004;
Havekes & Hoogkamer 2008). At sites with
high predation pressure or cattle trampling,
nest success was lower, 26–53% in Greylag
Geese (Kristiansen 1998) and 46% in
Canada Geese (Johnson & Sibley 1993).
Much lower rates of nest success were
found on the island of Texel in the
Netherlands, where extensive population
management was carried out and only
5–31% of all Greylag nests were successful
(Hondshorst & Voorbergen 2005). Fewer
studies have used the number of broods as
a proxy for nesting success, as we did in our
study. Kampp & Preuss (2005) estimated
an average nesting success of 60% for
Greylag Geese in an urban population in
Copenhagen, using the high density of
marked birds in their local populations to
monitor the breeding performance of
individual breeding pairs. Lensink (1998),
who used similar data as we did (also
without ringing, but taking only territorial
pairs and broods into account) recorded
18–68% successful pairs in Greylag Geese
which is in line with the 15–40% in our
