90
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
Does every goose count?
Pitfalls of surveying breeding geese in urban
areas
CHRISTINE KOWALLIK
1
* & KEES KOFFIJBERG
2
1
Biologische Station Westliches Ruhrgebiet, Lösorter Strasse 119, 47137 Duisburg,
Germany.
2
Friedhofstrasse 66b, 46562 Voerde, Germany.
*Correspondence author. E-mail:
Abstract
The size of local breeding populations of Greylag Geese
Anser anser
and Canada
Geese
Branta canadensis
at a suburban site in Northrhine-Westphalia, Germany, was
assessed between 2010 and 2012 using four different methods: nest surveys, counts of
territorial pairs and two types of brood counts. For both species, nest surveys
generated the highest estimate of breeding numbers. Geese recorded as territorial
pairs made up 50–75% of the apparent nesting pairs (73% of all nesting Greylag
Geese and 60% of all nesting Canada Geese in an area surveyed extensively in 2011).
Numbers of broods recorded never exceeded 50% of the number of apparent nesting
pairs. Moreover, the number of broods observed was heavily dependent on fieldwork
intensity, with most broods found during highly frequent (twice-weekly) counts that
allowed effective monitoring of the fate of individual broods, even without using
individual marking. When broods are monitored less frequently, one has to rely on the
maximum number of broods observed simultaneously in determining the number of
pairs with young, which in our study represented only 10–25% of the apparent nesting
number. Although nest counts may provide the highest estimate of breeding goose
abundance, they may be impractical or undesirable (
e.g.
because of disturbance to
other breeding birds). In such cases, territorial pair assessments may be the preferred
method, if separation of breeding and non-breeding birds is not made too
conservatively. For instance, only those birds that obviously behave as non-breeders,
by leaving the nesting areas to feed on nearby agricultural fields during daytime, should
be excluded from breeding numbers. Although counts of the total number of broods
can contribute to measures of reproductive success, they can considerably
underestimate the number of goose breeding pairs.
Key words
: breeding bird census, brood counts, Canada Goose, Greylag Goose, nest
counts.
