Pitfalls of surveying breeding geese 93
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
and Hunting Service from the city of
Duisburg (Stadt Duisburg 2011, 2012).
These measures aimed to reduce the number
of eggs to two per clutch, in order to reduce
clutch size without affecting the number of
nesting pairs. Island nest sites were visited
2–3 times during March–April to search for
active nests;
i.e.
, nests with clutches or
freshly-hatched eggs. During each visit,
Greylag and Canada Goose nests were
marked and counted, and clutches reduced
to two eggs. The nesting pair was usually
found (and species identified) on flushing
the incubating bird from the nest; sometimes
by observing its partner defending the nest.
The total number of nests was derived from
successive visits to the colony. Empty nests
were regarded as being hatched successfully;
new nests recorded during successive visits
were counted as being recently settled by
new pairs. It was assumed that where
clutches had been reduced, the pairs stayed
on the same nest (B. Voslamber, pers.
comm.). In some cases, as in Lake
Uettelsheim in 2011, the total number of
active nests of Greylag Geese and Canada
Geese coincided with the maximum number
of nests found on a single survey day.
Goose counts
In order to monitor goose numbers and
breeding success, a small research project
was initiated by the Biologische Station
Westliches Ruhrgebiet (Biologische Station
Westliches Ruhrgebiet 2011, 2012; Keil
et al.
2012). Goose counts were conducted at the
three study sites during the breeding seasons
of 2010–2012, from March–July inclusive.
The most comprehensive data were
collected in 2011, with three counts each in
March, June and July and twice-weekly
counts during the early stages of the
gosling-rearing period in April and May. In
2010, counts started from early May
onwards, but thereafter followed a similar
pattern to 2011 with twice-weekly counts in
May, and three counts per month in June
and July. In 2012, a total of 10 counts were
carried out from March–July (twice a
month). During the incubation period (in
March), observation frequency was similar
to that of 2011, but observations were less
frequent during the early gosling-rearing
period. All counts were carried out from
lake shores, using a 20–60× spotting scope
and 8–10× binoculars. Geese on the lake
and feeding on the surrounding lawns were
all counted, and their status (
i.e
. the number
of solitary birds, guarding males, and pairs
with or without a brood) was recorded. One
count took about 1–3 hours, depending on
the size of the survey site. During the
gosling-rearing period, broods were
monitored by assessing the number and age
of goslings (the latter with help of
photographs, which illustrated gosling size
for different age categories).
Interpretation of observations
Numbers of territorial pairs were determined
until mid-April, by summing the number of
pairs observed (including those with a nest, if
visible from the bank of the lake) and the
number of guarding males around the
breeding island during a survey day. Only
solitary birds close to the breeding island
were recorded as guarding males. Single birds
and loose gatherings of geese at larger
distances elsewhere on the lake or the banks
of the lake were assumed to be non-breeders,
