94 Pitfalls of surveying breeding geese
©Wildfowl & Wetlands Trust
Wildfowl
(2013) 63: 90–104
as proposed in the German breeding bird
census manual (Südbeck
et al.
2005).
The number of successful breeding pairs
was assessed by frequent monitoring of
broods, recording the number of broods
with goslings of different ages (indicated
by their size) present on each occasion. Due
to high count frequency, the fate of the
broods within the study area could be
assessed quite reliably without marking them,
because broods recorded in previous counts
could
be
distinguished
by
their
size from new, recently hatched broods
(but see Discussion). Also, when broods were
missed during one visit (which occurred
twice for the Canada Geese) they were
quickly relocated during subsequent visits.
Methods and observation effort were
similar in all three study areas. However, the
surveys were subject to some practical
problems, which occurred to differing
extents both during the different years of the
study and at the different study sites. For
instance, nest counts at Lake Toepper were
regarded as incomplete because one small
breeding island was totally inaccessible. In
2012, nest counts at Lake Toepper and Six
Lakes were unavoidably carried out too early,
which led to an incomplete nest count of
Canada Geese. At Lake Toepper, it was
presumed that exchange of broods with
nearby breeding sites took place, making an
accurate analysis of the fate of single broods
difficult. Hence, our results focus on the
dataset from Lake Uettelsheim, where such
problems did not occur and which was
surveyed extensively in 2011. Data from 2012
and the other two study sites are presented to
put the results of Lake Uettelsheim 2011 in a
broader context. Data from 2010 were too
incomplete to enable a good comparison of
the study methods.
Results
General results
Breeding Canada Geese and Greylag Geese
occurred at all the three study sites in each
year. Total numbers of breeding birds and
the ratios of Canada Geese to Greylag
Geese differed considerably between the
sites, although they were rather stable (with
a slight decline in Canada Geese) within
each site over the 2010–2012 study period
(Fig. 2). At Lake Toepper and Lake
Uettelsheim, the Greylag Goose was the
most abundant species; at the Six Lakes in
Duisburg-Wedau the breeding population
was dominated by Canada Geese. Egg laying
by Greylags started by the end of February
or early March, with eggs being laid well into
April. Hatching was observed from the end
of March onwards and most goslings
fledged in June. Canada Goose breeding
phenology followed about one month later,
with laying starting in the second half of
March and replacement clutches being
found well into May. First broods were
observed by the end of April and these
fledged in July. Over the 3-year study, the
number of breeding Greylag Geese at a
single lake ranged from 15–42 pairs,
compared with 6–40 breeding pairs per lake
recorded for Canada Geese, both based on
territory assessments (Fig. 2).
Greylag Goose
Breeding phenology at Lake Uettelsheim in 2011
At the start of the breeding season, in
March, pairs were mainly observed at the
