Wildfowl 63 - page 102

96 Pitfalls of surveying breeding geese
©Wildfowl & Wetlands Trust Wildfowl (2013) 63: 90–104
Numbers breeding at Lake Uettelsheim in 2011
During a visit to the breeding island of
Lake Uettelsheim on 28 March, 44 active
nests were found, indicating a breeding
population of 88 adult individuals (method
IV in Fig. 3). In some nests, large clutches of
up to 16 eggs implied egg-dumping by
several females, suggesting that even more
individuals were reproductively active locally
than the 44 nests alone would imply. On
using all observations that indicated breeding
to assess territory occupancy, 32 territorial
pairs were found to be present on 24 March
(i.e. 64 adult individuals, method III in Fig. 3),
consisting of 13 pairs, two pairs with an
observed nest and 17 guarding males around
the breeding island. These territorial pairs
represented 73% of the apparent nesting
pairs recorded. An estimated 21 pairs (42
adults) were seen with a brood during the
detailed monitoring of the fate of individual
broods in April–May (method II in Fig. 3).
Hence, only 48% of the apparent nesting
pairs were seen with goslings. If only the
maximum number of successful families
seen at once was considered, the numbers
breeding would have been estimated at only
Figure 3. Phenology of numbers of Greylag Geese counted at Lake Uettelsheim, Duisburg, in 2011,
separated according to status and age-groups. Numbers below the x-axis refer to the total numbers of
goslings (including fledged young) counted. Also given is the size of the breeding population (expressed
as the number of apparently paired individuals), derived from four different methods: I number of
broods observed simultaneously, II total number of broods when taking age-differences of broods into
account, III territorial pairs and IV number of nests found (see text for further explanation).
I
II
III
IV
40
20
0
20
40
60
80
100
01-Mar 14-Mar 24-Mar 04-Apr07-Apr11-Apr15-Apr18-Apr21-Apr25-Apr28-Apr03-May06-May09-May12-May16-May19-May24-May27-May31-May07-Jun 16-Jun24-Jun01-Jul10-Jul 21-Jul
Number of individuals
Parents Paired Paired with nest
Breeding / guarding Single birds / flocks Goslings