HAVEKES F & M HOOGKAMER (2008) High brood survival and adoption in an urban population of Greylag geese Anser anser in Zoetermeer, Zuid-Holland. LIMOSA 81 (4): 139-147.

Westerpark is a 150 ha large suburban park in Zoetermeer, Zuid-Holland, The Netherlands. It consists of circular waters, several islands, a mosaic of small meadows and woods, two lawns, and a sheep pasture. Numbers of Greylag Geese breeding in Westerpark have increased rapidly since the colonisation of the park in 2001 (Fig. 2). In an attempt to explain the rapid growth of this population, we determined nesting success and brood survival in 2008. A total of 68 nests were found, almost exclusively on the largest (1.5 ha) island, 28 of which hatched completely, 15 hatched partly and 25 did not hatch. Mean clutch size was 6.4 (± 2.5 SD). Clutch size significantly and negatively influenced hatching success. We found evidence of egg dumping and nest parasitism. Incomplete hatching of nests (mostly involving all but one egg hatched)may have been the result of nest parasitism during incubation. Female geese seemed incapable of discriminating their own eggs from illegitimate ones or even from golf balls and were inclined to take these into their nests. Nest predation by Carrion Crows or small mammals seemed to affect Greylag Geese much less than Egyptian Geese Alopochen aegyptiacus; none of five nests of the latter on the same island hatched any young.
      Gosling survival of Greylag Geese in Westerpark was very high (85% fledged, based on frequent counts of brood size), due to good feeding conditions and lack of predation. Families foraged on meadows along the waterside, on lawns and in May and June primarily on the sheep pasture. We estimated the geese to cause an increase in grazing pressure comparable to 33 sheep. There was a high incidence of adoption within the population and c. six pairs lost all goslings to other families. Adoption caused an increase in brood size from the initial value of 5.3 ± 1.7 at hatching to a final value of 5.9 ± 3.6 in May. Adoption may be the result of density dependent interactions between families, as subordinate parents disappear and their goslings join another, preferably larger, brood. It remains to be determined whether our findings are specific for urban environments or may be found elsewhere as well. High average brood size in May, combined with a high standard deviation, is an indication for the occurrence of adoption within any Greylag Goose population.

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