Temporal and spatial expansion of the Egyptian goose Alopochen aegyptiacus in The Netherlands, 1967-94

In this paper, the temporal and spatial expansion of the Egyptian goose in the Netherlands are described and analysed. The species bred near The Hague for the first time in 1967. In 1983 a second settlement developed in Drenthe. Both settlements expanded. Together, they contained about 1340 breeding pairs in 1994. For both settlements a linear relationship exists between the square root of the area occupied and time. In both cases, population growth is exponential. The bird behaves as a resident species. Thus, in winter numbers in The Netherlands also increase exponentially. The Egyptian goose seems to be sensitive to severe winters, which cause a high mortality. The observed velocity of range expansion is compared with the velocity as calculated with the expansion model of Van den Bosch et al. (1990). The observed velocity was about 3.0km per year, which is about 20% lower than expected, but not significantly different. Population growth was estimated using a projection matrix. In the first 10 years after its settlement near The Hague, the actual population growth was larger than the calculated growth. There is evidence for good breeding success during the first years due to mild winters between 1972 and 1978. In the second settlement, Drenthe, the same rapid population growth occurred. Here, a low but regular influx of birds from a nearby city park was probably the main factor. In the near future, further temporal and spatial expansion can be expected, in the direction of Germany and Denmark in the east and Belgium and France in the south. Towards the east the severity of the winters might limit further range expansion, possibly coinciding with the 0 degrees isotherm in January.