VALK R VAN DER (1996) The possible influence of Foxes Vulpes vulpes on the breeding biology of the Tufted Duck Aythya fuligula in the Amsterdam Waterworks Dunes. LIMOSA 69 (3): 97-102.

Tufted Duck Aythya fuligula The Dutch breeding population of the Tufted Duck started increasing about 40 years ago. Nowadays the species breeds in almost all low areas of The Netherlands. The Fox appeared in the dunes in about 1980 and the number of duck breeding pairs decreased strongly (Fig. I). However, since 1987 the number of breeding pairs increased again. This paper describes the influence of predation by Foxes on the start of egglaying and on breeding succes in Tufted Ducks in the Amsterdam Waterworks Dunes. A new (statistical) method is introduced to calculate the date of start of the egglaying from sex-ratio counts in the field. The start of the egglaying is calculated assuming that the number of males per counting date is binomially (N,P) distributed. Before the breeding period sex-ratios of 50%-52.5% are expected. Deviations of these values can be tested statistically by means of the normal Binomial hypothesis test procedures. A confidence coefficient 1-0:=0.95 and 0.90 is assumed. If the sex-ratio is increased significantly in spring, females have incubating. The start of egglaying is in the pentads 29 and 30 (21 May - 31 May), hardly varying among years (Tab. I). With an incubation period of 24 days, newly hatched chicks are to be expected in the period 14-25 June. Pulli are first observed when about one week old. This results in an expected first observation date for Tufted Ducks with pulli in the second half of June. Before 1981 this observation date is in accordance with the field data, but after 1981 a delay of one or two weeks into July is found (Tab. 2). Though partly due to a series of harsh winters, this effect is mainly ascribed to predation by Foxes. After 1988 there is a clear improvement of breeding success (Fig. 2) and an increase in the number of breeding pairs (Fig. I). Furthermore, in the most recent years pulIi have been observed one week earlier than in the years immediately after the all-time low. It is assumed that these changes are caused by a decreased predation pressure by Foxes. A series of mild winters could also have an influence. Probably it is more difficult for the Fox to find food along the banks of trenches due to expanded vegetation. Another possible reason for decreased predation pressure can be an increased number of Rabbits. The local Rabbit-population has increased since 1986. A decrease in predation pressure could also be ascribed to a decrease of the Fox-population. The (incomplete) data in Tab. 3 do not suggest this. The conclusion can be drawn that 10 to 15 years after first appearance of Foxes, a new equilibrium between Tufted Ducks and Foxes has arisen.

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