Ardea
Official journal of the Netherlands Ornithologists' Union

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Opdam P., Burgers J. & Muskens G. (1987) Population trend, reproduction, and pesticides in Dutch Sparrowhawks following the ban on DDT. ARDEA 75 (2): 205-212
Dutch Sparrowhawk populations declined during the 1960s due to adverse effects of organochlorine pesticides. Almost immediately after the bans on the use of these compounds in agriculture, populations started to recover. The effect of DDE on reproduction was often assumed to be a major factor responsible for the decline. We followed the reproductive output of several populations during the period 1970-1983 to test the assumption that an improvement of the breeding performance was responsible for the recovery of the population. Because breeding performance is governed by several factors other than organochlorines which, in their turn, may be connected with the amount of pesticides taken up by the birds, we also studied the effects of age of breeding birds, food supply as measured by landscape attributes in the feeding range, eggshell thickness and clutch size. The proportion of successful clutches increased quite suddenly from 60-70% during 1970-1975 to 70-80% from 1976 onwards, but Sparrowhawk populations had started to grow already before. We concluded that the recovery of reproduction had no effect on the breeding population, and suggest that the recovery of the Sparrowhawk was caused by a decrease in mortality due to disappearance of dieldrin and related compounds from the environment. The role of DDE in suppressing the Sparrowhawk populations is questioned. Breeding performance was governed by both male and female age. Immature pairs performed least well; adult pairs were most successful, whereas mixed pairs were intermediate. Reproductive output decreased with the area of woodland surrounding the nesting site and with the distance to the nearest village. DDE content in eggs and shell thickness were related to brood size and general breeding performance. A threshold value for the effect of shell thickness is suggested. Multiple regression techniques were used to account for the combined effects of various factors on breeding performance. We concluded that eggshell thickness and amount of woodland in the hunting range were the best predictors of mean brood size and breeding success. It is likely that the increase in breeding success in the mid-1970s was possible after an increase of eggshell thickness.


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