DIX M, MUSTERS K & TER KEURS W (1998) Is the breeding success of the Blackbird Turdus merula declining in the Netherlands?. LIMOSA 71 (2): 41-48.

A recent study on the number of passerines ringed in The Netherlands suggested a decline of the population of Blackbirds Turdus merula between 1960 and 1986. The popular response was to assume that this was caused by an increase of predation of eggs and nestlings by Magpies Pica pica as a result of the increasing populatIon of the latter. We studied changes in reproductive success of the Blackbird to test this hypothesis, using data from the Dutch Ringing Centre (1973-92). The ratio between fledglings and adults caught in the month of August gives an idea of the reproductive success in the preceding breeding season. This ratio can be seen as the resultant of the number of nests per adult per season, the number of nestlings per nest and the survival of fledglings (Fig. 1). To get an idea which ofthese factors could be responsible for a change in reproduction-ratio, data on the yearly average number of ringed nestlings per nest were analysed. This number should not change if Magpies are the cause of the decline, as they usually leave the nest empty. Because of known differences in reproduction between rural and urban Blackbirds, as well as in population change between woodland and open area Magpies, a division in four regions was made: 'woody rural', 'open rural', 'woody urbanised' and 'open urbanised' (Fig. 2).
      The results show that the reproduction-ratio in rural areas has increased to the level of urban areas, which has remained stable (Fig. 3, 4). The mean number of nestlings per nest in urban areas appears to have risen to the level of the rural areas, in which it remained stable (Fig. 5 & 6). The differences in reproduction success between rural and urban birds have vanished. In other words, if there has been any increase in predation by Magpies or other predators at all, it has not lead to an decrease in reproductive success of Blackbirds. The decrease in the number of birds caught must have other causes, like increased mortality in fledged and adult birds.

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