EDELAAR P (1999) Comment on Is the breeding success of Blackbird Turdus merula declining in the Netherlands?. LIMOSA 72 (2): 66-67.

In a recent article, Dix et al. (1998, Limosa 71: 41-48) showed that increased predation of eggs and nestlings is not likely to be responsible for the observed decline in numbers of ringed Blackbirds. Indeed, the breeding populations seems to have rather increased. Unfortunately this paradox is left unresolved, but a change in Blackbird behaviour is dismissed.
      I discuss how several studies have shown that Blackbirds around human settlement have genetically reduced migratory behaviour. With the increase of the proportion of The Netherlands occupied by human settlement, it seems quite possible that overall, migrating Blackbirds have been replaced by non-breeding individuals. This would lead to lower catches per effort, despite a population increase.
      Since numerous studies have documented a change in migratory behaviour, time-series analyses involving migrants should always consider the possibility that changes in numbers are (partly) due to changes in migratory behaviour, not (just) due to changes in population size.

[free pdf] [dutch summary]