Nest site selection in the Blackbird was investigated in two urban parks in Szczecin from 1997 to 2003. The age structure of the tree stands, the area of shrub coverage and the number of predators (apart from squirrels) were similar in both parks. 95% of the nests discovered at the beginning of the breeding season were found again in June and July. Any increase in the heights of the nest sites in successive periods of the breeding season and any changes in the type of vegetation selected for nest construction were recorded. In April, Blackbirds most often used coniferous trees. At the start of the season, when deciduous plants began sprouting leaves, Blackbirds preferred those whose leaves appeared earlier. But later in the season, no difference was found between the numbers of nests in trees developing their crowns earlier or later. The shorter period of nest use in conifers is probably due to their selective penetration by corvids. The selective penetration of such trees by predators probably reduces the frequency of nest building in them between the first (pentads 1–3) and second (pentads 4–6) period of the breeding season, despite the fact that they provide better concealment for nests. The selection of nest sites by the Blackbirds in this study confirms both the predator-pressure and the nest-concealment hypotheses.
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