The biology of individually colour-ringed European Blackbirds inhabiting two city parks in Szczecin (NW Poland) was studied in 1997-2003. In each park one to three observers watched the behaviour of Blackbirds every other day from dawn till afternoon (6-8 h a day); in this way almost all of their nests were discovered. 35 cases were recorded where pairs re-used their own nests, as well as two cases where the nest of another Blackbird pair and one Fieldfare nest were re-used. In 33 out of 81 cases observed, the re-use occurred after the brood had been successfully reared, while in two cases out of 378, female Blackbirds initiated the second breeding attempt in the same nest after the loss of the first one. The re-used nests were better concealed (80 ± 19% and 69 ± 18%, respectively) and had been built at greater heights than those abandoned after breeding (9.0 ± 5.0 m and 6.5 ± 4.2 m, respectively). No shortening of the interval between successive clutches was noted in the case of nest re-use (re-used nests 36.8 ± 4.9 days, newly-built nests 37.7 ± 8.0). The more and more frequent re-use of nests where breeding had been successful, their better concealment and higher sites, and also the lack of any differences in the intervals between successive clutches of pairs occupying old nests and those building new ones before the next breeding attempt, suggest that in the investigated population the basic reason for nest re-use was the insufficient number of safe nesting sites.
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