The study was carried out in 1999–2000 in a 405 ha area urban habitat and revealed one of the highest densities (56.8 pairs/km2) for the species. Breeding density was best predicted by the diversity of trees in the plot and was not limited by the presence of tall trees. Birds used 69% of the available tree species and preferentially selected half of them. Taller tree species were preferred to shorter ones and coniferous trees were preferred to broadleaved ones. Nests higher above the ground were more likely to be successful than nests lower down. The greater the nest height, the earlier the Magpies laid eggs and the more fledglings they produced. Pairs nesting in conifers laid earlier than those in broadleaved trees but there were no significant differences in clutch-size, hatching or fledging success between the two habitats. The nest site preference was not well explained by the success rate. Regardless of species, not only tall trees, where success was greater, were preferred but also smaller ones, despite the lower success rate. Nests in conifers were no more successful than those in broadleaved trees, despite the marked preference for conifers. Magpies that succeeded in their final breeding attempts in one season were less likely to make a contrasting nest-site choice the following season. The lack of a clear adjustment between the nest-site selection pattern and breeding success was hypothesized to be due to two non-exclusive mechanisms: 1) either there are no strong selective pressures at any type of nest-site and on the behavioral plasticity of Magpies or else selective pressures may change during the season; 2) the dependence of nest-site selection on bird quality.
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Vol. 37 • No. 2