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The forest Blackbirds in the Białowieża National Park constitute a migratory, monogamous, double-brooded population with a low frequency of inter-male and inter-female aggression. During most years they meet good feeding conditions reflected in: a large clutch size (4.5, in May even 4.8), high amount of food brought to nestlings, intensivemid-day feeding them, rarity of nestling starvation. Nesting losses (50-92%, mean 68%) were in 98% caused by predation or a threat of it. An average nest produced 1.3 nestlings, at a variable nesting success (8-50%). On average 2.5 young per pair fledged yearly; the most successful pairs reared 8-9 young. Nesting losses varied (50-81%) between habitats, seasons and years. The predators switch to other prey: in a low-rodent year they killed four times more Blackbird nestlings than in a rodent year, and in high-caterpillar years smaller predators left more bird eggs undestroyed. Breeding ecology of Blackbirds in the primaeval forest is shaped mostly by a strong predation pressure varying in space and time.
In the Białowieża National Bark, NE Boland, the Blackbird populates the whole extensive forest as a forest-interior species. The following mean densities were recorded in primaeval stands: 2.5 pairs/10 ha in riparian ash-alder, 2.2 in oak-lime-hornbeam and 0.7 p/10 ha in conifer-dominated stands. An approximate territory size was 1.1 -4.3 ha, usually 22-2.7 ha, which is 11 -43 times smaller of that found in Oxford Botanical Garden and some С-European urban parks. The density along the forest edge was slightly higher in some years only. In primaeval stands the species nests in trees, often in tree-holes, on average 5.3 (0.3-24) m above the ground. Nests placed above 7.5 m were twice more successful than the lower ones. The Blackbird's nesting in low bushes appears to be a feature reinforced under anthropogenic conditions, like its preference for the forest edge or its foraging in the open. It is argued that the Blackbird was primarily a species of old high-stemmed lowland and submontane forest.
In urban areas of Łódź (Central Poland), the vertical and horizontal distributions of the Blackbird Turdus merula and Song Thrush Turdus philomelos nests were investigated in relation to running lanes and paths as places of penetration of humans. A significant effect of human penetration on vertical and horizontal distribution of Blackbird nests in parks was found. The park population of Blackbirds, unlike forest-breeding ones built nests more distantly from the lanes, lower, and under the shelter of shrubs. In the Blackbird park population, 74.2% of the nests were built in shrubs. The mean nesting height was 1.18 m. Distribution of distance between the nest and lanes are negatively skewed to = -0.57). The reverse was true of the forest population of this species where81.9% nests were built in trees. The mean nesting height was 1.29 m. Nests of the forest population of Blackbird were distributed randomly. Differentiation in nest sides between urban and forest populations of Blackbird is a statisticaly significant. Only one pair of the Song Thrush was recorded in a parks. The forest population of the Song Thrush nesting in the trees, mean nesting height was higher than those of the Blackbird. The presence of people in thef orest had a significant negative effect on the horizontal distribution of Song Thrush nests. Therefore, human disturbance is considered the factor that prevents this species from colonising urban parks (contrary to the situation in West European countries).
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