A study of Tree Sparrows was conducted near Warsaw, central Poland. During the breeding season, nest boxes were checked to record the presence of Tree Sparrow nests and broods. Nestlings, juveniles, and adults captured in mist nets were ringed with different combinations of colour rings to identify their age during visual observations in the autumn sexual display period. Before the autumn display, breeding nests were dyed in order to identify nest material added during the autumn display period. In winter, nest boxes were inspected to catch the birds roosting in them at night. The study was conducted in optimal and marginal habitat types. In the optimal habitat during the autumn sexual display, adult birds were much more abundant than in the marginal habitat. During the breeding season, 41% of the nest boxes were occupied in the optimal habitat, compared with 8% in the marginal habitat. The respective figures during the autumn display were 95% and 45%. Autumn nests were built in 83% and 12% of the nest boxes, respectively, and in winter, 35% and 7% of nest boxes, respectively, were used by birds for night-time roosting at night. The autumn display continued from early September to the end of October. For roosting at night in winter (November-March), Tree Sparrows selected nests according to their insulating quality. Most often they roosted in nest boxes containing nests from the breeding season with autumn nests built over them, then, in descending order of frequency, in nest boxes with autumn nests built in empty boxes, in boxes with breeding nests, and in completely empty boxes. Among birds roosting at night and captured on the first survey in winter, 86% were represented by pairs that had built those nests during the autumn display. Young birds that did not build autumn nests typically roosted at night in tree crowns. This implies that the construction of autumn nests is primarily a consequence of the autumn sexual display, and secondarily may be an adaptation for winter survival. The winter survival rate was significantly higher in juvenile Tree Sparrows that were found in nest boxes on winter nights than in those that were not.
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