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The evolution of organisms leads to the elimination of behaviors that are costly in terms of energy. One of such behaviors in the Tree Sparrow Passer montanus is the autumn display during which these birds construct nests. The purpose of this paper is to find out if this behavior is a part of the strategy for winter survival.The 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. Before the autumn display, breeding nests were dyed in order to identify nest material added in the period of autumn display. Nestlings in nest boxes, juveniles, and adults captured in mist-nets were banded with different combinations of color bands to identify their age during visual observations in the period of autumn sexual display. Juveniles and adults caught in mist-nets were classified as molted or not molted birds. In winter, nest boxes were checked to catch the birds roosting in them at night. Autumnal sexual display in Tree Sparrows is similar to the spring display. Both adults and juveniles leave the breeding colony in August and return after molt. They form pairs, copulate and build nests in fall. The autumn display is continued from the first ten days of September to early November. The number of Tree Sparrows participating in the autumn display increases with the percentage of the birds that completed molt in the population. In the first half of September, 16% of the population completed molt, while 99% in mid-October. On the average, adult birds formed pairs on 11 September (SD: 7.7 days), and juveniles on 17 September (SD: 8.0 days). Nest construction was started, on the average, 14.2±8.7 days after pairing. The advance in nest building was dependent on the time of pair formation. Intensive nest building took place in the last 10-days period of September and in October. Early in November, nest building ceased with the onset of cool weather. The last birds to pair did not construct complete autumn nests and in winter they roosted in shrubs or in tree crowns . The building of autumn nest as a consequence of the autumn display, serving as a roosting place in winter, can be a consequence of natural selection promoting this behaviour.
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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