Structure and seasonality of bird communities in the Sierra de Gredos (Avila, Spain)
Authors: SÁNCHEZ, A.
Published: Volume 38(2), December 1991. Pages 207-231.
Original Title: Estructura y estacionalidad de las comunidades de aves de la Sierra de Gredos
In this paper I describe the composition, structure and seasonal patterns of the bird communities of the main habitats of the Sierra de Gredos (Ávila, Spain). The censuses were carried out using the transect method in spring (May-June) and winter (December-February) in the years 1987-1989. The altitude is an important factor in the bird community organization, with additional effects derived of the different vegetation physiognomies. The influence of the altitude is slighter in spring, but very outstanding during the winter, specially on species richness. In the latter period a great number of birds disappear from the high and mid-mountain habitats without a remarkable wintering, long-distance migrants being an important part of the breeding community. On the other hand, the low latitude habitats are important wintering quarters for many birds that acid up to the residents, the transaharian fraction being much smaller. So, there are two kinds of seasonality in these mountains. A summer seasonality which peaks in the montane shrublands, with many summer migrants that are not replaced in winter; and a winter seasonality, characteristic of the mediterranean habitats which are occupied in winter by a great number of birds absent in spring. The most stable habitats from an ornithological point of view are the mid-altitude forests. The reduced proportion of long-distance migrants in the pine-woods and the relatively low seasonality of these habitats are discussed. The patterns in seasonality and the abundance of transaharian migrants in Gredos seem to obey more to purely bioclimatic factors (productivity) than to physiognomical ones (vegetation structure), and they fit well to the «bottleneck hypothesis» (relative productivity differences between seasons).