Vegetational versus topographical effects on forest bird communities: a test in the Cantabrian Mixed Forest Ecoregion (Spain)
Published: Volume 57(2), December 2010. Pages 285-302.
The physiognomy and floristic composition of vegetation shape the structure of bird communities on a local scale. However, the usefulness of these features in identifying suitable areas for birds can be obscured by other factors, such as altitude, climate and regional landscape patterns. This occurs on large spatial scales (e.g. across latitudinal gradients), and also on smaller, regional scales (e.g. mountains). We explored how topography and vegetation affect breeding and wintering bird communities in mountain forests of the Cantabrian Mixed Forest Ecoregion (Spain), an area where many Palaearctic forest birds reach the southwestern boundary of their ranges. The work was carried out on nine extensive seminatural forests distributed along the northern slopes of the Cantabrian Mountains from near 400 m to 1,300 m a.s.l. Birds were censused in 78 line-transects 500-m long. Ten physiognomic and floristic variables were measured in each transect. The effects of forest variables, altitude and slope orientation on bird richness and abundance were examined with principal component analysis and general linear models. The results support that, at the study scale, the effects of vegetation on bird communities are very scanty and limited to a positive effect of oak stands on bird richness in winter. Altitude was an outstanding predictor of bird distribution, especially in winter, with the lowest-level forests having the highest bird richness and abundance. This pattern differed quantitatively among species with different foraging niches, since trunk gleaner birds were hardly affected by altitude, whereas the numbers of ground searchers decreased severely. The conservation of the Cantabrian bird communities requires the strict protection of the entire altitudinal forest range, and particularly the existing low altitude forests. Likewise, there is an urgent need to conserve all of the extant oak forests at whatever altitude.