Relationships between vegetation structure and breeding bird densities in fallow cereal steppes in Castro Verde, Portugal

Abstract

Fallow fields represent a large proportion of cereal steppes in southern Portugal. A study of the bird populations using fallows in the Castro Verde region during the breeding season was made during spring 1996, with the objective of characterizing the bird community and describing relationships between bird density and vegetation structure, for selected species. For each of 50 transects, birds were counted once and variables related to vegetation structure were measured. Multivariate analyses were used to explore the relationships between bird density and habitat characteristics. The bird community of fallows was characterized by the numerical abundance of two species – Calandra Lark Melanocorypha calandra (8.5 birds/km) and Little Bustard Tetrax tetrax (4.5 birds/km) – which represented 60% of the total number of birds detected. Of the 28 bird species observed, only four others reached average relative densities greater than 1 bird/km: Great Bustard Otis tarda, Cattle Egret Bubulcus ibis, Corn Bunting Miliaria calandra and Short-toed Lark Calandrella brachydactyla. The variables influencing bird density were related mainly to vegetation height, cover by bare ground and presence of shrubs. The results suggest that agricultural management practices which promote the simultaneous presence of fallows with different habitat characteristics will increase species diversity at the local level. The models built were also used to predict changes in bird populations as a consequence of changes in grazing intensity and land abandonment.