Migrant Passerine Birds in European Forest Successions in Relation to Vegetation Height and Geographical Position

Abstract

(1) The proportion of pairs of known trans-Saharan migrant species in the breeding population of passerine birds was examined in relation to vegetation height, geographical position and climate in twenty European studies of forest succession. (2) The proportion of migrant pairs was consistently low in the tallest vegetation, highest in vegetation 1-2 m high, and higher in northern and central Europe, than in southern and western Europe, respectively. (3) Using stepwise regression analysis, vegetation height and longitude were found to be the best predictors of the proportion of migrants; this was the case for the complete data set and for coniferous successions which were analysed separately. (4) The breeding densities of trans-Saharan migrants were greatest in the middle stages of succession at a vegetation height of 1-10 m. (5) Five main groups of migrant assemblages were identified in terms of successional stages and region; early successional stages (vegetation height < 1 m) supported distinct assemblages from the later stages with those in southern Europe being strikingly different to all others. (6) Diversity (measured by a dominance index) of migrant assemblages was lowest in western Europe and it tended to be lower in later than in early successional stages. (7) In western and southern Europe the diversity of migrant assemblages was greatest where migrants were abundant relative to other species, but this did not appear to be the case in northern and central Europe.