This paper examines the pattern of variation of the percentage of tropical migrants (PMT) in Iberian bird communities (Table I). PMT varies significantly among the different habitat types, being higher in shrublands and broadleaved woods than in conifer woods. There seems to exist a trend towards the highests PMT to occur in the different biotopes of the supramediterranean stage, as compared to comparable habitats in other bioclimatic stages. Results are interpreted in view of several proposed hypotheses (MACARTHUR, 1959; HERRERA, 1978a; BILCKE, 1984). It is suggested that the greater cover and complexity of pinewoods in winter, besides the particular nature of their foliage, can promote sedentarism and specialization in its residence avifauna that can be constraining the colonization og these woods by birds wintering in tropical Africa.
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