ECOLOGICAL TRANSITIONS IN BIRDS - THE CAS E OF PALEARCTIC MIGRANTS TOTHEIR AFRO-TROPICAL WINTER QUARTERS

Environmental pressures that migrant birds experience when changing fr om their breeding area in the Palearctic to their winter quarters in t he Afro-tropics raise questions about the nature and plasticity of eco logical niches. Though some species apparently keep the same ecologica l niche during their annual cycle, others must change it either along trophic dimensions or along habitat selection characteristics. The nec essity to conduct demographical and population studies is stressed, ta king into account factors acting during each stage of the annual cycle , not only during the breeding season. The study of ecological transit ions in the Afro-tropics-Palearctic migration system bears a real sign ificance for conservation biology.