The Peregrine Falcon Falco peregrinus is a specialized predator of birds. It occurs almost worldwide but is generally uncommon. In many areas, it is sympatric with one of a complex of similar, less specialized, congeneric species (subgenus Hierofalco, the desert falcons). Peregrine density and productivity tend to decrease with latitude, while desert falcons may be most successful in the tropics. This study compares the biology of Peregrines and sympatric Lanner Falcons F. biarmicus in South Africa, and examines the relative influence of resource limitation and interspecific competition with congeners on the natural regulation of Peregrine populations in tropical environments.