Prey availability influences habitat tolerance: an explanation for the rarity of peregrine falcons in the tropics

The density and productivity of peregrine falcon Falco peregrinus populatio ns correlate positively with distance from the Equator, while habitat speci ficity increases with proximity to the Equator. Low peregrine densities in the tropics may be a result of competition with similar congeners (e.g. the lanner falcon F. biarmicus in Africa), which replace them in many areas. A lternatively, tropical peregrines may be limited by resource deficiencies t hat do not affect their close relatives. Data from peregrine and lanner pop ulations in South Africa support the resource deficiency hypothesis, and th ere is no evidence to suggest direct competition between the two species. I n areas where prey are not spatially or temporally concentrated, or other,r ise particularly vulnerable to attack, morphological and behavioural specia lizations of peregrines probably restrict them to optimal foraging conditio ns. The relative dynamics of Arctic and temperate vs tropical prey populati ons is suggested as an important factor determining peregrine distribution globally. Populations of other widespread but particularly specialized avia n predators (e.g, osprey Pandion haliaetus) may be similarly controlled. Fo od limitation (in terms of a dearth of particularly vulnerable prey) in the tropics has resulted in specialization and rarity in peregrines and genera lization and relative abundance in similar congeners.