WHY FORAGING BIRDS IN TREES SHOULD CLIMB AND HOP UPWARDS RATHER THAN DOWNWARDS

Abstract

SummaryThis paper describes the energy cost of locomotion in birds foraging over vertical zones in trees. In particular, the energetically cheapest pattern for a bird flying among trees and moving within them is explored. For birds moving vertically by climbing and hopping (but not by flying) it should take less energy to climb and hop upwards in a tree and fly downwards to the next one, than to do the reverse. This is because part of the potential energy gained in climbing upwards may be used for subsequent horizontal progression to the next tree. For movements the other way, the potential energy is largely wasted during downward hopping and climbing within a tree.It is predicted that birds moving within trees by climbing and hopping (but not by flying) leave at a higher level than they arrive (whether the vertical movements within trees are along the trunk or among branches).These energetic considerations probably expose one selection pressure behind the morphology of the woodpecker—treecreeper type, which shows obvious adaptation for climbing upwards rather than downwards.