A MODEL TO EXPLAIN MOLT-BREEDING OVERLAP AND CLUTCH SIZE IN SOME TROPICAL BIRDS.

Abstract

The tendency for avian clutch size to decrease with latitude is well documented (Lack, 1947, 1948; Lack and Moreau, 1965; Moreau, 1944; Skutch, 1949, 1967a). Most of the hypotheses proposed to explain this phenomenon suggest that energy (either food itself or the time used to gather it) is in some way limiting. Lack (1947, 1948, 1966) indicates that mean clutch size represents the largest number of young for which parents can find adequate food. Because shorter days (Lack, 1947) and heavy rains (Foster, 1974a) decrease the number of daylight hours available for feeding, tropical birds presumably can feed fewer young than can temperate forms. Ricklefs (1970) postulates that increased intersp,ecific competition among tropical as compared to temperate predators causes a reduction in food abundance (insects) and thus results in low productivity of predators, i.e., low clutch size among birds. Cody (1966) suggests that tropical birds must devote proportionally mo,re energy to improving predator avoidance and competitive ability than do temperate forms, even at the expense of energy for reproduction, because both predation and competition are higher in the tropics. In view of the above, the large number of avian species in the New World and African tropics which molt while breeding is of interest. Although most investigators have considered molt-breeding overlap only incidental to other studies, this phenomenon has now been reported (Foster, 1974b) for 122 tropical species (excluding all Australian arid region birds). Molt, which usually is temporally independent of breeding is an energetically demanding process. Thus species which overlap molt and breeding form something of a paradox. These birds appear to have additional energy available during the breeding period which could be devoted to reproduction but is not. Organisms which do not utilize for breeding all energy potentially available for that process presumably should be at a selective disadvantage. The purpose of the present paper is to define a set of circumstances in which the channelling of energy into molt, even at the expense of breeding, may be selectively advantageous.