Rain, Feeding Behavior, and Clutch Size in Tropical Birds

Abstract

THAT avian clutch size generally decreases with latitude has been demonstrated repeatedly (Moreau 1944; Lack 1947, 1948; Skutch 1949, 1967; Lack and Moreau 1965). Several theories have been put forth to explain this. Two of the most diametrically opposed are those of Lack (1947, 1966) and Skutch (1949, 1967). Lack proposed that clutch size represents the average number of offspring that parents can nourish adequately so that the maximum number will survive to reproduce. In the tropics, presumably, parents can feed fewer young than can parents in temperate regions because of decreased daylength and increased competition and predation. Hence, they have lower clutch sizes. Adults still produce as many young as they can, and mortality is adjusted to balance natality. In view of this, the remarks of several ornithologists that tropical birds appear to have more time available for feeding young than they use are of interest (Skutch 1949, 1967; Wagner 1957; Miller and Miller 1968), particularly because Lack (1947) has suggested that decreased daylength in the tropics significantly decreases the time available for feeding young and thereby affects clutch size. Skutch on the other hand suggested that tropical birds do not rear as many young as they are capable of nourishing. According to his theory of adjusted reproduction, natural selection and density dependent regulating factors favor reduced clutch size in order to keep birth rate in balance with average annual mortality, which presumably is lower in the tropics. The birds do not produce as many young as they can, and natality is adjusted to balance mortality. One line of evidence that Skutch supplied to support this idea is an analysis of the time budget of tropical birds in relation to daylength. He showed that tropical parents should be able to feed at least one additional young. However his analysis contains an important fallacy in the assumption that all hours of daylight are equally usable by the birds. The present paper reviews Skutch's analysis to clarify the nature and usability of free time available.