Energetics of Growth and Maturation in Sympatric Passerines That Fledge at Different Ages

Abstract

We studied the energetics of growth and maturation of sympatric Western Bluebird (Sialia mexicana) and Ash-throated Flycatcher (Myiarchus cinerascens) nestlings in the San Jacinto Mountains, Riverside County, California. Both species fledge at a body mass of 27-28 g, but the flycatcher leaves the nest 4-5 days earlier than the bluebird. The growth curves of these two species become asymptotic by 14 days of age. Energy budgets, derived from estimates of CO2 production and energy accumulation in tissues, demonstrate that the average daily total metabolizable energy requirement of the nestling flycatcher is approxi- mately 22% (range: 16-32%) higher than the nestling bluebird. Most of this difference is due to a significantly higher field metabolic rate throughout the nestling period of the flycatcher. We attribute this higher energy expenditure to a greater daily allocation of energy to the metabolic processes and activity associated with maturation, which allows the flycatcher to fledge at a younger age than the bluebird. Because of the nestling flycatcher's higher daily energy requirements, adult flycatchers must gather up to 20 g more food each day than bluebird parents with the same number of young. Nestling mortality indicates that Ash- throated Flycatchers may be more susceptible to nest failure than Western Bluebirds during periods of poor food availability. Received 7 October 1988, accepted 7 July 1990. THE ENERGY requirements of young in rela- tion to the parents' ability to provide sufficient food to meet those requirements is an intrinsic relationship that affects the reproductive strat- egies of birds. Many factors influence the en- ergy requirements of growing birds. Differ- ences in body size, growth pattern, and the thermal environment of the nest site can affect the peak energy demands of a brood (Fiala and Congdon 1983, Ricklefs and White 1981, Mer- tens 1969, O'Connor 1975, Bryant and Gardiner 1979, Drent and Daan 1980). Our purpose was to compare the energy ex- penditure of sympatric nestling Western Blue- birds (Sialia mexicana) and Ash-throated Fly- catchers (Myiarchus cinerascens). The adult mass of both species is similar (range: 25-30 g), and nestlings fledge at a body mass ca. 27-28 g. The age at fledging differs by 4-5 days (bluebird: 20-21 days; flycatcher: 16-17 days). These two species have very similar breeding ecologies. The species share the same habitat, both are cavity nesters, and both are insectivorous dur- ing the breeding season. Because the breeding