Oecologia (Berlin) 65:387-393 Oecologia 9 Springer-Verlag 1984 Consequences of homeothermic capacity of nestlings on parental care in the european starling L. Clark* Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, USA Summary. The homeothermic capacity of chicks varied as a function of brood size, age, and air temperature. Commit- ment to brooding by parents also varied as a function of brood size, age of the young brooded, and prevailing air temperature. It was experimentally determined that parents altered their brooding commitment in direct response to the achieved mean homeothermic capacity of the brood rather than energy demands of the brood per se. Because larger broods achieved a given level of homeothermic ca- pacity earlier than smaller broods, parents spent less time brooding larger broods. This "freed" time represented an increase in potential foraging time by the parents. However, there was no evidence that parents used this potential in- creased foraging time to elevate the energy return to the nestlings. Other possible advantages of a facultative brood- ing response by parents are discussed. Atricial young begin life as ectothermic-poikioltherms then undergo a rapid physiological transition to endothermic- homeothermy. The ability of chicks to maintain body tem- perature (Tb) against a thermal gradient is initially due to thermal inertia, then subsequently to an increased capacity for thermogenesis and increased insulation (King and Farner 1961; Dawson and Hudson 1970; Dunn 1975; Marsh 1979, 1980). Prior to achieving a thermal indepen- dence (endothermic-homeothermy) altricial young are fre- quently brooded by their parents. Brooding functions to externally supply the heat energy necessary to maintain a high, constant T b for the chicks. This is important since physiological properties of the young, such as metabolism, digestive efficiency, and rapid growth are temperature sensi- tive (Brody 1945; King and Farner 1961). Hence, brooding behavior is presumably the result of selection pressure to provide a temperature environment which optimizes the rates of physiologic processes at a time when the chicks are incapable of doing so for themselves. Parents decrease their brooding commitment to the young as the chicks grow older and the homeothermic ca- pacity of chicks improves and/or the energy requirements of the chicks increases (Hudec and Folk 1961 ; Morton and Carey 1970; Askenmo 1977; Korpimaki 1978). However, workers have been unable to determine precisely the proxi- mate mechanisms responsible for this pattern. Recently, * Present address: Monell Chemical Senses Center, 3500 Market Street, Philadelphia, PA 19104, USA more attention has been directed towards the ontogeny of homeothermy and endothermy under ecologically realistic conditions. Chicks exist in the wild within a nest and with broodmates. This may substantially alter the heat flux char- acteristics of individuals, thereby affecting the onset of ho- meothermy (O'Connor 1975a, b; Dunn 1976, 1979; Clark and Balda 1981; Clark 1982). For example, in nestling star- lings (Sturnus vulgaris) of the same age, chicks within a brood of seven will attain a given level of homeothermy at least four days earlier than chicks within a brood of three when exposed to similar environmental conditions (Clark 1982). Nest placement in the habitat by parents may further influence the heat flux of chicks (sensu Walsberg and King 1978). To the extent that brooding takes time from foraging, a conflict may exist for the allocation of parental behavior (Wittenberger 1982; Johnson and Best 1982). Brooding pre- sumably minimizes the risk of death by exposure for young whose capacity for endogenously regulating T b is poor. But parents must also gather food adequate to support their own maintenance needs as well as those of their chicks. Whether or not these requirements together may exceed the capacity of the parents to fulfill them under certain conditions has not been demonstrated for altricial birds. The focus of this study is to determine to what extent star- lings alter the allocation of parental care in response to food demand and homeothermic capacity of chicks. Natu- ral variation in brood size, associated with variation in the onset of homeothermy provides an excellent opportunity to address the question of brooding and feeding commit- ment, and serves to shed light on mechanisms by which some brood sizes are more productive. The question of whether some conditions alter the limits of parental care or whether the evolution of the development pattern of chicks was affected by such time limitation considerations is to be addressed elsewhere. Materials and methods Laboratory experiments Handling of young. I obtained starling nestlings from two source colonies of 90 nest boxes each. Individually marked chicks of known age were removed from nest boxes ca. 45 rain prior to dusk and transported to a field laboratory at the Stroud Water Research Center, Avondale, PA in 1980 and Waterloo Mills Research Station in 1981. Trans- port to the laboratory took between 20 and 45 rain. I kept