Abstract
Seasonal changes in birds' energy requirements as well as in environmental food availability and quality, influence birds' capability to obtain and digest food (Karasov 1990). Increased metabolic rates, hypertrophy of the gastrointestinal tract, and increased absorption of nutrients may be viewed as responses ofwintering birds to physical and biotic seasonal habitats (Sibly 1981, Dykstra and Karasov 1992). In central Chile, the climate is seasonal with warm, dry summers and cool rainy winters; mean annual temperature is 22.1C and mean annual minimum temperature is 7.7C (di Castri and Hajek 1976). One of the most conspicuous resident birds of the mediterranean environments in central Chile is the Rufouscollared Sparrow Zonotrichia capensis (Emberizidae). This species is opportunistic, feeding on seeds and insects and showing seasonal dietary shifts according to food availability (L6pez-Calleja 1995). Individuals of this species also exhibit seasonal changes in metabolic rates and thermal insulation (Novoa 1993). Consequently, Z. capensis lives under marked seasonal variations in energy availability and requirements. Specifically, when nutritional demands increase during winter, birds may respond to the seasonal environment through behavioral, anatomical and physiological changes that allow the maintenance of a positive energy budget and consequently permit survival. The aim of this work is to examine the relationship between seasonal changes in diet, digestive tract morphology, and digestive efficiency ofRufous-collared Sparrows in central Chile.
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