Seasonal Metabolic Variation in Birds: Functional and Mechanistic Correlates

Abstract

The influence of seasonal changes in temperature and climate on metabolic rates in birds has been a topic of interest to ornithologists and ecophysiologists for decades (e.g., Hart 1962; Dawson 1958; Miller 1939). Because metabolic rates increase linearly with temperature in endotherms outside the thermal neutral zone, comparisons of metabolic rates among seasons or species require standardized measurements of metabolic rates. The most common of these standardized metabolic rates used for comparisons of energetics among seasons or species is basal, or resting, metabolic rate. It often serves as a baseline for comparisons of metabolic costs of activities within species, and for comparisons of the “rate of living” among species or species groups (e.g., Wiersma et al. 2007a; White et al. 2007; McKechnie et al. 2006; McKechnie and Wolf 2004; Trevelyan et al. 1990; McNab 1988; Bennett and Harvey 1987; Kersten and Piersma 1987). Theoretically, basal metabolic rate (BMR) is the minimum metabolic rate required for maintenance in endotherms. BMR is measured within the thermal neutral zone under postabsorptive digestive conditions during the resting phase of the daily cycle on resting, nongrowing, nonreproductive animals (McNab 1997). It is doubtful whether truly BMRs can ever be achieved in the laboratory, so the term resting metabolic rate (RMR) is often used to refer to such measurements, even when the standard conditions for BMR have been met. Here, I will revert to the standard terminology and consider BMR as the metabolic rate measured under the standard conditions listed above, recognizing that this may not, in fact, represent truly basal rates.