Abstract In endotherms, metabolic performance is associated with a wide array of ecological traits, including species distribution. Researchers have suggested that the northern boundaries of North American passerines are limited by their ability to sustain the high metabolic rates required for thermoregulation. Black-capped chickadees (Poecile atricapillus; BC) are year-round residents in most of Canada and the northern half of the United States, whereas Carolina chickadees (Poecile carolinensis; CA) are found exclusively in the southeastern United States. These species hybridize along a narrow contact zone that has been moving northward at a rate of about 1.6 km per decade, coincident with warming temperatures in Ohio. The location of the chickadee hybrid zone in Ohio closely matches air temperature isotherms, further suggesting that metabolic rate may correlate with distribution in these species. We tested the hypothesis that distribution patterns of chickadees are linked with their rate of metabolism. For populations of BC and CA chickadees, we measured basal metabolic rates (BMRs) and cold-induced peak metabolic rates from areas that differ in winter temperatures and supplemented this information with data from other studies. Although our findings suggest a general relationship between lower air temperatures and higher metabolic rate among black-capped chickadee populations, this trend was not robust across all locations. There was no significant relationship between lower air temperatures and metabolism in Carolina chickadees. Within Ohio, hybrids had a significantly higher mass-corrected BMR than either parental species. We suggest that the mtDNA-nDNA mismatch of hybrids may produce less efficient mitochondrial protein complexes, which in turn affects the efficiency of ATP production, thereby increasing rate of oxygen consumption to meet ATP demands.
