Interindividual variability in body composition and resting oxygen consumption rate in breeding tree swallows, Tachycineta bicolor

Basal metabolic rate is one of the most widely measured physiological traits. Previous studies on lab mice and field-caught lizards suggest that individuals with relatively high basal metabolic rates or standard metabolic rates have relatively large masses of metabolically active tissues (e.g., heart, kidney, liver). As these are energetically expensive organs, there may be variability between breeding seasons dependent on, for example, availability of prey and capacity for energy intake. We present data from breeding tree swallows (Tachycineta bicolor) collected over two successive seasons. There was no difference between years in resting oxygen consumption rates, although there were significant interannual differences in the masses of all organs and tissues except the pectoralis. Interindividual differences in the masses of the kidney and small intestine explained 21% of the variation in oxygen consumption rates. Although individuals with relatively high resting oxygen consumption rates had relatively large, metabolically active kidneys, they had relatively small intestines and pectoral muscles. This is in contrast to all previous studies on mammals and to the single interspecific study of birds. Oxygen consumption rate also correlated positively with hematocrit. Our results suggest that assumptions of consistent positive relationships between resting oxygen consumption rate and organ masses cannot be extended intraspecifically for birds.