BURNESS, G.P.*; YDENBERG, R.C.; HOCHACHKA, P.W.: Physiological origin and reproductive consequence of variation in daily energy expenditure: a study of breeding tree swallows

We studied tree swallows (Tachycineta bicolor) provisioning dependent young to test for relationships among parental daily energy expenditure (DEE), the masses of internal organs/tissues, and indices of breeding success. We predicted: (1) parental DEE would increase with brood size and nestling condition, and (2) adults with high DEE would require large internal organs and high metabolic capacities in their pectoral muscles. We measured the growth rate of nestlings from natural broods of 5, 6 and 7 over a 4-day period, followed by parental DEE using the doubly labeled water technique. Adults were then dissected and internal organs were weighed. As an index of capacity for flux through specific steps in various metabolic pathways, we measured the activity of citrate synthase, pyruvate kinase, HOAD, and lactate dehydrogenase in the pectoral muscle. The growth rate of nestlings was independent of the brood size in which they were reared. This indicated that adults did not trade-off nestling quality for quantity. Despite predictions, parental DEE was also independent of brood size. We hypothesize that adults differed in foraging efficiency and matched their clutch size to their own individual provisioning abilities. Among individuals with the same sized broods, in one of two years there was a positive relationship between DEE and brood mass, suggesting reproductive benefits of an high DEE. There was no relationship between an individual’s DEE and the size of any internal organ or the metabolic capacity of the pectoral muscle. Consequently, a previous suggestion that large internal organs allow for the maintenance of high-energy budgets is not supported.