Physiological and biochemical correlates of brood size and energy expenditure in tree swallows

Intra-population variation in many fitness-related traits (e,g, clutch size ) is often attributed to variation in individual parental quality. One poss ible component of quality is the level at which each individual can expend energy while provisioning dependent young, We used breeding tree swallows ( Tachycincta bicolor) to test whether adults with large, natural-sized brood s and/or nestlings in good nutritional condition had relatively high daily energy expenditures (DEEs). Adults with high DEEs were predicted to have la rge internal organs and high metabolic capacities, We first measured the gr owth rate of nestlings in natural broods of five, six and seven over a 4-da y period and then measured parental DEE using doubly labelled water. Adults mere then dissected for analyses of body composition and to determine maxi mum enzyme activities in the pectoral muscle. Although the total mass gain of large broods was greater than that of small broods, parental DEE was ind ependent of brood size. We hypothesize that adults matched their clutch siz e (and consequently, brood size) to their individual foraging efficiencies. When statistically controlling for the effects of brood size, in one of tw o years there was a positive correlation between DEE and brood mass. This s uggests that among individuals rearing the same-sized broods there were rep roductive benefits of a relatively high DEE, There was no correlation betwe en either brood size or DEE and the mass of any internal organ or the metab olic capacity of the pectoral muscle.