Environmental conditions can exert a strong influence on the growth and energy demands of chicks. We hypothesized that postnatal growth in a cold, aquatic environment would require a high level of energy metabolism in semiprecocial Red-throated Loon (Gavia stellata) chicks. We measured body-mass growth and daily energy expenditure (DEE) of free-ranging chicks in the Arctic. We used daily gains in body mass and DEE to estimate daily metabolizable energy (DME, kJ day-1) and total metabolizable energy (TME, kJ chick-1). Chicks gained body mass quickly, with a logistic growth rate constant 57% greater than the allometric prediction, yet were at only 60% of adult body mass at fledging. Males grew at a rate similar to that of females but for a slightly longer duration and so reached an asymptotic body mass 23% greater, and tarsus length 8% longer, than that of females. Chick growth performance was similar between first- and second-hatched chicks within broods of 2, which suggests that food availability was not limited. DEE increased in proportion to body mass, and DME peaked at 1,214 kJ day-1 on day 25 posthatching. Over the average 49-day postnatal period, TME was 49.0 MJ, which is within the range of error of the allometric prediction. Parents provided 58.6 MJ as food to meet this energy requirement. Given this chick energy requirement and the range of energy content of prey observed in the chick diet, selecting prey with higher energy content would greatly reduce adult provisioning effort. Red-throated Loon chicks did not have a high postnatal energy requirement, but rather grew quickly and fledged at a small size-with the effect of reducing the length of the postnatal period and, consequently, parental energy investment in chicks.
