Evidence for within-individual energy reallocation in cold-challenged, egg-producing birds

Recent studies have shown that the metabolic cost of avian egg production involves a 16-27% increase in metabolic rate (MR) above non-reproductive basal or resting values (BMR and RMR, respectively). To determine how the metabolic cost of egg production interacted with the costs of other essential processes (such as cold acclimation and active heat production), we measured the MR of non-breeding and egg-producing zebra finches (Taeniopygia guttata) while (a) warm-acclimated (to 19-21 degrees C) and measured within their thermoneutral zone (at 35 degrees C), (b) cold-acclimated (to 7 degrees C) and measured at thermoneutrality (at 35 degrees C, i.e. not actively producing heat), and (c) cold-acclimated and measured below thermoneutrality (at 7 degrees C) (i.e. during active heat production). The metabolic cost of egg production was small (24% above BMR) compared with the additive costs of cold acclimation and active heat production (224% above BMR). Exposure to low ambient temperatures was accompanied by an increase in seed consumption (by 72%) and a decrease in locomotor activity (by 72%) compared with warm-acclimated, non-breeding values. By contrast, egg production in heat-producing females was associated with an 11% decrease in MR and a 22% decrease in seed consumption compared with non-breeding thermoregulating values. Our data suggest that while the increase in MR associated with egg production is small in relation to the birds' capacity to increase MR in response to other energetically demanding processes, the addition of egg production to these metabolically costly activities may be enough to necessitate the use of energy-saving strategies, such as internal energy reallocation, to cope with the additional energetic demands.