SALVANTE, K.G.*; VEZINA, F.; WILLIAMS, T.D.; Simon Fraser Univ., Burnaby, BC, Canada; Royal Netherlands Institute for Sea Research (NIOZ), Den Burg; Simon Fraser Univ., Burnaby, BC, Canada: Is avian egg production really costly?

Recent empirical studies have shown that the metabolic cost of egg production in passerine birds includes a 16-27% increase in resting metabolic rate (RMR). However, it is not known how this cost of egg production compares with other energetically costly functions, and whether these costs are additive. We investigated the metabolic costs of acclimation to low ambient temperature and egg production in a captive population of zebra finches (Taeniopygia guttata). Zebra finches were acclimated to either warm (21°C) or cold (7°C) ambient conditions. We used a repeated measures approach to compare the RMR of females measured as non-breeders and during egg production. RMR of the cold-acclimated females was measured at both thermoneutrality (35°C), to assess the metabolic cost of maintaining thermoregulatory function, and at 7°C, to assess the added cost of heat production. These costs were then compared to the added metabolic cost of egg production. We confirmed previous studies in that egg production was associated with a 27% increase in RMR in warm-acclimated females measured at 35°C. However, this increase was small compared to the 98% increase in RMR of cold-acclimated, non-breeding females (representing the cost of cold-acclimation), and the 228% increase in RMR of cold-acclimated, thermoregulating, non-breeding females (representing the cost of cold-acclimation and thermoregulation). Interestingly, in cold-acclimated or actively thermoregulating females there was no measurable additive effect of egg production, as there were no differences in RMR when females were measured as non-breeders and during egg production at either thermoneutrality or at 7°C. Therefore, in light of the large energetic capacity of these birds, the metabolic cost of producing eggs in favorable conditions may be minimal.