VEZINA, F.; WILLIAMS, T.D.; Simon Fraser University, Burnaby, B.C., Canada: What Drives the Metabolic Costs of Egg Production in Birds?

Body composition in vertebrates is known to show phenotypic plasticity, and changes in organ masses are usually rapid and reversible. One of the most rapid and reversible change is the transformation of the female avian reproductive system prior to breeding. Although many studies have investigated variation in basal metabolic rate (BMR) through relationships between organ masses and BMR, no study to date has related the effect of plasticity in the avian reproductive machinery to BMR. We compared body composition of female European starlings ( Sturnus vulgaris) between reproductive stages over 3 years and investigated the pattern of changes in organ mass during follicular development and ovulation. In laying females we analyzed the relationship between organ mass and resting metabolic rate (RMR). Our analysis revealed marked variation in organ masses between breeding stages but with no consistent pattern among years except for kidney and pectoral muscle. An important finding is that the growth and regression of the reproductive organs is very rapid with the oviduct gaining 62% of its 22-fold increase in mass in only 3 days. Reabsorbtion is just as rapid and begins even before the final egg of the clutch is laid. On average, 42% of the oviduct mass is lost before laying the final egg. In laying females, 21% of the variation in RMR is explained by the mass of the oviduct (r2 = 0.15) and heart (r2 = 0.06). We suggest that the non-reproductive organs are more affected by changes in ecological conditions than the reproductive state itself and that the activity and maintenance cost of the oviduct is high enough that there has been selection to regress this organ as soon as possible after the last ovulation.