Energetic Consequences of Sexual Size Dimorphism in Nestling Red‐Winged Blackbirds

The energy budget of nestling Red—winged Blackbirds (Agelaius phoeniceus) was determined using doubly labeled water (3HH18O) to measure field metabolic rate (FMR) and body component data to measure growth energy. Sex—specific measurements permitted the evaluation of the effects of this species' substantial sexual size dimorphism on FMR and total energetics. FMR averaged CO2 release of 5.12 mL°g—1°h—1, or 0.129 kJ°g—1°h—1, with no significant differences between the sexes. Daytime FMRs of CO2 production (5.34 mL°g—1°h—1) were higher, but not significantly so, than nighttime FMRs (4.45 mL°g—1°h—1). Water influx averaged 0.95 mL°g—1°d—1, with daytime rates (1.22 mL°g—1°d—1) significantly higher than nighttime (0.40 mL°g—1°d—1) rates. Total assimilated energy from hatching to fledging was 1014 and 797 kJ for male and female nestlings, respectively. The sexual differences in total energetics reflected differences in body size of the nestlings and suggest that there is a greater cost to the parents in raising males than in raising females.