Field Metabolism of Free-living Female Savannah Sparrows during Incubation: A Study Using Doubly Labeled Water

We measured field metabolic rates (FMR) of female Savannah sparrows (Passerculus sandwichensis) during the incubation period for a resident population living in Baja California, Mexico. We used a single-sample variant of the doubly labeled water (DLW) method that did not require extended capture time for equilibration of isotopes and hence reduced stress. Simultaneously with the DL Wmeasurements, measurements of operative temperatures were made by means of taxidermic mounts. After calculating standard operative temperatures for each female that we injected, we partitioned FMR into maintenance metabolism (basal plus thermoregulatory costs) and the energy available for other activities such as foraging and providing heat to eggs. Carbon dioxide production averaged 128.9± 12.2 mL CO₂/h, or 76.1 kJ/d, for incubating female sparrows (N = 16). Female mass averaged 18.7±1.0 g; all females lost mass during FMR measurement, with an average change of -2.6%. Values for maintenance metabolism ranged from 42.0% to 68.5% of FMR, averaging 55.1%. Though incubating females had FMRs similar to those of females feeding nestlings, their available foraging time was reduced by more than 50%. Because of this reduction in foraging time and added energy expenditure to warm eggs, we suggest that, during incubation, considerations of energy balance may be important to the female, whose mate provides little or no assistance.