Summary

We used the doubly labeled water (DLW) method to measure field metabolic rate (FMR) and water turnover during winter and summer in a very small (6.5 g) insectivorous desert passerine bird, the verdin (Auriparus flaviceps). Concurrently, we monitored weather conditions and used time-activity budget data of free-living birds and laboratory data on resting metabolic rate to construct time-activity laboratory (TAL) estimates of daily energy expenditure (DEE) and to partition the verdins' energy budget into thermoregulatory, activity and basal components. The FMR of adult verdins was 33.6+/−0.9 kJ day(−)(1) (mean +/− s.e.m.; N=14) in winter and 22.8+/−0.45 kJ day(−)(1) (N=7) in summer. FMR correlated negatively with the mean standard operative environmental temperature (T(es)) prevailing during the measurement period. TAL analysis produced DEE estimates that corresponded on average to within −0.9+/−2.4 % of our DLW-measured FMR values (range of individual values −18.3 % to +14.3 %). Metabolic expeditures for cold defense were 19.5+/−2.1 % of DEE in winter (computed assuming substitution of exercise thermogenesis for thermoregulatory costs in active birds). In the summer, thermoregulatory costs amounted to 9.0+/−0.4 % of DEE for keeping warm and 1.0+/−0.1 % of DEE for keeping cool in the heat. Activity costs were 21.0+/−0.5 % of DEE in winter and 17.5+/−0.1 % of DEE in summer. The water efflux of nonbreeding adult verdins was 3.9+/−0.2 ml day(−)(1) (624+/−22 ml kg(−)(1)day(−)(1)) in summer (N=5) and 3. 4+/−0.2 ml day(−)(1) (498+/−26 ml kg(−)(1)day(−)(1)) in winter (N=14). The water economy index (WEI; water efflux per unit FMR) of verdins was higher in summer (0.17+/−0.01 ml kJ(−)(1)) than in winter (0.10+/−0.01 ml kJ(−)(1)) and correlated negatively with mean T(es).