Seasonal changes in energy and water use by verdins, Auriparus flaviceps

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).