Field Metabolic Rate and Food Consumption of Two Sympatric Hummingbird Species in Southeastern Arizona

We compared the field metabolic rate (FMR) and behavior around sugar-water feeders of sympatric territorial and non-territorial hummingbirds in the Chiricahua Mountains of southeastern Arizona during July 1987 and 1989. The territorial species was the Blue-throated Hummingbird (Lampornis clemenciae; mean mass 8.77 g) and the non-territorial species the Black-chinned Hummingbird (Archilochus alexandri; mean mass 3.67 g). FMR (CO2 production) and water turnover were measured using the doubly labeled water technique. FMR averaged 81.7 kJ/day (n = 4) in L. clemenciae and 29.1 kJ/day (n = 4) in A. alexandri. Mean mass-specific FMR was higher in L. clemenciae (18.36 ml CO2 g-' hr-') than in A. alexandri (15.58 ml CO2 g-' hr-'). This might be due to higher activity costs associated with the aggressive territorial behavior of L. clemenciae. Water influx averaged 1,734 ml kg-' day-' in L. clemenciae (n = 5) and 1,728 ml kg-' day-' in A. alexandri (n = 6). These values represent turnover rates equivalent to 185% and 245% of body mass per day respectively and are consistent with measurements made on other hummingbird species. Where these species coexist in the Chiricahua Mountains, L. clemenciae is a dominant territorial species whereas A. alexandri appears to be non-territorial, acquiring energy by robbing nectar from L. clemenciae territories. Lampornis clemenciae is highly aggressive against conspecifics, but appears to ignore intruding A. alexandri. Lack of territorial defense behavior in A. alexandri might reduce field energy expenditures and contribute to a lower FMR than L. clemenciae. We hypothesize that the lack of territorial aggression by L. clemenciae against A. alexandri is due to the high quality, abundance, and predictability of their food source which eliminates the profitability of such aggression. We also suggest that the intense aggression exhibited by L. clemenciae towards conspecifics might be motivated by factors relating to fitness other than defense of a food source.