Interplay between the energetics of foraging and thermoregulatory costs in the green-backed firecrown hummingbird Sephanoides sephaniodes

Abstract

In response to the interplay between variation in food quality and energetic demands, the foraging behaviour of captive green-backed firecrown hummingbirds Sephanoides sephaniodes was studied. Hummingbirds were exposed to two temperatures (25°C vs 15°C), two food qualities (0.5 vs 0.75 m sucrose solutions), and two costs of feeding (birds were provided with feeders with and without a perch). Food selection and consumption were measured, as well as time budgets and metabolic rate while feeding. We predicted that when given a choice, birds would minimize the cost of feeding by selecting feeders with a perch and with a high sugar concentration. However, rather than increasing energy consumption when energy availability was low and thermoregulatory demands were high, hummingbirds remained perched. They reduced feeding and spent most of their time perching. Our results identify a novel behavioural and physiological strategy in hummingbirds. These birds seem to shift their foraging behaviour depending on thermoregulatory and feeding costs. When these costs are high, rather than matching them with increased energy consumption, hummingbirds reduce energy costs by reducing activity. They seemed to adopt the following strategy: when food quality was high and thermoregulatory demands were low, they adopted a high-expense lifestyle. In contrast, when thermoregulatory costs were high, they adopted an energy conserving strategy even when food quality was high. We hypothesize that limitations imposed by physiological processes may explain why animals do not forage during all available time and why under some circumstances they choose foraging behaviours with lower rates of net energy gain.