Comparative energetics of divergent foraging modes: a doubly labelled water experiment on hummingbird competition

Abstract

Two tropical hummingbird species with divergen foraging modes and competitive strategies were tested for energetic and behavioural responses during food competition. In a large flight cage, heterospecific pairs competed for limited food (sucrose solution) delivered to five artificial flowers (feeders) arranged in two food-density treatments. When feeders were tightly clumped, the dominant territorial species (the steely-vented hummingbird, Amazilia saucerottei) actively defended them, thereby reducing access by the subordinate ‘traplining’ species (the fork-tailed emerald, Chlorostilbon canivetii). As a consequence, the dominant species maintained relatively high mass-specific rates of energy intake (J/g/h) and a balanced energy budget; whereas the trapliner maintained low food intake, high energy expenditures, and suffered negative energy balance. When feeders were widely dispersed and infefensible, the trapliner obtained food at a higher mass-specific rate than the territorialist, but again suffered negative energy balance as a result of excessive energy expenditure. In contrast, the territorialist had low intake and expenditures, thus maintaining energy balance. Regardless of treatment, both species changed their behaviour in similar ways over the feeding day, reducing (1) visit rate to feeders, (2) diversity of feeders visited, (3) overlap between cagemates in feeders visited, and (4) time spent flying. After repeated interactions, each cagemate eventually concentrated its foraging on a disjunct subset of feeders, thus enhancing foraging profitability while partially mitigating the negative energetic impacts of competition for limited food.