Distinguishing Energy Maximizers from Time Minimizers: A Comparative Study of Two Hummingbird Species

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Abstract
SYNOPSIS. The potential reproductive success of a food energy maximizer increases with foraging time, while that of a foraging time minimizer increases with time spent in nonforaging activities given a set energy requirement has been met. How can these foraging “goals” be distinguished for nonbreeding animals in the field? If individuals of two species occupying the same habitat consume the same foods, face similar foraging constraints, and have similar meal sizes (food intake per foraging bout), then relative to a time minimizer, an energy maximizer should: (1) spend more time foraging, with greater foragingbout frequency, but no difference in foraging-bout duration; (2) spend less time sitting, with lower sitting-bout duration yet greater sitting-bout frequency; (3) gain mass more rapidly, if net energy intake results in mass accumulation; and (4) exhibit no other differences in time budgeting. These assumptions and predictions were verified by population- and individual-level comparisons of immature males of two species of nectar-feeding hummingbirds studied over three field seasons. The results suggest that, relative to each other, migrant Rufous Hummingbirds are energy maximizers and nonmigrant Costa Hummingbirds are time minimizers. Despite significant differences in time budgeting, by far the most striking difference between the species was that the Rufous gained mass four to eight times as rapidly as the Costa. This was due to the Rufous entering torpor at night, resulting in relatively little overnight loss in body mass. These patterns underscore the importance of measuring net energy intake as directly as possible (in this case by fat accumulation) in testing foraging theory. Indirect measures (such as time budgets) may not always provide the resolution necessary to detect important energetic differences between different foragers.

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