Optimally Foraging Hummers

photograph of Rufous Hummingbird by Rohan Kamath
Do animals gather food efficiently -- that is, in a way that maximizes their intake per unit of effort? This question has long interested ecologists and has led to the formulation of optimal foraging theory, which describes what sorts of behavior should be observed if the foragers are feeding with a minimum of wasted effort. For example, an animal defending a feeding territory should make that territory large enough to maximize its energy "profit." Territory size would be optimal when there is the greatest possible difference between the amount of energy that could be obtained from the territory and the expenditure of energy required to gather food from it and patrol and defend it.

So much for what should happen according to the theory. But does the theory
reasonably describe what actually goes on in nature? It has proven quite difficult to test optimal foraging theory in the field. Among other things, periodically catching an animal to measure its weight gain (or loss) under different conditions is almost certain to modify its behavior. But biologists R Lynn Carpenter, David C. Paton, and Mark A. Hixon found an ingenious way to test optimal foraging theory, using Rufous Hummingbirds. These hummers establish feeding territories during stops on their 2,000-mile migration between their breeding grounds in the Pacific Northwest and their wintering habitat in southern Mexico. They zealously guard those territories, driving off hawkmoths, butterflies, other hummers, and even bees that might compete for the nectar. In addition, they deplete the nectar resources around the periphery of their territories as early in the day as they can, in order to out-compete other nectar-sippers that might try to sneak a drink at the territory edge.

When half of the flowers in a territory were covered with cloth so the birds could not drain them, Carpenter and her coworkers found that the resident hummer increased its territory size. This showed that territoriality was tied to the availability of nectar, and that the bird could in some way assess the amount of nectar it controlled. Then, by substituting a sensitive scale topped by a perch for the territory-holder's traditional perch, they were able to measure the bird's weight each time it alighted. The researchers found that the hummers optimized their territory size by trial and error, making it larger or smaller until their daily weight gain was at a maximum.

In this case of migrant-territorial hummers, theory accurately predicted how a bird behaves in nature. But when hummingbirds are not migrating, things seem to get more complicated. For example, hummingbird student William Calder of the University of Arizona observed that, near the Rocky Mountain Biological Laboratory in colorado, territories were larger than usual in 1986. That was a year when hummers were rare and, as a result, nectar resources were superabundant. Apparently males made their territories larger in order to increase their chances of finding a mate. So theory that explains behavior in one set of circumstances may have to be modified before it accurately predicts what will happen in other circumstances, as when reproductive and foraging strategies cannot simultaneously be optimized.
SEE: Territoriality; Migration; Hummingbirds, Nectar, and Water.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.