Hummingbird Foraging Bouts
photograph of a Rufous Hummingbird by Rohan Kamath
other small animals that are "warm-blooded" (that is, like
us, maintain a high body temperature by generating metabolic
heat), hummingbirds need a prodigious energy intake. In
spite of this, Rufous Hummingbirds don't feed constantly. In
fact, they make only 14-18 foraging bouts per hour, each
taking less than a minute. For the remainder of the hour the
bird perches quietly. Ecologists William Karasov, Duong
Phan, Jared Diamond, and Lynn Carpenter discovered the
reason for this inactivity.
They found that the hummers pass nectar through their digestive tracts very rapidly -- average transit time through the gut is less than an hour. In this short time, they are able to extract about 97 percent of the sugars from the nectar. But why do they sit
around so much "doing nothing" when they could be sipping
more nectar? The answer was discovered with a clever
experiment using radioactive isotopes as tracers to follow
what happens to the nectar. In fact, the "resting" hummers
aren't "doing nothing" -- they are emptying their crops
(specially modified parts of the digestive system that store
food immediately after it is taken in).
They apparently wait until the crop is about half empty before foraging again, and it takes about four minutes for this to happen (which would account for the roughly 15 bouts of nectar gathering per hour). They forage only as often as required to keep up with the rate at which the crop can pass nectar into the rest of the digestive system; more frequent foraging would carry a high energy cost but provide no further benefit. While it is emptying its crop, therefore, the bird conserves energy by remaining immobile.
The hummers don't have room to take in any more nectar until the crop is partly drained. What limits the rate of crop emptying is not yet clear, but it is probably how fast the intestine can absorb the sugar, or how fast the stomach can acidify the crop contents (an important step in digestion). As Diamond and his colleagues say, ". . . despite external appearances, hummingbirds may be energy maximizers, taking in energy as rapidly as their digestive processes permit."
Optimally Foraging Hummers;
Hummingbirds, Nectar, and Water.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.