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From WildBird magazine, May 1993, with permission:

Hummingbird Feeding

Researchers Studied Hummingbird Foods and Feeding and Question Using the 4:1 Sugar-Water Ratio In Feeders

by Reed Hainsworth, Ph.D. and Larry Wolf, Ph.D.

Obtaining the food needed to live from day to day is a fundamental part of life for birds. Imagine small hummingbirds discovering a large amount of food in one place, such as a feeder. For them a feeder is supernatural. Within a very short time at a feeder, a small, hungry hummingbird can solve its immediate requirements for food.

The very size of hummingbirds makes their survival an even bigger adventure. Hummingbirds must eat more than their weight in food each day, and they fulfill this need by eating often. Because their survival depends critically on eating frequently more than any other animal - they continually face the danger of starving.

Hummingbird Meals

How much and how often do hummingbirds eat? When we studied hummingbirds in the laboratory, we found that they, like humans, eat meals. A meal is a relatively quick and large intake of food, which is followed by time when no feeding occurs while the energy that has been consumed is used. In the lab, hummingbird meals are easy to observe because the birds fly from feeders back to a perch, and they do not come back to a feeder until they are ready for their next meal.

An X-ray of a Magnificent Hummingbird shows what happens to a meal once it is eaten. Food initially passes to an elastic sac in the neck called a crop, which serves the same storage and supply functions as a stomach. Small amounts of food empty from the crop and pass to the Intestine, where sugar is assimilated into the blood.

Measurements of excreted fluids show hummingbirds digest all the sugar from sugar-water meals. How often hummingbirds eat meals, and the amount they eat in a day, depends on the energy content of food. Hummingbirds feed on a variety of flower nectars with caloric values that may vary from 10 to 82 calories per meal (1/100 fluid ounces).

We found that when using a relative rich sugar solution, a three gram male Ruby-throated Hummingbird ate five meals an hour. For each meal he consumed a little less than 1/100 of a fluid ounce.

When we diluted the food by one-half, the Ruby-throat continued to eat the same volume for each meal, but he ate 14 meals an hour, or one meal every four or five minutes. The crop emptied more rapidly when the energy (sugar) content of its food was lower. Each meal weighed about one-quarter gram, so with 14 meals an hour, the three-gram bird ate 3.6 grams, or more than his weight in one hour! Over a 12-hour daylight feeding period, this hummingbird ate 43 grams of sugar water, or 14 times his weight in food. Even with the richer food, he ate 5.4 times his weight in a day.

The Impression from this frantic eating schedule seems to confirm that a hummingbird might very quickly starve to death if it does not eat in a short time. How, then, do these birds manage to survive overnight without eating?

To find out, we measured the amount of energy they used compared to the energy they ate. We measured energy they used while they perched and while they hovered, and we found a three-gram hummingbird used 15 times more energy in a minute to hover than to perch. When we added up the energy a hummingbird used after it ate a meal, we found it went back to eat again before it had utilized all the energy it had eaten. Some energy from each meal was saved and stored as fat.

Energy storage keeps a hummingbird from starving, but not for long. The energy stored by the end of a day usually is just sufficient to survive overnight.

What happens if a hummingbird cannot feed enough, or if it is cold and more energy must be used to keep warm overnight? Fortunately, hummingbirds, like hibernating mammals, can lower their body temperature overnight to conserve energy.

However, we found that hummingbirds do not lower their body temperature unless there is a danger they actually may starve. Even with their abilities to save some energy and to conserve energy in an extreme crisis, the impression is that small hummingbirds face big problems because they must eat often.

One way to help solve the problem is to eat energy-rich food; a hummingbird can store more energy from each meal, so their survival problems are reduced by feeding on rich foods. Hummingbirds spend most of their feeding time visiting flowers to eat nectar. Is it a rich food?

Nectar

To the ancient Greeks, nectar was the drink of the gods, thus you might think nectar is pretty special. Actually flower nectar is a simple fluid composed mainly of water and sugar. Anyone who has sucked a honeysuckle or petunia blossom can testify to nectar's sweetness.

Analysis of the nectar from 124 plant species showed that it is composed of a combination of sucrose (table sugar), glucose and fructose. None of the nectars contained only glucose or only fructose. When we gave hummingbirds a choice between feeders containing sucrose and those with only glucose or only fructose, they preferred the sucrose.

Nectar also contains very small amounts of protein, and sodium and potassium salts. However, hummingbirds get most of their protein by eating small insects for a short time each day.

We were interested to know the sugar concentration, or the amount of sugar dissolved in a volume of water contained in lower nectar. This information would show how natural foods influence hummingbird feeding, and what sugar concentration to mix so hummingbird feeders provide the same food values that hummingbirds get from flower nectar.

We found that sugar concentrations differ widely among plant species, so no single sugar-water concentration is representative of all flower nectars that hummingbirds eat. The lowest sugar concentration we found was 10 calories in flowers of Iris missouriensis in the mountains of southeastern Arizona, while the highest, 82 calories, was for a Salvia in the Sonoran Desert in the same region.

The mean average sugar concentration for 65 plant species was 32 calories; the highest concentration was more than twice that, while the lowest was more than three times less than the average. This means hummingbirds will eat more or less frequently depending on the sugar concentration of nectar in the flowers they visit.

Sugar Solutions

Considering such large differences In nectar sugar concentrations in flowers, it is possible to mix different solutions of sugar and water to achieve different goals, while still providing food similar to what the birds obtain from plants. Backyard birders have two major goals: to provide food to attract hummingbirds so they continue to visit, and to maintain feeding frequencies so it is easier to watch and enjoy the birds' behavior.

A hummingbird Is more likely to stay at a feeder when it first arrives if the feeder contains a relatively rich sugar solution. A 60 calorie solution can be mixed for this purpose with equal volumes of sugar and water (1:1 ratio). This high concentration is important to replenish energy reserves during migration, and to fuel the territorial exploits of males and nesting activities of females.

Once hummingbirds have been attracted with a rich sugar solution for two or three weeks, a lower concentration will increase their feeding activity and still provide sufficient energy. To promote high rates of feeding activity, mix one part sugar with four parts water (1:4 ratio). This 10-calorie solution is similar to lower sugar concentrations in nectar produced by some plants.

It will seem like there are many more hummingbirds visiting your feeders because each bird will feed 10 to 12 times an hour in comparison to two or three times an hour with richer food. If you wish to make the change from high to low sugar concentrations more gradual, the "average" 35-calorie concentration can be mixed with one part sugar and two parts water (1:2 ratio).

It Is not necessary to always provide the same concentration as the average found In flowers. Like a feeder for seed-eating birds, a hummingbird feeder is efficient because a bird can find and eat a meal very quickly. Although a higher caloric food in a feeder is more efficient for the birds, it decreases their feeding activity. It helps if neighbors coordinate changes in sugar-water concentrations because hummingbirds always prefer a higher sugar-water concentration. By studying the feeding behavior and physiology of hummingbirds in relation to flower nectar sugar concentrations, it has become obvious there is no best or most healthful feeder solution. Regardless of what sugar-water concentration you use, be sure to keep your feeders clean and your nectar fresh for the birds.

Dr. Reed Hainsworth and Dr. Larry Wolf are Professors of Biology at Syracuse University in New York. They have been studying hummingbird physiology and ecology for 25 years in the United States and tropical America.


 

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