Diet and Nutrition

Birds eat many things that seem none-too-appealing to us: beetles, flies, spiders, earthworms, rotting fish, offal, poison oak berries, weed seeds, and so on. Not only that, most birds have diets that are quite monotonous -- some passerines may go for weeks on a diet composed largely of grasshoppers, Brants dine almost exclusively on eelgrass, and Snail Kites rarely if ever taste anything but snails. In spite of this, the nutritional requirements of birds are not very different from ours; they need proteins, fats, carbohydrates, vitamins, and minerals.

Carbohydrates and fats are used primarily as energy sources, but proteins -- more specifically the nitrogen-containing amino acids that are the building blocks of proteins -- are needed for construction of tissues, enzymes, and so on. Reproduction, growth, and molting all require more nitrogen than simple maintenance of the body, and proteins are the source of that nitrogen. Birds, such as Red-winged Blackbirds, that are omnivorous (eating both plant and animal food) increase the proportion of protein-rich animal food they eat in the breeding season. Many that are herbivorous (primarily eating plant foods), such as sparrows, may subsist for much of the year on a relatively low-protein vegetable diet, but in the breeding season they take as many insects as possible, and often provide their young with a diet comprised entirely of insects.

Similarly, wood warblers, which are considered carnivorous (dining mostly on animal food), will feed themselves and their young virtually exclusively on insects in the breeding season. Like many thrushes and other more omnivorous species, they may have berries and other plant foods as a substantial portion of their fall-winter intake. And nectarivores, such as hummingbirds, must also catch insects to provide protein to balance their energy-rich but nitrogen-poor intake of nectar, especially when breeding. It is, of course, no miracle that protein-rich food sources just happen to be more abundant during the breeding season. Just the reverse -- evolution has timed the breeding season so that it occurs when the needed nitrogen can be obtained.

Birds' mineral requirements seem much like ours. Calcium, which is needed in large quantities for egg production, is a critical mineral nutrient for reproductively active female birds. It is thought that shortage of calcium may restrict the reproductive output of vultures, which devour only the soft, calcium-poor parts of carcasses. This may be the reason that some African vultures, as well as our Black and Turkey Vultures, supplement their diets with small vertebrate prey that can be swallowed whole. Calcium, is of course, also critical for reproductively active female human beings, who must produce large amounts of calcium-rich milk. Avian needs for vitamins are also similar to the human requirements. Unlike us, however, many birds manufacture vitamin C in their kidneys or liver, or in both.

As with human beings, what birds eat is determined by more than their brute nutritional requirements. Various kinds of learning play an important role. For instance, once a bird has discovered a certain kind of palatable prey, it may form a "search image" for that prey and specialize for a time in eating it. Experiments also indicate that a bird's feeding preferences can be influenced by its diet as a nestling. The best-documented learning pattern in birds is the speed with which Blue jays learn to avoid foods that make them sick. Many monarch butterflies contain heart poisons, cardiac glycosides, which they obtain as caterpillars from the milkweed plants they feed on. A jay that has never seen such a monarch before will eat it and then suffer a bout of vomiting brought on by the glycosides. Subsequently the bird will not touch a monarch butterfly or even a viceroy butterfly, which closely mimics the monarch. Here again bird behavior is much like human behavior. A person who gets violently ill soon after eating a particular food may be unable to stomach that food again, even if he or she knows that the sickness was caused, not by food, but by a virus. Nausea is a powerful teacher for both mammals and birds.

Changing availability of different kinds of food can also be a crucial factor in what is eaten. Many birds will opportunistically switch to a new food source that suddenly becomes abundant. The immortalized sea gulls that saved the Mormons' crops from a locust plague did that; when seventeen-year cicada ("locust") broods emerge, many birds switch from whatever they have been eating to gorge on cicadas. Hobbies (Eurasian falcons that look like small Peregrines) are reported to catch and eat more swifts in cool cloudy weather than in warm sunny weather. Apparently the lack of insects in the gloom weakens the swifts, making them easier prey for the Hobbies.

SEE: Breeding Season; Hummingbirds, Nectar, and Water; Hoarding Food; Drinking.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.