Determining Diets

photograph of a Yellow Warbler by Tom Grey
Considering how many people are interested in birds, a surprising amount remains to be learned about exactly what they eat. Determining by observation what birds consume is both tedious and difficult. It may be evident that a warbler working over bark or leaves high in a tree is catching insects, but which insects, and how many, cannot be assessed readily. A Willet or Marbled Godwit probing in shallow water along a mud bar occasionally may be seen swallowing a worm, but how can we record what kind it was, or what else the bird may be feeding on that is simply too small to see?

Because of such problems, ornithologists long relied on careful examination of stomach contents to assess feeding habits. This procedure is reasonably accurate, although, for example, marine worms are often omitted from diet descriptions because they are digested
too rapidly to be identified. The main exception to the need to dissect stomachs are the birds, especially owls, that regurgitate the remains of their meals as pellets that can subsequently be collected and analyzed. Very large samples of diets can be obtained this way. It has been estimated that some 30,000 prey items of Long-eared Owls have been identified from North America, and more than 300,000 from Europe. This makes the food habits of these owls the best-known aspect of their ecology.

Stomach-content analysis is less commonly used today because large numbers of birds would have to be killed to develop a detailed knowledge of the diet of even a single population. We know that individual birds, like individual people, differ in their food intake at any given time, and that the average diet of a population will often change dramatically throughout the year. Many species that are primarily insectivorous specialize in devouring one common kind of insect at one time, switch to another insect species when the first kind of bug completes its flight season (or is decimated by predators), and then move on to others until insect activity becomes less common in the fall. The birds then may begin to supplement their diets with vegetable matter such as berries, or else depart for southern climes where insects are still active (and where many species that eat insects in the north eat primarily, if not exclusively, fruit).

Birds frequently form "search images"; i.e., they learn to find certain abundant (although often camouflaged) prey and then specialize in eating that prey as long as it remains abundant. Thus the diet of a population shifts dramatically as increasing numbers of individuals form new search images. Diets may change from week to week or from year to year as the abundance of food items changes. For example, when there is a mass emergence of 13- or 17-year cicadas, many birds will gorge on those superabundant insects. A naive observer sampling at that time might conclude that, say, the Red-winged Blackbird was a specialist on cicadas.

To make assessment even more difficult, diets differ from place to place. Yellow Warblers in Nebraska eat many more grasshoppers than do Yellow Warblers in Massachusetts, where gypsy moth caterpillars and plant lice are more favored. We suspect that winter diets of Yellow Warblers in Central America are quite different from spring and summer diets on the breeding grounds. Obviously, Peregrines living near the sea dine much more often on seabirds than those living inland, and inland Peregrines eat more doves than their coastal cousins. It is no surprise that Herring Gulls near big cities eat a lot more garbage than those on pristine coasts.

Nevertheless, there are limits to the variation in the diet of a species. Herring Gulls do not feed on plant lice, and Yellow Warblers eat neither fish nor doves. Birds, being adaptable creatures, usually dine on a spectrum of different kinds of foods, but even closely related species tend to have different spectra. Observations of foraging behavior and studies of stomach contents have outlined major features of the diets of North American birds, but detailed knowledge is lacking for many species. Information reported in the literature is often based on small samples of birds recorded in too few places and limited portions of the year. To get a thorough picture of the diet of a single population would require examination of the stomach contents of dozens of birds at numerous different times throughout the year, and repeating the process over a series of years. To document the diet of a species thoroughly, such studies would have to be carried out on many populations over the species' entire range. It is the prospect of such a slaughter that makes ornithologists forgo the use of stomach-content analysis as a method for fine tuning knowledge of feeding habits (although when specimens are collected for other scientific purposes, the contents are always recorded).

Yet it is important to understand the similarities and differences of avian diets in much greater detail. If students of birds continue to record and publish careful observations of feeding habits, much of the needed information will accumulate gradually. During the breeding season, the nature of the diet tends to be of basic scientific interest, because, for example, one often wants to know whether two species breeding in the same place are competing for food. Scientists studying a population sometimes are able to assay diets by gently placing pipe cleaners around the throats of nestlings, so that they cannot swallow but can still breathe. The food delivered by the parents can then be retrieved and analyzed. This technique must be left to experts so that the nestlings are not injured or deprived of too much food. It is, however, often possible to identify what is being fed to nestlings by observation alone. So if you spend time monitoring nests, note what the parents bring to the young. If you are not f with the major groups of insects, bird watching can introduce you to another fascinating group of organisms. A good book to start with is A Field Guide to the Insects, by D. J. Borror and R. E. White (Houghton Mifflin, Boston, 1970).
SEE: Pellets; Diet and Nutrition.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.