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.