Many
of the essays in this book are loaded with "weasel" words --
birds appear to do this, or seem to behave in a certain
manner, or may respond in a particular way for the following
reason. Surely, one would think scientists could be more
definitive about the biology of such a well-known group of
animals. But they cannot. One reason for the
uncertainty is the difficulty of finding out what a rapidly
flying animal does. Two White-throated Swifts that touch
briefly in flight may appear to be copulating, but how
(short of bringing them down for a biological assay) can we
be sure? How can we even be sure that the two birds tumbling
in midair as they sweep past us down a canyon at forty miles
an hour are a male and a female? Finding answers in such
cases may require detailed, logistically difficult, and
time-consuming research involving such things as netting,
trapping, examining and banding birds, photographing them
with high-speed cameras, and often clever experimental
alterations of the birds or their environments. Suppose, after completing
such a detailed study, a scientist is convinced that
White-throated Swifts in one part of Arizona copulate in
midair. How far can the researcher generalize such findings?
Does it mean that White-throated Swifts everywhere copulate
in midair? Do all other species of swifts copulate in
midair? Ornithologists think all swifts are aerial maters.
In fact, they assume that swifts are the only birds that do
so on the wing. But they cannot be certain of either point.
Maybe one obscure population of Palm Swifts in Asia actually
mates only in their nests. And just possibly an Amazonian
hummingbird, on occasion, copulates aerially above the
canopy. That's why the authoritative Dictionary of Birds
says, "Swifts are apparently unique in copulating in the
air" (our emphasis). In short, statements by scientists are
always provisional; they represent the best current
evaluation of the situation, but are always subject to
revision. A related problem is that
there are more than 9,700 species of birds, and, as we know,
they are a rather diverse lot. Suppose, for instance, we
wished to know whether female birds prefer males that hold
superior territories. Just determining whether females of
one species in one habitat have that preference is not a
simple task. The territories of males must be mapped and
criteria to judge territorial quality must be found. For
example, to compare territories within a population of
insect eaters an ecologist may be required to do many
careful censuses of insect abundance during the breeding
season, without disturbing the breeding birds. Finally, of
course, the female choices must be evaluated, most likely by
counting the number of nests and the number of young fledged
within each male's territory, often a difficult task in
itself. So much for finding the
answer for one species in one locality -- how about other
localities? And, since male birds of most species are
territorial, what about other species? Obviously, it would
be a mistake to generalize from the behavior of one
population living in one habitat to all species of birds in
all habitats. It would be necessary to see if female choices
were made not only in a variety of species, but also in
different habitats. If this were done, a pattern should
emerge slowly, and with it an opportunity to draw tentative
conclusions. Further studies may tend to confirm or refute
those initial conclusions, and after enough studies have
been completed, a consensus will emerge. For instance,
territoriality in most male birds is so well documented that
it is treated as fact. That females tend to mate with the
holders of the better territories is not so well
established, but many ecologists think that it eventually
will be. Learning about bird
communities can be even more difficult than drawing
conclusions about the behavior of single species. Suppose,
for example, we wished to know whether Downy and Hairy
Woodpeckers were competing in three square miles of forest.
One way to find out would be to remove all the Downy
Woodpeckers in one square mile, all the Hairy Woodpeckers in
another, and leave the third square mile undisturbed. Then
comparing the fledging rates in the three localities should
provide substantial information on the degree of
competition. The logistics of doing such an experiment would
be daunting, but more important, we and many others would
consider its impact on the birds to raise serious ethical
questions. Physical scientists generally ask whether the
knowledge that might be gained from an experiment would
justify the effort; biologists must also consider an
experiment's effects on the organisms and natural systems
being investigated. In our essays we try to
indicate how well established the conclusions are and,
wherever possible, to describe the evidence on which they
are based. The length of the treatments generally reflects
how much is known about the birds, although the length of
some of the most thoroughly studied species had to be
shortened because of space constraints. You should be
cautioned that there often appears to be more certainty in
the data from poorly studied species; for instance, if only
a few nests have been found, the range of heights above
ground or clutch sizes will almost always be smaller than if
hundreds of nests had been examined. You may be disappointed by
how little is known about so many aspects of avian ecology
and behavior, even though many birds have been thoroughly
studied. But looking at the bright side, that means that
there are numerous opportunities for birder and
ornithologist alike to add to the understanding of these
fascinating creatures. SEE: Territoriality;
Polygyny. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.