Some
of the most abundant birds in North America were
deliberately introduced to our continent. We owe the
presence of the extremely successful (and often pestiferous)
European Starling to William Shakespeare. Toward the end of
the last century, "The American Acclimatization Society" had
the goal of establishing in the United States every species
of bird mentioned in the works of the immortal Bard of Avon.
Unfortunately, in Henry IV, Hotspur proclaimed, "Nay, I'll
have a starling shall be taught to speak nothing but
'Mortimer'...." North American birds and people have been
suffering ever since. The starlings' impact on
native birds, in some cases, appears to have been severe.
Starlings also devour grain put out for cattle in feedlots,
damage crops, foul buildings and walkways with their
droppings, and may be involved in transmitting
histoplasmosis, a serious fungal disease of human
beings. A great deal of money has
been spent trying to control starling populations, and many
of the birds have been killed. In the 1960s, one program in
California designed to alleviate starling depredations on
cattle feed resulted in the slaughter of some 9 million
birds, but left 5,000 starlings in the area alive to
reproduce. In spite of such massive efforts to reduce the
numbers of descendants of those birds introduced by the
Acclimatization Society, starlings are today ubiquitous on
the North American continent except for the Far
North. It is interesting to
speculate on why some introduced birds thrive, while others
do not. The Crested Myna, a close relative of the starling,
has never spread beyond the Vancouver, British Columbia,
area, where it became established during the last century.
Apparently it has remained localized because it retains
incubating habits more suitable to its tropical homeland
than to the temperate zone, thus limiting its reproductive
success. More mysterious is the case
of the House (or English) Sparrow and its close relative,
the Eurasian Tree Sparrow. The House Sparrow formed a
relationship with Homo sapiens shortly after people in the
Middle East first settled down and started farming. The
sparrows are thought to have originally been migratory, but
they appear to have lost that ability over many generations
as they evolved the habit of overwintering close to
settlements and feeding on grain stores, garbage, and other
materials made available by human activities. Sparrow
populations grew especially large in cities when horses and
their seed-rich droppings were common. Several attempts were made
to introduce the House Sparrow to North America, both
because people considered it attractive and because it was
hoped that the bird would help control insect pests. The
first successful introduction was made in Brooklyn shortly
after 1850, and like the starling the House Sparrow spread
rapidly, taking only 50 years to occupy suitable habitats
over the entire United States. In contrast, the Tree Sparrow
was established in St. Louis in 1870, but for almost 100
years remained largely confined to that area. Around 1960 it
began to spread, and has since occupied adjacent parts of
Missouri and Illinois, but it has shown none of the
colonizing vigor of the House Sparrow. On other continents
where both species have been introduced, the House Sparrow
invariably has been the most successful. Ecologists still do not
understand why the two species have had such different
colonization histories. The House Sparrow is somewhat
larger, and a bigger bird may compete better with native
finch-like species, but the size difference is not great.
Pure luck might be invoked to explain the difference in the
United States. For instance, the original stock of the House
Sparrow might have contained a sample of the genetic traits
of that species better suited for survival here than did the
initial Tree Sparrow immigrants. Or the Tree Sparrow may
have had the misfortune to bring more of its diseases with
it. However, since the House Sparrow seems a more vigorous
colonizer wherever it has been introduced, as the sole
explanation chance alone seems unlikely. It is possible that
the House Sparrow simply out-competes the Tree Sparrow in
areas of human disturbance. Outside of their native
communities in Eurasia those may be the only areas invasible
by either. Not all introductions of
birds have been intentional. Escapes and releases of caged
pet birds have caused a rash of introductions of parrots,
parakeets, doves, and others into the United States --
mostly in Florida and southern California. The demise of the
Carolina Parakeet ended the natural occurrence of parrots
north of Mexico, but it remains to be seen whether any of
the related introduced species will build large, sustainable
populations in North America. Finally, there are birds
that have invaded North America in historic times under
their own steam. Of these, the Old World Cattle Egret is the
best known and most successful. Around 1880 the egret
reached Suriname on the northern coast of South America, and
it arrived in Florida about 1940. It is now firmly
established over the eastern and southwestern United States,
thriving in the pastures and among the cattle herds
established by Homo sapiens. The spread of the egret is,
of course, just a recently observed example of a natural
process that has gone on for as long as there have been
birds (indeed, as long as there have been organisms); i.e.,
species crossing barriers and extending their ranges into
new areas of suitable habitat. It is a mistake to regard
distribution maps as final. We should always be on the alert
for expansion and shrinkage of the area occupied by a bird
species -- it is a normal process. SEE: Incubation
Time;
Feral
Birds;
Urban
Birds;
Population
Dynamics Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.