Avian Invaders

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.