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Exotic Birds in Urban Environments: Part 1

November 15, 2007
An individual charged with counting birds in an urban environment might find himself with a job too big for any one person. While the notion that areas with high human densities are bird deficient is true when one thinks in terms of diversity, actual numbers of individual birds are quite high. The abundance of food in the form of refuse, the easy accessibility of nesting sites afforded by construction and the vacant niche left by birds less able to deal to the altered environment allows some species of birds to thrive in urban areas in a way they are unable to do in more rural environments. By and large, such urban birds are exotics, birds introduced either accidentally or intentionally by humans from other continents. The most successful and therefore most globally widespread of these species are the European Starling (Sturna vulgaris), the Rock Pigeon (Columba livia) and the House Sparrow (Passer domesticus). All three species readily adapt to new disturbed habitats in ways native species are unable to do, and because of that, have expanded from in some cases less than a hundred birds to sustain populations crossing North America. 

These three particular birds have been long-established, however. The Rock Pigeon, whose original wild range is restricted to remote areas of Europe and the Middle East, was domesticated several thousand years ago. The domestic version was originally brought from Europe as early as the 17th Century, and was well-established as a agricultural bird. Additionally its strong homing instinct made the species a valuable tool for communication, and pigeon breeding and showing was a popular avocation at the time in Europe and eventually in the United States as well. In the United States we are familiar with the many color variations of the feral version of this bird, whose wild populations are still supplemented by escapees from captivity. The House Sparrow, widespread across Europe, Asia and the Middle East, was introduced in the late 19th Century to control outburst of canker worms in several cities across the northeast United States. Reports are unclear from that time as to exactly how many populations were introduced, but the bird quickly spread and became a pest itself, feeding on agricultural fields in great flocks across the country.

The most familiar introduced species, and arguably the most successful in North America, is the European Starling. A native of Eastern Europe whose range had been expanding naturally into Britain over the last few hundred years, the species was introduced famously into Central Park in 1895 because some individuals felt the birds of Shakespeare’s sonnets would add as much to North American avifauna as the actual sonnets added to English literature. From a stock group of 40 birds, the population has grown to nearly 200 million in only a century, crossing the entire continent north of Mexico. These three birds are excellent examples of the type of birds that, when introduced, can rapidly increase their populations to the point where they are self-sustaining. Their global populations are testament to the ability these species in particular have to survive and thrive. By and large these birds, while wildly different in appearance, share the following characteristics that enable their populations to explode.

  • Rapid Reproduction: All three species are prolific breeders, capable of producing multiple broods in a single year. The Rock Pigeon often even laying new eggs before the first fledglings have left the nest. House Sparrows in particular can raise up to 3 clutches of up to 7 eggs per year, theoretically quintupling the population over a single year.
  • Effective Dispersal: While none of the species have an extensive migratory pattern in the United States, flocks of juveniles often travel long distances to new feeding areas. The members of the Pigeon family, Columbidae, are strong flyers and excellent dispersers, traveling quickly over long distances. Starlings and House Sparrows often travel miles to gather together in large roost site in the autumn and winter. Increased land use for agriculture and spilt seed at these sites allowed easy access to food sources for all three species, and they were all quick to take advantage of this new food source as they expanded westward.
  • Rapid Establishment: All three species are hardy birds, able to survive both excessively cold and hot environments. They seem to have little issue with living near humans, and the advantages of such a lifestyle are obvious. If a bird can eat a wide range of foods and nest in a varying array of disturbed habitat then that species will be able to take advantage of areas that other birds are simply unable to live in so readily. Rock Pigeons seem to find ledges on buildings similar enough to the cliffs favored by the wild type birds to make them regular city dwellers. Starlings and House Sparrows prefer cavities but also will nest in protected locations such as rafters, gutters, roofs, ledges, eaves, or above pipes and ductworks on buildings. Both species will aggressively turn out native birds from their nests on occasion, and unlike other native cavity nesting species, have no problem nesting in close proximity to others of their species, a characteristic shared with the ledge nesting Pigeon.
  • Aggressive Competition: Mentioned above but important enough to merit its own consideration is the aggressive nature with which Starlings and House Sparrows especially, defend nest and territory. Because both species are largely non-migratory, they begin nesting in late winter. In doing so they beat out migratory birds like Purple Martins (Progne subis) and Tree Swallows (Tachycineta bicolor) to preferred nest sites and defend such nest site vigorously once established.

The combination of these factors has led to a very successful population of these three exotic species. Another important, but less obvious characteristic, is time. All three of the classic exotic species were among the first exotics established in North America and all three have had at least a hundred years to establish themselves. In every case, even the exotics that became widespread and abundant did so after failing to survive multiple earlier introductions. The European Starling for instance, failed at least eight previous attempts at introduction before succeeding in 1895. The House Sparrow’s introduction at several different site probably helped its chances a great deal, and the Rock Pigeon was a widely used agricultural species before it finally caught on as a self-sufficient population. Approximately 97 of the 1000 bird species in North America are exotic. So which of these species will we be able to look back on as we look now at Starlings, Sparrows and Pigeons?

Part 2 continues here.

  1. slybird permalink
    November 15, 2007 7:23 pm

    Great summary. Where are you going with the rest of it?


  2. November 15, 2007 11:28 pm

    Oh, you’ll see, it was a long paper…

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