Urban and suburban wildlife

Animals and plants in cities and suburbs are categorized as urban wildlife. As cities and suburbs grow ever larger and displace natural habitats, many city and suburban landscapes have become more attractive for certain kinds of wildlife, or at least urban wildlife has become more noticeable. Urban wildlife consists of an eclectic and unlikely mix of escaped pets (mostly exotics and caged birds), feral animals, furtive and temporary intruders from adjacent natural habitats, and species whose natural ecology and behavior enables them to fit within human-modified landscapes and tolerate living in close proximity to humans.

Urban landscapes present a seemingly stark and forbidding environment for wildlife. The horizontal pavement of streets and sidewalks is punctuated by rising angles and arches of concrete and steel, which, in turn, are topped by wood and metal rooftops. Overhead, a maze of telephone, power, and cable lines limits vertical movement, while vehicle and foot traffic pose a constant threat to surface movement. All these edifices and connecting corridors and lines result in a complex, vertically structured environment within which some animals struggle to maneuver yet to which other animals quickly adapt. In addition to monotonous and often dangerous structural diversity, urban wildlife is subject to elevated and often almost continuous noise and disturbance and is constantly exposed to an enormous variety of residential wastes (garbage, litter, excess water, salts, and sewage), vehicular pollutants (lubricants, greases, gasoline, hydrocarbons, and nitrogen oxides), and chemical wastes (pesticides, paints, lead, mercury, and contaminants).

Despite the forbidding features offered by urban habitats, a surprising variety of wildlife manages to exist on a more-or-less permanent status. In fact, some wildlife can be found even in the most degraded forms of urban blight. Ailanthus altissima, also commonly called tree-of-heaven, is one of many opportunistic trees and shrubs that take root and grow given a bare minimum of soil and nutrients. A simple linear crack in the pavement of a sidewalk, a little-used roadway, an unused parking area, or a vacant lot can trap enough windswept dirt to offer a growing substrate for Ailanthus altissima, and similar hardy plants. Each Ailanthus altissima, in turn, provides food and shelter for equally tough and adaptable wildlife, ranging from the variety of invertebrates that colonize and feed upon Ailanthus altissima to birds and mammals that take shelter or find food in its branches and foliage. Similarly, every invading sprig of grass, wildflower, shrub, or tree, however large or small, creates its own suite of microhabitats, which offer colonization opportunities for other plants and animals, the whole ultimately contributing to an overall increase in urban biodiversity.

Characteristics of Urban Wildlife

The tree-of-heaven is an example of those plants and animals able to tolerate the most extreme urban conditions, but in reality, most urban wildlife derives a number of benefits from living within the confines of cities and suburbs. Far from being homogenous expanses of concrete, most urban centers are a patchwork of different habitats—residential, commercial, and industrial buildings, warehouses, power stations, vacant lots, detached gardens, rooftop gardens, and alleyways—that each offers innumerable opportunities for wildlife. Many urban areas also have a number of limited access areas that animals are quick to adopt for shelter and breeding places; these include fenced-in lots and boarded-up buildings, along with a rabbit warren of underground tunnels, ducts, steam and water pipes, basements, and access ways.

City lights extend foraging time and opportunities, allowing wildlife to hunt for food not only throughout the day but also during much of the night, as needed. Urban nooks and crannies offer an extensive variety of microhabitats that differ fundamentally in size, microclimate, and other structural features. These microhabitats serve primarily as shelters and breeding sites for city wildlife. Many birds, such as house sparrows (Passerdomesticus) and European starlings (Sturnus vulgaris), nest in crevices, cracks, nooks, niches, and sheltered rooftops. Rock pigeons (Columba livia) and starlings hide in sheltered enclaves offered by bridge abutments and supports, archways, and other edifices. While these birds adapt well to their new environment, scientists have found that urban birds have higher levels of stress hormones than their wild counterparts, but these findings are inconsistent between species.

The most adaptable wildlife is quick to find and take advantage of subtle advantages offered in urban habitats: Many birds cluster around chimneys and roof reflectors or in shelters afforded by lee sides of rooftops during harsh cold and windstorms. Others are equally quick to obtain warmth by sitting on poles, rooftops, or other elevated perches to orient toward sunlight, while at ground level, animals gather near gratings, vents, and underground heating pipes.

Urban wildlife quickly concentrates in areas where potential food is made available, for instance, during trash pickup, and then just as quickly disperses to find new food sources. Most urban wildlife forage opportunistically as scavengers, specializing in finding and consuming all bits of discarded food, raiding trash cans, and concentrating at the waste collection and disposal centers. Thus, the rubbish dumps, found in or immediately adjacent to every large city, attract an amazing diversity of small mammals and birds. Feeding on the scavenged food of urban areas and bird feeders is much more efficient because it requires less energy to find or catch and is usually available throughout the year. However, some animals may become dependent on this food source, and young animals raised in this environment may struggle to adapt and find food in their natural habitat. Others may become obese. Eating high-calorie, processed foods from trash cans requires no hunting, making the calories they expend decrease and the calories they ingest increase.

Because of the need to find and exploit temporary food resources, some of the most successful urban animals forage in loose groupings or flocks—the more eyes, the more searching, and the more feeding opportunities can be identified and exploited. Solitary and nonsocial species often do less well in urban environments because they lack the collective power of the group to find food and shelter and avoid enemies.

The availability of a year-round food supply—however tenuous and temporary—along with the presence of an enormous variety of safe shelters and breeding sites promotes a higher life expectancy, which partly or mostly balances the higher vehicle-related death rates to which urban wildlife is continuously subject.

Parks and open spaces provide the only true refuges of natural habitats set deep within urban and suburban landscapes. Such open-space habitats function as ecological islands in a sea of urbanism. Most are managed habitats rather than entirely natural and, like the urban environment that surrounds them, are usually subject to constant disturbance from adjacent traffic, noise, and other forms of pollution. Economically, since most open-space parks are set aside and maintained for a variety of recreational purposes rather than as natural habitats, the wildlife that colonizes these unnatural natural habitats must have an unusually high tolerance for human presence and recreational activities of all kinds.

Sources and Types of Urban Wildlife

For some urban wildlife, the landscape is merely an artificial version of their natural environment. Thus, for pigeons, the ledges, cracks, and crevices of buildings and bridges represent an urban version of the cracks and crevices of cliffs and rock outcrops used for roosting and nesting in their native habitats. Similarly, the short-eared owls (Asio flammeus) and snowy owls (Bubo scandiacus), that show up in winter to stand as silent sentinels at airports, golf courses, and other open areas are simply substituting these managed short-grass habitats for the tundra habitats preferred by snowy owls and the coastal marshes hunted by short-eared owls. Their summer replacements include a host of grassland nesting species, such as grasshopper sparrows (Ammodramus savannarum), killdeer (Charadriusvociferus), and upland sandpipers (Bartramia longicauda), which find these managed habitats to be ideal substitutes for the native grasslands, which they displaced or replaced.

Many bird inhabitants of urban and suburban environments are exotics that were deliberately or inadvertently introduced into urban areas. The three birds with the widest urban distribution in North America, the pigeon or rock dove, European starling, and house sparrow or English sparrow, all fit within this category. The introduction of the European starling into North American cities and suburbs resulted from the dedicated efforts of the American Acclimatization Society of the late 1800s. The goal of this society was the successful introduction of all birds mentioned in the works of Shakespeare into North America. The character of Hotspur in Henry IV makes brief note of the starling, so the society repeatedly attempted to introduce the starling into New York's Central Park until they were finally successful. Since then, the starling has become the scourge of cities and suburbs throughout much of North America. Starlings damage and despoil crops and dirty buildings with their droppings.

The association between house sparrows and urban centers is apparently very old. Evidence suggests that they abandoned their migratory ways to become permanent occupants of some of the earliest settlements along the Nile and Fertile Crescent, a trend that has continued to this day. Sparrows and starlings share certain characteristics that enable them effectively to exploit urban and suburban habitats; both are aggressive colonizers and competitors, able to feed opportunistically on grains, crops, discarded bits of garbage, and other food supplies.

Avian occupants also include an increasing diversity of released caged pets. Thus, urban locales in Florida, Southern California, and along the Gulf Coast support an ever-increasing diversity of parakeets, parrots, finches, and lovebirds, all stemming from caged pet birds either deliberately released or lost as escapees.

Feral animals, mostly dogs (Canidae) and cats (Felidae) represent another important source and component of urban wildlife. Feral dogs revert to primal adaptive behaviors, gathering in loose packs that usually forage and take shelter together, but have limited success because almost all cities in developed countries have ongoing measures to control and remove them whenever found. Feral cats are often more successful because they are secretive, mostly nocturnal, and can better exploit available urban food sources. The role of other feral animals as urban wildlife, mostly escaped pets, is not well known.

Humans and Urban Wildlife

The attitude of urban dwellers toward urban wildlife varies greatly. For many humans, urban wildlife offers a welcome respite from their otherwise dreary and mundane surroundings. Urban wildlife in all its forms and colors can be aesthetically attractive, even beautiful, and is also compellingly interesting. For example, the nesting of a pair of red-tailed hawks (Buteo jamaicensis) in New York City’s Central Park sparked a remarkable interest in birdwatching in the city. All facets of the pair’s courtship and nesting were watched and reported in newsprint, novellas, and even a book, Red-Tails in Love (1998). Other animals, while not nearly as large, conspicuous, and glamorous in their color and disposition, also elicit interest. Urban wildlife adds lively color and contrast to the otherwise monotonous gray and grime of streets and sidewalks. Part of the attraction is that urban birds are usually already sufficiently tolerant to be semi-tame in spirit, easily seen and observed, and, in some instances, easily attracted. Strategically placed bird feeders and birdhouses also attract these birds.

Public attitude toward urban predators varies considerably. Some people find them attractive and interesting, and even put out food for them. Others consider them pests or potentially dangerous and avoid them. During rabies outbreaks or public scares, most urban wildlife is targeted by various control programs to remove unwanted animals.

Suburban Wildlife Habitats

The vast sprawl of suburbs across the landscape offers many types of wildlife yet another habitat opportunity to exploit, either as residences or as temporary components of the search for food or shelter. Like urban areas, suburbs offer a range of differing habitats. The simplest suburbs are merely extensions of urban row houses with minimal yards, but there is an increasing progression toward more open and natural yards in outlying suburbs that merge with rural areas and natural habitats. The larger and more diverse yards at the edges of suburbs often help blur the distinction and diversity between human landscapes and natural landscapes.

Ornamental trees, shrubs, flowers, gardens, and lawns that characterize almost all suburban habitats provide artificial habitats that can increase wildlife diversity. Again, the chief wildlife benefactors are species that can best ecologically exploit the unnatural blend of woodland, edge, and meadow that suburban landscapes offer. It is no accident that some of the most common components of suburban wildlife include thrushes, such as robins, finches, cardinals, titmice, blue jays, crows, and many other similar birds. All these species respond to the structural components of the suburban landscape, which provides suitable substitutes for their natural landscapes.

The blend of ornamental and garden vegetation offered by most suburban landscapes offers food for a diversity of what were once considered less tolerant wildlife. Deer, wild turkey, grouse, and a host of other animals, large and small, make periodic forays into suburbs in search of food. Crepuscular and nocturnal wildlife is much more likely to exploit food sources offered by suburban landscapes than diurnal wildlife, which is more at risk because of its high visibility during daylight hours.

Well-wooded suburban habitats that attract a variety of wildlife also attract an increasing number of predators. American kestrels (Falco sparverius), Cooper’s hawks (Accipiter cooperii), barn owls (Tyto alba), screech owls (Megascops), and little owls (Athene noctua) provide but a small sampling of birds of prey that nest deep within urban and suburban environments, taking advantage of open-space habitats deep within cities and quickly exploiting unused areas within most suburbs. Terrestrial predators are almost equally common, but most are nocturnal or nearly so. Consequently, their contact with humans is limited. Many urban predators are mistaken for neighborhood pets and left alone or avoided: Coyotes (Canis latrans) are often mistaken for dogs, especially when seen in twilight. The wily coyote is equally at home in suburbs, joining a host of small and medium-sized mammal predators, such as foxes (Vulpes spp.), and scavengers, such as opossums (Didelphis marsupialis), raccoons (Procyon lotor), and striped skunks (Mephitis mephitis). These urban predators share many behavioral attributes. All are omnivorous and feed on a wide variety of natural foods, such as fruit, small birds and mammals, insects, and invertebrates, such as beetles, grasshoppers, and earthworms. Bears are also adapting to city life in much of North America. However, bears that live in urban areas move less and are heavier animals, which leads to premature births and early deaths.

The foraging and food habits of urban predators sometimes conflict with human concerns. Urban foxes hunt and kill cats, especially kittens, if given the opportunity, while the larger and stronger urban coyote may kill and eat cats and dogs.

Conservation and Management of Urban and Suburban Wildlife

Urban wildlife must be much more closely managed than wildlife of natural environments because urban and suburban habitats attract an enormous number of pest species as well as interesting and beneficial species. Introduced species, such as starlings, may also transmit histoplasmosis, a fungal disease that attacks human lungs. Other birds may also be harbingers, carriers, and vectors of various diseases, the most notable of which are the parrots and parakeets, which transmit parrot fever or psittacosis. Rats and mice (Rodentia) carry and spread disease and despoil both residential and public buildings and other structures.

The growing interest in urban wildlife has stimulated innumerable programs to promote beneficial wildlife. Both public and private organizations and agencies have embarked on a variety of programs aimed at remodeling existing habitats and even creating new habitats for urban wildlife.

Programs aimed at creating new or modifying existing urban habitats come in a variety of categories, such as linear parks, greenways, urban wildlife acres programs, backyard gardens, and treescaping streets and roadways, all of which create biodiversity, which provides attractive habitats for colonization by additional animals and plants. Modification of existing habitats to increase animal biodiversity includes “critter crossings,” roadside habitats, backyard gardens, and arbor plantings, all of which provide refuges, shelters, breeding sites, connecting corridors, and safe havens that promote the welfare of urban and suburban wildlife.

Many existing open-space habitats are also being modified. Urban renewal commissions continue placing new and more restrictive regulations on the use of pesticides and fertilizers on golf courses, reducing the incidence and intensity of nonpoint pollution from the golf courses and reducing the incidence of wildlife poisoning. These steps cannot help but increase the biotic potential of golf courses for supporting local biodiversity.

Principal Terms

Anthropogenic: Originating from human sources, such as aerosols and other pollutants

Biodiversity: Variety of life found in a community or ecosystem; includes both species richness and the relative number of individuals of each species

Exotics: Organisms, usually animals, that have been deliberately or inadvertently introduced into a new habitat, such as monk parakeets in New England or brown snakes in Guam

Feral Animals: Domestic animals that have reverted to a wild or semiwild condition, such as cats, dogs, or caged birds that have been released or escaped and now survive in the wild

Morphology: Development, structure, and function of form in organisms

Open Space: Natural or partly natural areas in and around cities and suburbs, such as woodlots, greenbelts, parks, and cemeteries

Urban Wildlife: Generally, the nondomestic invertebrates and vertebrates of urban, suburban, and urbanizing areas; may include domestic animals that have escaped and are subsequently feral

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