Urban and metropolitan areas often include some semi-natural lands inhabited by wild animals and plants. They may range from sizable parks (e.g., Philadelphia’s Fairmount Park, comprising 4100 acres), to undeveloped land fragments of only a few acres that have so far escaped conversion to commercial or residential use. Not to stretch the definition too much, leafy suburbs might also be included here for their canopies, especially if not sprayed, provide habitat for birds, moths and other species. None of these lands can be described as wilderness or perhaps not even as “natural”, given man’s pervasive influence, but as green space, open space or low (human)-density development, they have some conservation value approximately in proportion to their area.
Large mammals, may be entirely absent from smaller areas for lack of adequate habitat despite the increasing reports of white-tailed deer, coyotes, armadillos and even mountain lions visiting residential areas. But in general, species diversity is reduced and skewed towards pest species in disturbed areas. Bird diversity, for example, is commonly lower (and the proportion of generalist species higher) in small tracts than in similar-sized areas within undeveloped landscapes. Many causes may underlie this impoverishment. Following is a partial list of these causes, leaving out what may be the source of the gravest loss, the large-scale obliteration of natural lands to make way for malls, subdivisions and so forth. Detailed discussion of some of these may come in future posts, however, for now, consider that :
Ground-level and low shrub nesters (e.g., Ovenbirds, other warblers, and wrens) face threats to nests and young from feral dogs and cats, rats, and wandering family pets, and, of course, from natural predators.
Hole-nesters suffer from reduced nest site availability and food supply because of dead limb removal for “aesthetic” reasons and to reduce risk to person or property. Woodpeckers, titmice, crested flycatchers, and nuthatches are some of the species affected.
Fruit-eating forest birds are confronted with loss of native fruit and seed sources due to competition from non-native plants (such as introduced honeysuckles, Lonicera spp.), of lesser nutritive value.
Declines in prey populations (rodents, birds, insects, reptiles) as a result of causes discussed here reduce food availability to other links in the food web including raptors, owls, shrikes and other predatory birds.
Loss of large trees from logging or fires diminishes nest site availability for many birds including hawks, owls and herons.
Reproductive success of many forest species such as thrushes, vireos, and many warblers is reduced, sometimes severely, due to brood parasitism by Brown-headed Cowbirds whose numbers have soared in some areas over recent decades.
Competition for nest cavities from more aggressive non-native birds including particularly the European Starling but also the English Sparrow (Passer domesticus), and in the St. Louis area (to which it has been restricted for over a century), the Eurasian Tree Sparrow (P. montanus). Especially affected are tree swallows, red-headed woodpeckers, and bluebirds.
The high edge: interior ratio of forest fragments often results in forest interior birds suffering increased egg and chick predation from species adapted to agricultural and disturbed landscapes like grackles, starlings, and corvids. Small forested areas may effectively have no interiors, i.e., nest thieves and brood parasites can reach all nests.
Lead poisoning of many wild ducks, loons, swans and other aquatic birds, as well as doves, hawks, eagles and vultures occurs when they ingest lead shot or lead fishing weights which accumulate in hunting grounds, water bodies and connected wetlands. Pesticide residues often accumulate in wintering flocks of crows, dickcissels and bobolinks found in or near agricultural lands. Accumulation of pesticide residues in the tissues of raptors that have fed on contaminated prey has declined in recent decades but not disappeared.
Intensification of farming especially in hayfields and grain fields, has greatly reduced the suitability of these critical lands for grassland bird nesting and foraging.
Degradation of wintering habitats of migratory birds in the tropics results from land clearing, conversion of forests to plantations, and exposure to pesticides.
Collisions between migratory birds and buildings, spotlights, communication towers, electrical wires, and ceilometers kill millions of migrants annually.
Since the 1990s, West Nile Virus has become a source of mortality of wild birds, especially but not only, corvids (crows and related species).
Human-produced noise adversely affects bird communication and reproduction, and foraging, especially that associated with combustion engines and electricity generation. This impact may seem less important than the previous threats but it is not trivial as a later post will suggest.