A 2007 study of urban birds and their close relatives in the wild in 73 major cities across the world found that the native ranges of urban birds are wider in latitude and elevation than those bird species that have not invaded cities. The House Sparrow, for example, was native to Europe, most of the Middle East, a large portion of Asia and a bit of Africa. The native range of the European Starling is similar but smaller. Urban birds tend to be generalists and have considerable behavioral, physiological and ecological flexibility.
Feral pigeons and House Sparrows have been associated with humans for thousands of years so the populations of these birds are higher in urban than nearby rural habitats. But even European Blackbirds which only invaded cities less than two hundred years ago have urban populations two orders of magnitude higher than adjacent woodlands, their native habitat. Overall, urban bird populations average 30 percent higher than nearby rural populations of the same species. Since the 1950s, 47 bird species living in the inner city of Warsaw, Poland, have diminished in number or disappeared as the city grew, but 37 species increased their populations and twelve new bird species colonized the city. About 114 bird species utilize cities or suburbs worldwide for part or all of their lifespan. Gulls, vultures, ravens and crows hang out at garbage dumps and along roadways looking for scraps. Hummingbirds, towhees, chickadees, nuthatches, sparrows, doves, finches, jays and woodpeckers frequent bird feeders. Falcons, hawks, and owls even nest in cities as there is abundant prey for them. George and Gracie, a Peregrine Falcon pair, have been nesting on a San Francisco building since 2005; in 2014 they nested in a 30th floor planter on a financial district skyscraper. There is a famous Red-tailed Hawk in New York City, dubbed “Pale Male” because of his light colored head, that has nested on an apartment building overhang on Fifth Avenue for 24 years with a new female about every three years. That he has survived that long is amazing as 75 percent of Red-tailed Hawks die before their first birthday.
The populations of year-round city resident birds have held steady or showed a strong increase over the past 40 years, especially birds like vultures, turkeys, gulls, and the House Finch. Human food scraps are super-abundant, and although domestic predators (dogs and cats) are common, predation pressure is low because dogs and cats also have human food to rely on and birds have an infinite number of hiding places. Migratory birds which only use urban areas for nesting, such as Common Nighthawks and Chimney Swifts are on the decline. The competition for food and nesting sites with the growing populations of sedentary urban birds is too great for migratory ones. As cities grow there are fewer and fewer niches for birds that only visit for a few months of the year.