Urbanization, the process by which towns and cities grow to accommodate increasing populations, has a major impact on bird diversity worldwide. As natural habitats are replaced by human developments, the composition of bird communities changes dramatically. Understanding how urbanization affects birds can help guide conservation efforts to protect declining species.
Birds are an important part of ecosystems, contributing to services like pollination, seed dispersal, and pest control. However, human activity has caused severe declines in bird populations globally. The leading threats to birds are habitat loss, climate change, pollution, collisions with man-made structures, and predation from invasive species. Urban areas intensify many of these threats due to the density of buildings, impervious surfaces, introduced predators, and other hazards to avian life.
Yet cities also provide opportunities for some species to thrive. Urban areas contain a mosaic of habitat types including parks, gardens, vacant lots, and street trees where birds can find food and nesting sites. Species adapted to human proximity and disturbance tend to increase with urbanization, while those that rely on natural or specialized habitats decline. Overall, urbanization reduces native bird diversity, shifts community composition towards non-native species, and threatens populations of range-restricted and human-sensitive native birds.
Changes in Bird Abundance and Diversity
Studies consistently show that increasing urbanization leads to lower bird species richness, or the total number of species present. For example, a survey of breeding birds across France found that highly urbanized areas had 30% fewer species on average than nearby rural areas. Declines can be even more dramatic in tropical regions. One study in Brazil documented 64% fewer species in urban sites compared to adjacent Atlantic Forest.
In addition to hosting fewer species overall, cities favor bird generalists over specialists. Generalist species, such as house sparrows and European starlings, can utilize a wide variety of urban resources. Meanwhile specialist species become locally extinct as their niche habitats disappear. Forest-dependent birds are especially vulnerable to urbanization in tropical regions given the extensive deforestation for housing and infrastructure.
While native diversity drops, urban bird communities become dominated by a few highly adaptable species. Rock pigeons, house sparrows, European starlings, and barn swallows are common urban exploiters worldwide. Introduced species often make up a substantial proportion of urban bird diversity. For example, over 25% of breeding birds in New York City are non-native species introduced by human activity.
Changes in Abundance of Particular Species
Urban adapters that thrive in developed areas show population increases with greater urbanization. Species often considered “nuisance birds” in cities include:
- Rock pigeons
- European starlings
- House sparrows
- Canada geese
- Ring-billed gulls
- House finches
Meanwhile, species sensitive to human disturbance and habitat loss decline in abundance. Neotropical migratory songbirds, including flycatchers, swallows, woodpeckers, and warblers, are especially vulnerable. For example, Baltimore oriole populations decreased with urbanization in North America. Urban-sensitive birds also include raptors, such as the red-tailed hawk and Cooper’s hawk, which require natural habitats for nesting and hunting.
Mechanisms Behind Shifts in Diversity
Several key mechanisms drive changes to urban bird communities:
Loss of Natural Habitats
Clearing land for development destroys and fragments natural habitats. About 75% of the Earth’s land surface has been altered by human activity, mostly since the 1950s. Birds specialized on particular habitat features lose out as cities expand into surrounding landscapes. Loss of forests and wetlands hit species like woodpeckers, flycatchers, and marsh birds especially hard.
Proliferation of Buildings and Impervious Surfaces
Buildings and paved surfaces create physical hazards for birds navigating urban areas. Collisions with buildings and windows are a major source of avian mortality. Impervious surfaces like concrete increase stormwater runoff which degrades water quality in urban streams. Dark-colored pavement also raises temperatures in the urban heat island effect.
Abundance of Human Food Sources
Several species capitalize on human food sources like garbage, pet food, bird feeders, and hand feeding in public spaces. Abundant artificial food allows urban exploiters to thrive at unnaturally high densities. Reliance on foods like bread and popcorn can also be unhealthy for birds.
Introduction of Non-native Species
Human trade and travel fosters introduction of exotic bird species to new regions. Some species, like European starlings and house sparrows, become invasive and outcompete native birds for nest sites and food. Domestic cats also kill billions of birds in the U.S. every year, exacerbating declines especially for ground nesting species.
Noise, Light, and Chemical Pollution
Cities create noise pollution from vehicles, sirens, machinery, and other technology. Noise interferes with bird communication and ability to locate prey by sound. Excessive night lighting can disrupt circadian rhythms. Chemicals and toxins from industry, traffic, and landscaping also degrade urban habitat quality.
Consequences for Avian Conservation
Urbanization threatens bird populations locally and globally. More than 1 in 5 bird species worldwide is imperiled, with urban expansion a leading driver of declines. Protecting birds in cities is an important conservation strategy and brings ecological benefits to urban residents.
Birds provide essential functions like controlling insect pests, pollinating plants, and dispersing seeds. Diverse bird communities also contribute aesthetic, cultural, and recreational value to city parks and green spaces. Abundant native birds improve quality of life and connect urbanites to nature.
Making cities more bird-friendly through planning and design interventions can balance growth with conservation. Urban bird diversity depends on preservation of natural habitats and restoration to support sensitive species. Sustainable design and policies that reduce hazards will help populations of both common and threatened birds persist alongside urban development.
Key Research Findings
Scientific studies around the world consistently demonstrate large-scale shifts in bird communities with urbanization:
- Native species diversity declines by 20-60% in urban versus rural areas
- Specialist species are lost while generalist species increase
- Non-native species make up a greater proportion of urban diversity
- Grassland and forest specialists are most negatively affected
- Building collisions kill hundreds of millions of North American birds annually
Case Study: Bird Diversity in Phoenix, Arizona
Phoenix provides a typical example of how urbanization drives bird community change. Arizona’s capital is located in the Sonoran Desert and has rapidly expanded over the past century. A study sampling breeding birds across an urban gradient in Phoenix revealed shifts from native desert species to human-adapted urban exploiters:
| Bird Group | Response to Urbanization |
|---|---|
| Native desert birds | Declined in abundance with increasing housing density, roadways, and impervious surface. Examples include cactus wren, curved-bill thrasher, and Gambel’s quail. |
| Birds adapted to suburban habitats | Most abundant at intermediate housing densities. Examples include mourning dove, Anna’s hummingbird, and black-chinned hummingbird. |
| Urban-exploiter species | Increased with urbanization. Examples include rock pigeon, European starling, and house sparrow. Accounted for over 50% of individuals in highly developed areas. |
The study confirmed urbanization in a desert landscape drives decline of specialized native species while enabling generalists to dominate.
Conservation Recommendations
Protecting avian diversity in urban areas involves both preserving natural habitats and designing new developments to support birds. Recommendations include:
- Establish nature reserves, parks, and greenbelts to conserve habitats for sensitive species
- Protect and restore riparian areas which are disproportionately important for desert birds
- Promote native vegetation and avoid invasive exotic plants in landscaping
- Reduce glass facades on buildings and install bird-friendly window treatments
- Add nest boxes, soil beds, and rooftop gardens to create habitat diversity
- Dim night lighting and use wildlife-friendly wavelengths to reduce disruption
- Conduct environmental education to connect residents to local birdlife
Conclusion
Urban expansion threatens bird populations by degrading and eliminating natural habitats critical to avian life cycles. While some species adapt to urban environments, overall diversity declines as specialist natives disappear and invasive generalists take over. Protecting parks and greenspace allows urban planning to balance growth and bird conservation. Making cities bird-friendly helps preserve threatened populations and brings benefits like pest control and aesthetic value to residents.
