Sunday, May 20, 2012

Building the Biodiversity of Urban Birds

Image from the Cornell Lab of Ornithology archive
            City construction destroys the habitats of countless species, requiring them to either relocate to another area of wilderness or adapt to the new urban environment that has sprung up around them in order to survive.  Plants, being unable to move, have neither of these options, but there are a host of animals that have become ubiquitous to the city landscape.  Among these animals are many species of birds, but although some of them may be thriving in their new urban environments, others would benefit tremendously from a concerted effort on the part of city dwellers to build as much habitat as possible into their new surroundings.  As cities swallow up more and more of the land surrounding them, retaining urban bird biodiversity is critically important to ensuring the continued survival of these species.

            What makes the difference between birds that flourish in cities and birds that struggle?  One hypothesis is that birds with bigger brains have the intelligence and mental flexibility to adapt to the new environmental pressures—such as noise, traffic, lack of nesting sites, and predatory cats and dogs— of the urban landscape, and to take advantage of its benefits—such as increased warmth due to the “heat bubble” of cities and readily available food in the form of garbage and road kill.  Examples of a few such birds are the pigeon, the peregrine falcon (which nests on the crossbeams of bridges), and the crow.  Crows have been shown to be able to learn new behaviors from other crows, and to be particularly innovative in figuring out difficult problems for themselves.  Crow populations are increasing within cities, and they don’t seem to be in any hurry to leave.  Here is the link to"The Amazing Intelligence of Crows" a video of a TED talk by Joshua Klein including, among others, an example about how crows use traffic to crack nuts.
            Most birds, however, are threatened by urban sprawl and the destruction of their natural habitats.  Birds lose nest sites, foraging cover, and food and water sources when their homes are turned into buildings.  Ground nesters are particularly at risk in the city, as they require a lot of shrub growth under which to hide their eggs, and dense bushes are less likely to be planted in parks and yards than trees and ornamental flowers.  Bird species that occupy small, specific ecological niches are far less likely to be able to survive in the city than generalist species. For this reason, native bird biodiversity is low within cities, as are the numbers of native birds within a species that manage to live there; native birds are much more likely to have their only food sources, the only trees on which they nest, or even their entire habitats wiped out by urban sprawl.  In general, one study found that birds that do better in cities are those that include plant material in their diets, have a large natural geographic range, are tree nesters, and are not long-distance migrators.  (Full paper found here)    
Image from City Parks Blog    

            Parks and leftover forested fragments are, perhaps unsurprisingly, the bird biodiversity hotspots of cities.  In fact, even the “built” environment of a park—this being further within the built environment of the city—can be home to a surprisingly large number of bird species.  What’s more, we can continue to construct the park environment so that it benefits the greatest number and diversity of birds.  Increasing the number and area covered by trees appears to be the most influential change that can be made in order to positively affect bird biodiversity.  The type of tree matters as well.  It has been found that bird biodiversity in parks is at its maximum when there is an even number of evergreen and deciduous trees, providing the greatest variety of habitats.  This, in addition to the planting of shrubs of different heights, provides the vertical layering of plants that is extremely important as foraging cover and nest hiding places, as well as “corridors” that can conceal a bird as it travels between tree and ground.  Comparatively small areas in which there is a lot of vertical layering may have a greater effect on bird biodiversity than larger areas in which there is no vertical layering (i.e. trees surrounded by grass, as in the picture on the left).  (Full paper found here)
            The shape of parks matters as well.  Long, thin parks have fewer bird species than do parks with a lower edge-to-area ratio.  Parks and inner-city forest fragments with more edge habitat can only sustain the types of birds that are able to live in an edge area, causing lowered biodiversity.  Wooded streets may positively contribute to biodiversity by essentially increasing the size of the park to which they may be attached. By building parks with less edge and planting trees and shrubs along the streets leading up to them, we can engineer an environment that is more desirable to birds.
Image from
            City residents’ yards are also possible bird nesting and feeding sites, however there are several factors that affect the chances of a bird making its home in one.  The first is what types of plants are present.  Native birds are far more likely to nest in someone’s yard if there is a wide variety of native plants there, as well as plants of different heights.  A large expanse of lawn negatively impacts bird presence; by breaking up the lawn with shrub and tree “islands,” birds will have less exposure to predators while foraging for food.  Putting up nest boxes is also a good way to attract birds.  Cities naturally have fewer trees than rural environments, and if cavity-nesting birds struggling to find a site are able to lay their eggs in a nest box, bird biodiversity will be preserved that much more into the next generation.  Bird feeders, feeding tables, and water sources such as ponds or bird baths are other ways to substitute for resources these birds would traditionally have found in the wild.  Finally, making sure that pets are kept from hunting these birds is another way to promote bird biodiversity within the urban environment.
            Urbanization is a great threat to bird biodiversity, but with the right strategies, cities can be kept from destroying it altogether.  By constructing and modifying parks and yards to accommodate the greatest number of species, and by preserving and protecting the forest fragments and wetlands that remain undeveloped, we can “build” an environment that imitates those lost to urban sprawl and keeps bird biodiversity as high as possible.  


Dawson, Dan and M. Hostetler.  "Forest Remants: Conserving and Observing Bird Biodiversity in Urban Settings."  2010.  EDIS.    

Evans, Karl, D. Chamberlain, B. Hatchwell, R. Gregory, and K. Gaston.  "What Makes an Urban Bird?"  2011.  Global Change Biology: pp. 32-44.

 Fontana, Simone, T. Sattler, F. Bontdina, and M. Moretti.   "How to Manage the Urban Green to Improve Bird Diversity and Community Structure."  2011.  Elsevier; Landscape and Urban Planning: pp. 278-285.

Lerman, Susannah, and P. Warren.  "The Conservation Value of Residential Yards: Linking Birds and People."  2011.  Ecological Society of America: pp. 1327-1339.

McCaffrey, Rachel and R. Mannan.  "How Scale Influences Birds' Response to Habitat Features in Urban Residential Areas."  2011.  Elsevier; Landscape and Urban Planning: pp. 274-280.


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