So, urban birds as a topic can be approached from many angles but as the number of quasi-urban dwelling ravens that I encounter increases I find myself thinking more and more about urban birds and evolution and things of this nature. I mean, if nature typically selects for species that can survive the best in current conditions...what is happening to urban birds? Well, it seems logical that birds that shy from humans or are non-charismatic species that act conspicuously or aggressively will be selected AGAINST. Birds that perform well within cities, in monocultured agriculture fields, in suburbs and in early seral areas will continue to see population inclines while birds that depend on wild/natural/non-human affected areas, don't deal well with the toxins we proliferate and that are within reach of the feral and/or invasive predators that come along with urban expansion (and often follow the gravel roads into non-urban areas) will continue to see population declines.
The U.S. Census Bureau classifies an area as urban if the population density is at least 1,000 people per square mile at the time of census. Thus, there can be a spectrum of areas that are classified as urban. One of my favorite things about Oregon is that some of our urban areas are still incredibly diverse and strive to balance population growth with the natural world around them. All too often though urban areas are a homogenized area void of many things natural, concrete and steel ecosystems as one group from Stanford wrote in 1988. In this short piece they mention some characteristics of city dwelling birds that I hadn't really thought about before. Namely that birds that live in cities have to be adapted to vocalizations that are either at a frequency or decibel that are high enough for potential mates and other members of their flock to hear. Urban areas create problems for migrating birds in a number of ways. For waterfowl cities are often a varitable waste land of concrete, glass and steel. Any water that may be available is often polluted (though mallards, great blue herons and some other wetland/water dwelling birds can be found in urban areas). For passerines cities create a maze of glass, light and noise pollution, toxins and a myriad of other problems that lead to confusion, disorientation and ultimately death. Birds that live in cities have adapted to foraging for the food that we throw out, on the bits of plant matter that make their way through cracks in sidewalks and in urban parks.
The second leading cause of population declines that lead to a species being listed as endangered or threatened by the U.S. Fish and Wildlife Department is urbanization. The primary cause of federal listing as threatened or endangered is interactions with non-native species (Czech, et al. 2000). European starlings, house sparrows and house finches are all non-native/invasive species that often out compete native species and are found in urban areas, thus it can be surmised that there is overlap between urbanization and interaction with non-native species. In addition to non-native birds competing with native birds we also have feral cats, "outside" house cats too, that prey on birds. In fact, according to Czech, et al. of the 18 reasons that species become threatened or endangered at least eight of them are associated with urbanization (and all but two have to do with humans).
An Audubon study found that 45 of the at least 700 species of birds found in North America inhabit urban areas. Of these 44 are species of least concern and one is a species considered threatened or endangered. According to the study 20 of the 45 species have seen declines in numbers, and 23 of the 45 have seen increases in numbers, from 1966 through 2003. Of these species 10 can be also be found in natural aquatic/wetland ecosystems, 14 can be found in multiple natural areas, 15 in woodland areas, five in scrublands and one in grasslands. Homogenization of the natural world is a well documented effect of urbanization. Urban areas are homogeneic ecosystems that support growing numbers of non-native species. Biodiversity of bird species decreases as urban areas increase. In a study of "wooded streets" (streets with trees planted along the sidewalk) in Valencia, Spain Murgui found that these areas were home to only eight species in winter and 11 species in the breeding season, a mere 25% of the number of species found in urban parks (Murgui 2007). Based on the information above it can be surmised that the largest variety of species in urban areas will be found in urban parks. On a side note increasing numbers of urban community gardens helps increase avian biodiversity as does increased wildlife friendly landscaping. Despite these oasis of diversity non-native and human "friendly" species are on the incline while natives and non-aggressive species are being out-competed (populations are on the decline).
So, what can we do? We can develop landscape practices and plans that encourage native species at businesses, homes and public spaces. We can encourage community gardens and the development of green space. Finally, we can work at creating wildlife corridors, particularly through developing and managing natural areas such as city parks, so that non-edge dwelling species (species that will not happily inhabit areas at the edge of their habitat) can move from habitat patch to habitat patch. Additionally, we can spay and neuter cats and keep them indoors so that the feral cat populations won't decimate bird populations. Dimming city lights can help reduce confusion during migration, taking public transportation can reduce air and noise pollution, eat local, reduce, reuse, recycle, leave no trace when recreating outdoors, plant fruiting trees away from roadsides and be conscious of the things we do. Birds are amazingly diverse and many/most of us appreciate them for their diversity (life lists wouldn't be very interesting if there were only 45 ish species to watch). Urban sprawl may not be something we can completely halt but we can work at reducing the impact of urbanization on global avian populations.
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