Birds of a feather flock together when adapting to urban growth

Urban areas are the fastest-growing habitat on earth, but little is known about how plants and animals fit themselves into landscapes dominated by humans. Now, researchers from the University of Massachusetts Amherst and Arizona State University have shown that birds faced with urban development are a lot like people when choosing a place to live. Some species, including pigeons, thrive in the noise and confusion of city life, others prefer to stay firmly rooted in the country and some are comfortable in both environments.

'Urbanisation turns large areas of wild land into cities and suburbs, and has a profound effect on native species, changing where they live and how they interact,' says Paige Warren, an urban ecologist at UMass Amherst. 'Knowing how these organisms relate to each other and their environment is critical to developing sound conservation strategies.' The study focuses on Phoenix, Ariz., but Warren expects that similar patterns exist in cities throughout the United States.

Analytical work for the study was done by Jason Walker of Arizona State University. The team also includes Robert Balling, John Briggs and Elizabeth Wenz of Arizona State University and Madhusudan Katti of California State University, Fresno.

Using bird count data taken over a two-year period in Phoenix, and computer models developed for mineral mapping, the team illustrated some striking trends in the distribution of birds over the Phoenix metropolitan area, including surrounding agricultural land and remnants of the Sonoran Desert. Computer models generated maps showing where certain species of birds were likely to live, including the rock dove, or pigeon, the cactus wren and phainopepla, a bird native to the deserts and dry woodlands of the Southwest.

Anyone who has spent time in a major city can probably guess that pigeons showed a marked preference for urban life. 'Maps showed that pigeons, a flagship urban species found in cities around the world, adhered strongly to downtown Phoenix,' says Warren. 'There was a sharp decline in the probability of finding them in the outlying desert and agricultural regions.'

According to Warren, pigeons didn't migrate from their wild homes to conquer the big city by themselves. 'These birds were taken from the rocky coasts of Malta several hundred years ago and kept as pets in Europe, where they became genetically different from wild pigeons,' she says. 'Eventually they escaped into cities, where they adapted remarkably well, since they will eat almost anything and are well-suited to live on the sides of buildings.'

At the other end of the spectrum was phainopepla, a crested bird native to the desert and dry woodlands that feasts on mistletoe berries and insects caught on the wing. The probability of finding phainopepla in the desert was high, and this species showed no ability to penetrate into the city successfully. Because of this, phainopepla is sensitive to habitat loss from the conversion of desert to farms and developed areas, which has already reduced the number and size of breeding populations.

The cactus wren showed a different distribution, and emerged as an interesting intermediary species. 'The cactus wren is usually associated with the desert, since it builds nests in the protection of cacti and other thorny plants,' says Warren. 'However this native species was able to penetrate the urban ecosystem more successfully than phainopepla, and has been seen nesting in satellite dishes and other man-made structures.'

Computer models used in the study proved to be a useful tool to fill in the gaps between the observation sites where bird counts were taken, overcoming the difficulty of predicting where birds will be found in large areas of urban development.

'Usually we can look at the characteristics of a habitat, like food and water availability or types of shelter, and predict whether a certain species would be found there, but urban ecology is a young science, and we don't have a sense of what features an urban habitat should have to support different species,' says Warren.

This research was supported by the National Science Foundation, the Central Arizona-Phoenix Long-Term Ecological Research Program and the IGERT Program for Urban Ecology at Arizona State University. Bird census data was collected as part of long-term ecological monitoring by the Central Arizona-Phoenix Long Term Ecological Research project.