How Birds Perceive Landscapes
A new study reveals how certain species select sites from the air
Imagine a Carolina wren and a red-tailed hawk flying over your neighborhood. Now consider the resources the two differently sized species may perceive in the landscape below. How does each choose one area over another for foraging or breeding? How do their criteria differ? Why would a bird zero in on a patch of habitat in your yard instead of one in a similar yard on the other side of town?
Those are among the questions I have been investigating for six years, first as a graduate student at the University of Florida and more recently as a researcher at Arizona State University. Ornithologists have long agreed that what they call structure--bushes, trees, fields or some combination of habitat types--is the main criterion birds use when they select an area for nesting or foraging. Some birds probably evaluate a much larger area than other birds. And within a given area, different species may respond to varying patch sizes. But to which area and patch sizes do different bird species respond?
That question has puzzled ecologists and conservationists for generations: The scales at which various species respond to habitat structure are virtually unknown. For example, which are most important to a given bird: the trees in your backyard or all the trees in your whole neighborhood?
To start answering these questions, I studied bird communities in eight North American urban areas, which I chose because people have transformed these landscapes. And they have done so at a variety of scales--from backyards, for example, to neighborhoods, to whole cityscapes. Also, urban environments are an important factor in the future conservation of many species. Not only has urban sprawl grown into the paths of stopover sites on bird flyways, but the sheer volume of human development has changed the amount of area available for nesting or overwintering species.
For this project, I first sifted through other researchers´ work to find the exact locations of concentrations of bird species recorded in these urban areas. Studying hundreds of aerial photographs taken of the same places, I correlated the reports of birds on the ground with bird´s-eye views of tree-canopy cover at the same spots. I considered the density of trees at each of these locations at four different scales. In other words, I took into account different amounts of landscape surrounding the location of a concentration of birds. When I looked at the aerial photographs, I was amazed at how landscape structure changes as one goes from small to large scales. A site that is forested at a small scale--such as your yard--may be embedded in a highly fragmented landscape at a larger scale--such as your side of the city.
Although my results are preliminary, I do have several observations based on the work so far. First, the presence of smaller bird species tends to be correlated with smaller tree patches. This result is loosely related to research indicating that larger birds tend to have larger home ranges. However, exceptions to this trend are common. For example, many small neotropical warblers require huge tracts of forest to breed in, while other species of the same size, such as chickadees, can breed in small forest patches. Still, these warblers are breeding in a small patch; it is just embedded within a large forest.
Most of all, my results indicate that much more research is needed. The amount and the sizes of patches at several scales do have a profound effect on which bird species appear in your backyard, but we are still at the beginning stages of understanding what these scales are. For now, it´s probably safe to assume that a bird´s home range is a rough indication of the scale at which a bird responds to landscape structure. To attract many bird species (especially the large ones), people should not only think about the amount of structure in their own yards, but also the amount in their neighbors´ yards, in their neighborhood and sometimes even in their section of the city.
I am now concentrating on one urban area: the Phoenix Valley in Arizona, which includes the rapidly growing urban sprawl around Phoenix itself. This research is part of a long-term project, funded by the National Science Foundation and Arizona State University, that is investigating a variety of urban ecological factors. Phoenix is changing so fast that if we track species diversity over a long time, we can see when and where it is that birds decide, "This is a good place to come down, and that over there isn´t." And, I hope, someday the birds´ criteria for landscape design might even influence our own.