Where do birds go?

Review on Bird communities and the structure of urban habitats

“Okay,” I said. Then I thought of something, all of a sudden. “Hey, listen,” I said. “You know those ducks in that lagoon right near Central Park South? That little lake? By any chance, do you happen to know where they go, the ducks, when it gets all frozen over?

Ecologists have been focused on the distribution and richness of a species in the natural environment for a long time. How organism lives in their natural habitat has been intensively studies. Recently, with the rapid urbanization, ecologists have started to focus on how organisms live under the urban environment, a relatively “unnatural” environment for most of the organisms. Understanding how urban settings influence the dynamic of population helps the preservation of urban species. The urban environment brings both challenges and opportunities for the creatures it hosts. A studies from Lepczyk et al. (2017)^[1] shows that artificial green space is the major habitat for most of the urban species. Different from “natural” habitat, the urban green spaces, usually separated by artificial construction, are more fragmented and disconnected. The fragmented green space prevents organisms from switching between different types of green space and maximizing their fertility. Under the “natural setting,” where habitats are usually connected, one organism could choose a habitat with more resources during the growing season and then transport to other habitats with less competition to lay their eggs. The prohibition of habitat choice decreases the total fertility of each species and therefore fails to maximize the potential of urban habitat. However, for our focus species today, birds, their high mobility allows them to switch between habitats with ease. Therefore, the habitat bird chooses is aligned with their will, which maximize the urban habitat benefits. By taking a close look at the population dynamics of bird species, one can see how they relate to the structure of the habitat.

In a paper by Lancaster and Rees (1979)^[2], the authors explore how environment structural influences the overall diversity, richness, and density of individual species. They collected data by surveying the abundance and diversity of bird species under different habitat settings. The authors divide the habitat they surveyed into five major categories (i.e., commercial industrial, apartment, suburban, woodland, and rural) by both artificial construction coverage and foliage. The main finding from the study is that the foliage coverage is a positive indicator of bird species diversity while the artificial construction coverage is negatively correlated with bird species diversity. One interpretation (Emlen, 1974)^[3] of the negative correlation between artificial construction coverage and bird species diversity is that, since the urban habitat is fairly novel for bird species, it takes time for them to adapt toward “harmony with the new habitat” and occupy vacant niches in artificial construction. However, the authors refute this opinion by pointing out that the theory is based on the assumption that man-made features, which generate vacant niches, and therefore increase the habitat structural diversity. From the study, the author claim that most of the man-made features do not necessarily increase the diversity of the habitat. To find out the underlying causes of the negative relationship between bird species diversity and artificial construction coverage, the author then studied the bird species that have a higher density under urban settings than under rural settings, implying successful adaptation to the urban environment. From the survey, the house sparrow, rock dove, and American robin were found to be the species that dominated the urban environment.

Figure 1. Gang of four. From left to right, House sparrow (Passer domesticus), common starling (sturnus vulgaris), American robin (Turdus migratorius), and rock dove (Columba livia). Pictures credited to wikipedia.org

The authors found the success of the four bird species within the urban environment is related to the availability of food resources. The main food resource of the urban area is the food provided by people. The authors discovered a strong positive correlation between food provided by humans and the density of the four bird species. Because the quantity of food provided by humans exceeds the caloric requirement of the bird species, the availability of energy essentially become an unlimited factor.

The study brings many insights about the distribution of bird species under urban settings and also offers implications about bird preservation in the urban environment. In contrast, our ecology projects will focus on the diversity and richness of bird species under suburban settings. In our study, we will examine the bird habitats on the northern part of the Grinnell College campus. Grinnell College, located in the middle of Iowa and distant from the major cities, not only offers us an isolated environment to examine the human impact on bird species, but also a chance to verify Lancaster and Rees’ hypothesis on urban adaption of certain bird species. Since the Grinnell College campus is surrounded by “natural” environment, it is easier for bird to retreat from the area with intensive human interference (the area we observed) to the area with less human interference. Therefore, we can observe how human activity influence the habitat choice of bird. Also, we are interested in whether there exists any birds that take advantage of connected habitat, which differs from urban environment that Lancaster and Rees studied. For example, if there exist bird species that choose to forage in area with more human activity, which means more food, and then returns to remote area to rest without human interference.

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1. Lepczyk, C. A., Aronson, M. F., Evans, K. L., Goddard, M. A., Lerman, S. B., & MacIvor, J. S. (2017). Biodiversity in the city: fundamental questions for understanding the ecology of urban green spaces for biodiversity conservation. BioScience, 67(9), 799-807. ↩

2. Lancaster, R. K., & Rees, W. E. (1979). Bird communities and the structure of urban habitats. Canadian Journal of Zoology, 57(12), 2358-2368. ↩

3. Emlen, J. T. (1974). An urban bird community in Tucson, Arizona: derivation, structure, regulation. The Condor, 76(2), 184-197. ↩