Globally, industrialization is causing human populations to migrate into cities. As a result, urbanization is among the fastest growing land use changes. The necessity to see urban environments as true ecosystems in order to understand what these environments can provide for displaced species is a topic of growing importance.
Ornithologists are now attempting to determine what life history traits can predict a particular avian species’ probability of surviving an urban transition. To identify traits that make a species vulnerable to urban development, one group of researchers used a data set comprising avian responses to towns and cities for a <3,000 square kilometer, highly urbanized British region.
Previous research has found that migrating birds tend to disappear more quickly than permanent residents. Migrants may be disadvantaged when competing for more limited urban resources with permanent residents that have established territories. Migrants also seem to be more susceptible to mismatches in the timing of breeding and peak food availability. Such mismatches are becoming more common due to climate warming and increased climate variability, and are exacerbated in urban regions due to urban heat-island effects.
This recent study determined two new factors driving survival patters. The first important ‘survival factor’ is diet. Birds that feed on plant material have higher survival rates in urban environments than insectivores. High levels of “supplementary” vegetarian food supplies, including seeds and processed plant material, often characterize urban regions. Species that can exploit an entirely vegetarian diet may therefore be at an advantage over insect-dependent species.
The second important survival factor determined in this study is nesting location. Birds that nest above ground fare much better in urban environments than ground nesters. Obvious reasons for this include increased danger of ground nest destruction due to pedestrians, bicycles and automobiles. Ground nesters may also be at much higher risk of predation, due principally to the fact that they tend to produce broader, more open-cupped and vulnerable nests.
Studies quantifying traits that increase an organism’s survival chances due to urbanization are important not only in predicting which species will survive but in developing more conservation-friendly urban landscapes. For example, this study indicates that maximizing the availability of insect food sources, which could be achieved by increasing urban green space, may increase avian diversity. In addition to increased green space, the implementation or larger patches of green space may reduce intense resource competition that occurs on small, fragmented habitat patches. Improved suitability for ground-nesting species may be more challenging to implement, but is an important goal for city planners to keep in when designing new urban developments.
Evans et al. 2001. What makes an urban bird? Global Change Biology 17: 32-44.