The effect of urbanization on the plumage of white-crowned sparrows (Zonotrichia leucophyrs)

Background/Question/Methods

Human population growth is resulting in the modification of many habitats around the world. One of the most dramatic changes in habitat is in urban areas, which has caused many species to be extirpated from these regions. However some native species persist in urban areas, but they are under strong directional selectional pressure to adapt to local urban habitats. Recent studies on animal communication have shown that animals alter their acoustic communication in the presence of anthropogenic noise. One example is the white-crowned sparrow (Zonotrichia leucophyrs), which in cities, but not in nearby rural populations, has altered its song. Despite this surge in research, there has been little investigation into adaptations of other features in this native species in urban areas. We studied museum specimens of white-crowned sparrows from the San Francisco, CA area using a digital imagery analyzer, Image J, to develop quantitative data on plumage differences. We looked at plumage coloration in both urban and rural populations with the hypothesis that coloration on the back would be adaptive to the local environment for camouflage. We studied the effect this increased in homogeneity, and decreased native vegetation has had on the plumage of the white-crowned sparrow.

Results/Conclusions

We found significant differences in the dorsal (back) plumage of white-crowned sparrows between urban and rural populations in the San Francisco area. We found that the birds from rural populations have significantly higher color values for red (t-value = -3.30, DF=60, p= 0.002), green (t-value = -2.32, DF=60, p= 0.02) and black (t-value = -2.21, DF=60, p=0.03 ) color in their plumage than urban populations. Thus the urban population has duller plumage than rural populations. We attribute this difference to the relatively grayer and more homogenized urban environment. These differences in plumage coloration were found in both the recent and historical samples indicating the differentiation may have occurred over the past 100 years of urban development. Our findings indicate a newly observed adaptation to urban environments by native species, and suggest that many traits, in addition to communication signals, may be changing in response to urban selection pressures.