White-crowned sparrow http://www.flickr.com/photos/lostinfog/ / CC BY-SA 2.0 |
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In the middle of a snowstorm in Baltimore, there is a lack of sound. There are no moving cars, no birdsong in the trees, and the insulating snow dampens any cross-street conversation. Even the sirens seem more silent than usual. Luckily, we have the University of Maryland to remind us of the importance of urban sound. Published in the February issue of the Proceedings of the Royal Society B, David Luther from UM-College Park and Luis Baptista from the California Academy of Sciences report on their study on the effect of urban noise on white-crowned sparrows.
Human-made noise has long been suspected to have an impact on bird populations. Bird species are less abundant near highways and urbanization limits bird distribution. Noise has been shown to reduce nesting success and affect species interactions. Urban birds sing at a higher minimum frequency than rural birds to be heard above the low-pitched human-caused noise. (Think of the hum of the city.)
Birds, like humans, speak in vocal dialects that change with geographical region and often differ between neighboring populations. The alteration or change in learned songs between one generation and the next is described as the cultural evolution of songs. Some species of birds, such as the indigo bunting, are capable of changing dialects within less than a year, whereas others maintain their songs for decades or longer.
Luther and Baptista’s study is the first long-term study to document an increase in the minimum pitch of song over multiple generations of urban sparrows with a mixture of three dialects. By examining urban sparrow birdsong recordings spanning almost 30 years (1969, 1970, 1990, and 1998), they essentially observed the cultural evolution of sparrow song in a small San Francisco population. (Sparrows only live for 2 years, eliminating the possibility of recording the same individuals from the beginning to the end of the study.) Luther and Baptista found that the lowest frequency of birdsong rose over time with a shift to the dialect previously found within the most urban environment with the greatest ambient noise. This result of shifting dialect indicates an adaptation to the local acoustic environment over multiple generations. Songs (or dialects) change to a higher pitch to transmit more effectively in ambient noise.
So if urban noise affects bird dialects, does this mean that Baltimoreans will be speaking a higher pitch of Bawlmerese in 250 years? Maybe… if our breeding depended on it.
