Scientists know that bird song sounds different in cities than it does in rural areas. For example, birds tend to sing at a higher pitch in urban habitats because higher frequencies carry better over the rumble of traffic, construction, and other noise in cities.
But noise isn’t the only salient characteristic of city habitat, and other qualities of bird song also vary between city and country sites. Yet so far, few studies have investigated what other environmental features might drive how city birds sing.
A new study by Ohio State University researchers is the first to simultaneously assess multiple potential explanations for urban bird song changes. The researchers recorded the songs of 66 male northern cardinals in 9 forest patches—some heavily urbanized, some rural, some in between—in the Columbus, Ohio area.
Male cardinals sing both to attract females and to keep other males off their territory. Cardinals do well in human-dominated habitats, breeding successfully in both the city and the country. In fact, the birds can be found at four times higher densities in urban compared to rural areas, especially on sites with thick, shrubby understory vegetation. Many other generalist bird species also reach higher population densities in cities.
The researchers visited the study sites weekly from March through August, the cardinal breeding season, they report in a recent paper in Behavioral Ecology. Spectrograms of the recorded songs show that, as expected, urban cardinal songs are higher pitched, with minimum, maximum, and peak frequencies increasing along the rural-to-urban gradient. The data also show that urban territories are noisier and songs are higher pitched in noisier territories, all in line with previous findings.
Urban songs are also longer and faster than rural ones, the researchers found. Longer, faster songs have also been reported in previous studies of urban birds, although no one has investigated why these shifts occur. In fact, these findings have puzzled scientists because slower songs would be expected to carry better over urban background noise.
But the new study was able to pinpoint the reason. The population density of cardinals is higher in urban areas, and song length and speed increase along with population density. “The longer and faster songs of males from sites with high densities may be a behavioral consequence of increased territorial interactions between neighbors,” the researchers write. In other words, male cardinals in crowded urban environments have to defend their territories against more potential intruders, and this causes them to sing longer songs and sing more rapidly.
Most previous studies of urban bird song have only looked at how the physical environment differs in cities, and have missed the social context, the researchers point out. But social life in cities differs for birds just as it does for people. Future studies should investigate multiple possible explanations behind “urban” changes in wildlife behavior, they say.