It is well-known that birds engage in a vocal arms race with humans. To avoid being drowned out by the constant drone of traffic and industrial noise, songbirds have been increasingly pressured to make themselves heard in the urban jungle.
Some birds, such as the great tit, can easily rise to the challenge ?- this adaptable species has been shown to sing faster and at higher pitches in urban environments. However, not all birds are flexible enough to succeed on man's turf: a cruel reality that is reflected in the decrease in biodiversity near man-made roads and cities.
In the densely-packed woodlands, birds tend to sing slower and at lower pitches to prevent their songs from being blocked out by the foliage. On the other hand, the urban environment presents a novel set of challenges for chorusing birds ?- it is open, meaning that there are fewer surfaces to block sound transmission, and packed with low-pitched sounds. While males with deep voices are more successful in the woods, the opposite is true in the city.
There are several theories for why birds become city-smart. One hypothesis proposes that young birds drop low songs from their repertoires due to their inability to hear them above the city noise.
Another proposes that urban birds are forced to use higher-pitched songs because the low-pitched ones do not prompt the mating response. Either way, it is clear that birds face a number of challenges from living near an urban environment.
Studies have shown that birds from noisy areas respond less strongly to birdsongs from quieter areas, implying that breeding opportunities, and even mate recognition, are impeded by noisy environments.
To make things worse, many birds cannot significantly change their songs after the first year of life, limiting their ability to adapt to different environments. Each species of bird has their own way of overcoming man-made noise barriers. Nightingales cope by raising the intensity of their songs, blasting German cities with their 95-decibel ballads. Robins, on the other hand, reserve their singing for the night.
To add to this wealth of information, a recent study at the University of Copenhagen and the University of Aberystwyth identified an additional factor influencing urban birdsong - the physical structure of cities. Co-author Torben Dabelsteen of the Section for Ecology and Evolution at the University of Copenhagen's Department of Biology argues that the need for birds to drown out competitors does not sufficiently explain why birds sing at lower pitches in the city.
According to the researchers, birds cope with urban noise in one of two ways: directly, by raising their tones and staying away from noisy environments, or indirectly, by attempting to drown out anthropogenic noise. Through the use of controlled sound recordings, the team showed that higher frequency notes are transmitted across cities in the absence of noise from traffic. From this, Dabelsteen concludes that the physical structure of cities must contribute to the heightened song frequencies.
Cities are complicated and birds must take into account how different structures will transmit or reflect their notes. While birds can easily spot each other in the urban jungle, they must also learn how to communicate effectively by reducing echoes from buildings and narrow streets. City life is hard work for these birds.
Their country-dwelling cousins can attract females without having to sing at full force. In the woodlands, abundant trees and foliage distorts sounds and obscures birds from each other. Thus, country birds have learned to use these distortions to judge the distance to the nearest friend or potential mate.
Unfortunately, human activity does not only affect bird communication. Humans are a powerful selective agent that can affect animal communication, predator-prey relationships, and even the survival of an entire species. Whales and dolphins fight to hear each other above the sound of ships, insects attracted to artificial light become easy pickings for bats, and pests such as cockroaches and rats find comfortable hiding places in human dwellings. In this time and age, survival of the fittest means, for many animals, adapting to humans.