During the summer of 2008, birds were recorded in Washington, D.C. area backyards. Surprisingly, in areas with intense development (e.g. more pavement, buildings and roads), birds sang songs with a lower maximum frequency (the highest pitch of their song gets lower) and a narrower frequency range.
Lower-pitched sounds transmit (or travel) better in areas with a lot of hard surfaces and are less likely to be scattered, reverberated (echoed) or absorbed. This result may represent an adjustment birds make for better song transmission in urban environments.
However, in areas with higher noise levels (mostly from road traffic), birds sing songs with a higher minimum frequency (they shift the lowest pitch of their songs higher). Noises made by humans tend to be low-pitched, so this result may represent an adjustment birds make to avoid having the low-pitched portions of their songs masked by human noise.
After determining that bird songs differ in pitch in these various sound environments, we next set out to determine if the song adjustments birds make affect how the receiver (the bird that hears the song) responds.
In the summer of 2009, playback experiments were conducted. A bird song with certain pitch characteristics was played in the territory of a breeding pair of birds and their aggressive response was measured. There was no difference found in response to songs with the lowest pitch shifted versus songs with no pitch shift.
Birds did respond more strongly to songs with a lower peak frequency (more energy in lower pitches). These results may indicate that adjustments that affect minimum frequency (like the adjustments seen in noisy environments) may not be noticed or may not be important to receivers, while adjustments that affect how low pitched the song is (like the adjustments seen in response to urban development) may have an important effect on bird communication.
In the summer of 2010, a sound transmission experiment was conducted in 62 Neighborhood Nestwatch backyards. Since birds sing differently depending on whether they live in urban or rural areas, the hypothesis that birds sing songs that are best matched to the habitat where they live was investigated.
Different bird song types were played (songs sung by country birds and city birds) and recorded at different distances in 12 forested, 11 rural, 30 suburban and 9 urban sites. The goal is to determine how the song changes as it moves through each type of environment (i.e. how much is the song scattered, absorbed, echoed and masked by noise in different environments).
From this, it can be determined if the song types that birds sing in the city transmit better when played in the city and if song types sung in rural areas transmit better in the country. Stay tuned for results!
The results have been presented at several research symposia at Cornell University (where Jenélle now attends graduate school) and will be presented in September 2010 at the International Behavioral Ecology meeting in Perth, Australia.