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Noise Pollution and Bird Song

By David Bird

Some of our city birds may be changing their tunes in more ways than one. Studies in the mid-1990s in the Netherlands revealed decreased songbird densities near roadways and that traffic noise appears to be largely responsible. In short, some bird species are unable to adjust to living and breeding in areas with a lot of human-generated (anthropogenic) noise. However, some species are actually making changes in some aspects of their vocalizations to compensate for the excess noise from our lawn mowers, airplanes, industry, and - most of all - vehicle traffic.

William Wood and Stephen Yezerinac of Reed College in Portland, Oregon, have conducted the first study in North America of the impact of noise in urban environments on bird song, specifically in the song sparrow, and published their results in the July 2006 issue of The Auk. Their paper is titled "Song Sparrow (Melospiza melodia) Song Varies With Urban Noise."

It is not the first such study in the world - great tits in the Netherlands were demonstrated to sing at a higher pitch in urban environments about three or four years ago. Moreover, common nightingales on territory were also found to sing louder in areas with more traffic noise. Noise pollution is not always associated with humans. Around the same time, it was discovered that populations of little greenbuls living in densely vegetated African forests with little low-frequency noise had songs with lower minimal frequencies than those living in grasslands with relatively more low-frequency noise. The study by Wood and Yezerinac on song sparrows in Portland, Oregon, though, brings the problem a little closer to home.

Environment and Vocalization

First, some background on how a bird's environment can affect its vocal signals. Basically, ambient noises, especially those with similar frequencies, can significantly mask a bird’s song, affecting such aspects as its amplitude, frequency, rhythm, timbre, and call band-width. Burgeoning human populations have led to increasing urbanization, presenting "evolutionarily novel acoustic environments for many birds worldwide."

In short, this is an important issue and we have only recently begun to pay attention to it. Clear song transmission is critical to most birds because their reproductive success may depend on it. Song is important in resolving conflicts between males, allowing them to maintain their territories and repel intruders. Moreover, the quality of song learning in males and song repertoire size have both been positively correlated with female preferences when choosing a mate.

Why Song Sparrrows?

Wood and Yezerinac did not choose the song sparrow at random to investigate effects of urban noise. They liked the fact that this species is found over most of North America in both rural and urban habitats and that individual males generally sing between 5 and 13 different song types, cycling through the various song types, repeating one type for several minutes before switching to the next. Each song is composed of a series of notes and trills with frequencies ranging from 1,140 to 9,280 Hertz (Hz), which partially overlaps the frequencies of urban noise.

Song sparrows live in Portland, Oregon, year-round and begin their territorial singing around late February to early March. The study area chosen by Wood and Yezerinac encompassed territories of hundreds of song sparrows in habitats ranging from urban parks to residential yards. They recorded all song sparrows, but not those with territories within 12 feet of one another so as to avoid ambiguity. In the end, they recorded 28 different song sparrows, along with the prevailing ambient background noise.

Of the 28 different birds, 26 employed two or more song types. Two songs from each individual were selected at random and measured for high frequency, low frequency, frequency of greatest amplitude, duration, and the number of notes. They did the same for the buzz sounds in the calls, but not for the number of notes.

Results

"Song sparrows singing at noisier locations had higher-frequency low notes and had relatively less energy (amplitude) in the low-frequency range of their songs (1 to 4 Hz)," wrote Wood and Yezerinac. "These results clearly indicate that song sparrow songs varied in their low frequency in relation to levels of urban noise."

Because background noise in the study area was mostly at low frequencies, the masking of the middle- and upper-frequency portions of song was minimal. The most interesting finding was that the song sparrows did modify the low frequency part of their songs while keeping the other parts of their songs fairly constant. This same finding was also observed in the great tit study.

Possible Techniques

How could they do this? Well, Wood and Yezerinac offer several possibilities. First, they may have the ability to modify their songs between or during song bouts according to the prevailing noise. This is analogous to being able to reflexively increase their amplitude of vocalization in response to masking noise, which possibly evolved as a means to deal with environmental noise from sources such as wind or waves. Because song sparrows have a repertoire of 5 to 13 song types that differ in frequency, they might choose to sing those songs that are not as masked by the prevailing ambient noise. On the other hand, they might actually be able to change the frequency at which they sing particular song types. Wood and Yezerinac attempted to record the same roadside individuals at times of peak and low traffic flow, but they concede that their preliminary data are inconclusive at this time.

Changing Songs?

A more interesting possibility is that the individual birds in question undergo some sort of developmental or ontogenetic change that gives them a song repertoire that combats the noisiness of their territory. Here’s how that might work. Song sparrows live year-round in the Portland area and exhibit high levels of territory fidelity. According to neurological studies, each spring male song sparrows undergo a partial renewal of brain regions involved in song learning and production. Behavioral research also shows that along with this renewal comes subtle changes in bird song. So, if a song sparrow is able to tailor its song production according to the prevailing noise spectrum it experiences on its territory each spring, the success of being able to attract females and repel male intruders will essentially provide feedback and reinforcement to retain this ability.

Another related possibility is the fact that individual birds do learn more songs than they will eventually sing and will selectively lose some of them during the course of their second year of life. This is called selective attrition, and it could be affected by more than just matching the song of their territorial rivals. Ambient noise on the territory where the bird eventually settles also could be important.

Perhaps this ability occurs earlier in life, such as during the critical juvenile period of song learning. "Juveniles living in noisier areas may not hear, and thus not learn to sing, the low-frequency notes of the previous generation," Wood and Yezerinac suggest. "Song sparrows also may listen to their own calls during the crystallization period of song learning and make changes if the notes sound masked."

A final theory is that natural selection could be acting on the existing genetic variation in song frequency if indeed songs of various frequencies do differ in their effectiveness for communication depending on noise.

What It Means

So song sparrows have to sing at a higher frequency in a noisy environment. So what, you ask. Well, singing loudly does have a cost - increased rates of oxygen consumption and energy expenditure. Besides translating into being forced to acquire extra energy to sing louder, its inherent risks, such as predation, could also lead to a decline in individual health.

But Wood and Yezerinac leave us with a rather more ominous message. Not all bird species living in noisy urban environments possess the ability to adapt and change their song frequencies to maintain their populations. Those who cannot will simply depart from that habitat. As for the song sparrows themselves, it is known that females can discriminate between songs from individuals from other song sparrow populations as far away as 20 miles. With that ability, according to Wood and Yezerinac, "the possibility of speciation between urban and rural populations of song sparrows should not be overlooked."