I live in a relatively small town: the centre has three roads, no railway station and you can’t walk ten feet without running into someone you know. However, even in a small town like this you notice birds acting differently to how you might expect if they were in a more natural setting. For example, the other day I saw a crow swallowing a chicken kebab whole – surely there’s something wrong with that? I’ve also seen herring gulls mobbing someone’s car, the owner of which was attempting to hide inside with some fish and chips.
Another favourite example of an urban bird has to be ‘Sam’ the seagull. This story probably didn’t make world news, but it certainly was a talking point in Scotland:
In my personal experience, urban birds seem extremely versatile in their behaviour. However, a behaviour I had not considered until recently was that of their singing, and how it changes in an urban environment.
Human activities make noise which is generally low in pitch, and can be disruptive to birds in a number of ways. It can cause them stress, mask the sound of their predators, and interfere with their alarm calls and other acoustic signals they may use. When we are in a noisy environment, like a busy street or rowdy pub, we adjust the volume of our voice accordingly. Birds seem to do the same, singing louder than they usually would when in more urban areas. This is called the Lombard effect. Other bird species have been found to also sing at a higher pitch than what they usually would (at higher frequencies than the low-frequency anthropogenic noise). However, some researchers have proposed that birds may not be singing higher than normal as a response to the background noise, as this is not as efficient as just singing louder. Instead they have suggested that singing louder simply causes higher singing as a by-product, rather than that being the goal in itself.
In some cases it is easy to see how amplitude (loudness) and frequency (pitch) are physiologically related. For example, in non-songbirds like the ring dove, they have their call pitch determined by airsac pressure. As the airsac pressure also affects airflow and therefore amplitude, adjusting the frequency of a call also influences how loud it sounds. However, in songbirds the pitch of songs is not determined in this way, but instead through a particular set of muscles in the syrinx (the vocal organ of birds) which allow precise control of amplitude and pitch. So it seems that perhaps they could decouple these two components of their song when adjusting it to background noise. A recent paper by Cardoso & Atwell that came out in the journal Animal Behaviour looks into just this.
Using the dark-eyed junco, the researchers looked to see whether louder songs are sung at higher frequencies. These birds have songs which consist of a single trilled syllable, although some of them have multiple trilled syllables. The first and last syllables tend to be a bit quieter.
The dark-eyed junco singing it’s song:
Using a pre-existing database of over 1000 songs of 151 individuals, the researchers looked at these songs in detail. They compared the minimum and maximum frequencies (the highest and lowest notes) between the loudest and softest songs within a single bout of singing. They also looked at what frequency the song was the loudest for the loud and quiet songs (the peak frequency). They then looked at these three measures within a song, comparing the quiet first and last notes of a song to the louder remaining syllables of the same song.
The comparison between songs showed that louder songs actually had both a lower low-note and a higher high-note than the quieter songs. The loudest note was also a higher note in louder songs than in quieter songs. A similar pattern was found when looking within a single song. This shows that frequency and amplitude are able to vary largely independently of each other, and the frequency of songs is not simply determined by the amplitude.
A comparison between city-dwelling juncos and their more rural counterparts showed that the urban juncos did not have a higher top note than non-urban juncos, but their lowest note was higher. The loudest note of the urban juncos also tended to be a higher note than the loudest note of the non-urban juncos. Therefore, in this species at least, it seems that the birds do sing higher in an urban environment, but it is not the entire song that becomes higher, but instead the lowest notes. This could be because these parts of the songs would not be heard otherwise over low-frequency human noise. It also seems that these birds vary how high their songs are independently of how loud they are.
In a world that is becoming increasingly urbanised, it is important to understand how this will affect animals other than ourselves. As birds with higher-pitch songs tend to be less affected by human noise, for now it seems that the dark-eyed junco will be able to keep singing.
Cardoso, G.C. & Atwell, J.W. (2011) On the relation between loudness and the increased song frequency of urban birds. Animal Behaviour. doi:10.1016/j.anbehav.2011.07.018
Photo credit: mdf
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