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Birdsong & Noise

(Great Tits)

Great tit - a key study species (Photo: Maria Gill)
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Urban noise is something that we are all aware of - just open the window next time you're driving along a main road or near an airport.  I am interested in how and why birds adapt their communication strategies in response to background noise.  (Visit the Media page for links to coverage of this research on radio, tv and in the press).

This project includes our Nestbox Survey, involving regular monitoring of nearly 200 nest boxes in the University woodlands by a team of student volunteers, most of whom are taking our degree schemes in Animal Behaviour and Zoology. Female great tits are harder to locate than males, so providing them with a place to nest means that we know where to find them during the breeding season.

Great tit peeking out of nestbox (photo: EJ Mockford)Variation in song between noisy and quiet areas has now been shown in a number of studies.  Differences can be seen over relatively small distances, within a city or around a main road.  Birds seem to sing at a higher pitch (frequency) the higher the level of background noise - presumably so that they can be heard above all the racket. 

But not all birds are able to adapt - one study in Germany found that low-pitched species of bird were not found near to main roads, suggesting that man-made noise may lead to a decrease in biodiversity around towns and main roads.

Noise maps show this variation in noise around roads, railways and towns (see an example noise map of London, Swansea or Bristol).  As part of the EU's Environmental Noise Directive, the UK Government has recently drawn up noise maps of the UK and is required to keep these updated and to create noise action plans.  Further details of noise maps and how the EU directive is being implemented can be found here for England, Wales and Scotland.

Great tit song is often likened to a "tea-cher, tea-cher" sound (or a squeaky bicycle pump) and can be heard from late January until May.

Below is a picture of what great tit song looks like when we play it into a computer.  The "sonogram" of a rural great tit song is on the left and an urban one on the right.  Frequency (pitch) is measured in kHz (kilo Herz) on the vertical axis.  Note that the urban song is at a higher pitch, entirely above the 4kHz red line.  The rural song is spread across a wider range of frequencies, down as low as 2kHz in places
Sonogram of rural and urban great tit songs

 

 

 

 

Click here to listen to examples of rural and urban song.

Many birds respond strongly to song of their own species when it is played back to them over a loudspeaker in their territory.  They fly towards the source of the sound, hop around nearby, sing at the loudspeaker and may even peck at it.  The picture on the right shows a great tit peering down a tree trunk at a tape recorder beneath which is playing the song of another great tit. 

Only the male great tits sing.  But females judge males on the quality of their singing.  One aspect of their song is its pitch: females prefer lower pitched song.  So, what happens when the males raise their pitch in noisy areas?

Our research is investigating the differences in pitch in relation to background noise on both a small and large scale.  In a study of 20 cities across the UK, we have recently found that great tits sing at a significantly higher pitch in noisy urban areas than they do in quieter rural ones (see our peer-reviewed journal article on this work here: Mockford & Marshall 2009). 

We are also interested in the response of birds to songs sung at a pitch different to their own.  Computer editing songs and playing them back to other birds allows us to observe any variations in response.  We have found that birds from noisy areas respond less strongly to the song of birds from quieter areas.  And vice versa, even when the songs we play them come from an area in the same city just a mile or two away (see our peer-reviewed journal article on this work here: Mockford & Marshall 2009).  Since great tits can disperse up to 3km (1.8 miles) in their first year, this means that young urban males may have difficulty establishing a territory or attracting a mate if they move to an area with more or less noise than they are used to.

Transmission properties of song.  How well a bird song carries through the air varies depending on the environment in which they are sung. While you might think that city songs are best for the cities and rural songs travel best in the rural areas - you'd only be half right.

We have found that urban song degrades less in urban areas than does rural song. In other words, a bird perching in an urban territory will hear a better quality of song if its neighbour sings an urban song than they would hear if their neighbour sang a rural song. However, rural song also degraded less in urban areas than in the countryside. So the ideal place to sing a song seems to be the city Ė not good if youíre a country bird.

Curiously, urban songs travelled the best in both urban and rural environments - which begs the question, why do rural birds not sing urban songs?

The short answer is that we don't know. However, great tits, and other species, are known to measure how far away a neighbour is by how much its song has degraded. In cities, with few trees to block the view, vision may be more important than song degradation in determining how distant a neighbour is. In the countryside, where trees and other obstacles make spotting neighbours difficult, the degradation of songs might be useful - and thus birds may prefer to sing songs that do degrade so that they can better determine each other's location. (See our peer-reviewed journal article on song transmission here: Mockford, Marshall & Dabelsteen 2011)

You can see what the degraded song looks like in the sonograms below.

Click here to listen to rural & urban songs.  Can you tell the difference between rural and urban? Or between near and far?

A sonogram of songs showing how they degrade over 12 metres and 48 metres.

We are currently continuing our investigations of these variations in response to noise-adapted song and the implications these changes have on great tits' ability to communicate and breed successfully.Great tit on tree investigating taped song (photo: EJ Mockford)

Why is this important?  Noise has the potential to affect not only communication among members of a single species but between species.  If great tits cannot communicate efficiently, it may disrupt their breeding: it may reduce a male's ability to attract a female, or his ability to defend his territory against other males.

Such a behavioural barrier to breeding has implications for how urban and rural birds interact - will they eventually stop recognising each other?  Will geneflow be reduced between the urban and rural populations?  What will happen in small cities with small populations - will they suffer from genetic bottlenecks? 

These are speculative questions for the moment - further research is required to understand the consequences - for example, it is generally thought that great tits learn their song in their first year and can only make small changes after this.  The extent of their ability to make changes could be crucial in terms of their ability to adapt to noise levels in the areas to which they disperse to breed.

Communication is also important between different species of bird: not hearing an approaching predator, such as a sparrowhawk, could have disastrous consequences for the individual concerned.  Similarly, noise may disrupt a predator's ability to locate prey. No prey means no food which may mean a reduced ability to survive in urban or noisy environments.

While some species (the urban survivors) may be able to adapt to anthropogenic (man-made) noise, the decline of those species that canít adapt may decrease biodiversity around human settlement.  As mentioned above, one study in Germany found that only high-pitched species lived next to main roads - low-pitched species were simply not there.Juvenile great tit (photo: EJ Mockford)

And noise does not just affect birds.  Noise is important in the lives of mammals and amphibians too: from croaking frogs to bats' ultrasonic navigation, to underwater communication by marine mammals such as whales & dolphins.

In terms of Conservation, it is important to conserve appropriate habitats, bearing in mind that air, light and noise pollution are all important ecological factors - are nature reserves in noisy areas ecological cages rather than source populations from which residents are able to disperse?

These are just a few examples - man-made noise has a wide impact on the natural world.  Ongoing research here and elsewhere seeks to improve our understanding of its consequences.

 

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