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.
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
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 – not good if you’re 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?
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.
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 can’t 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.
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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