[tt] NS: City songbirds are changing their tune

Wed Apr 16 00:55:09 UTC 2008

City songbirds are changing their tune
http://www.newscientist.com/article.ns?id=mg19726491.400&print=true
[Thanks to Frances for this.]

28 March 2008
Ed Yong

DAYBREAK in the city. The brief quiet of the night gives way to the
low rumble of cars, trucks and industry, but one sound is notable
by its absence. Gone is the familiar dawn chorus, with its rich mix
of enchanting melodies and calls. In its place is a strangely
depleted music - abrupt, high-pitched and sometimes ear-piercing.
Welcome to the urban soundscape of the future.

This is no dystopian vision. It is the prediction of scientists who
have been studying the way in which noise pollution affects urban
bird life. The growing clamour of cities and roads may be annoying
to us, but for many birds it can mean the difference between life
and death. Background noise can mask both the sounds of approaching
predators and the alarm calls that warn of danger. They can also
rob individuals of reproductive success by drowning out the songs
that male birds use to attract mates and demarcate their territory.

The impact of this noise is now becoming clear. Some species simply
are not able to make themselves heard above the ever-growing racket
and are finding themselves squeezed out of the city. Others are
beginning to change the way they communicate. In the long term, new
species may evolve. If noise levels continue to rise, it seems
inevitable that urban bird life will change dramatically.

You can already hear the changes, if you know what to listen out
for. One giveaway is birds unexpectedly singing outside their
traditional peak times of morning and evening. At these prime times
of day, wind noise and turbulence are at their lowest, so sound
carries further - but not if you factor in the impact of rush-hour
traffic. Richard Fuller of the University of Sheffield in the UK
has found that some local robins have abandoned the traditional
dawn chorus and taken to singing at night to avoid the daytime din
altogether. This shift had previously been attributed to the
confusing effects of light pollution, but Fuller's analysis reveals
that daytime noise has a much stronger effect: the parts of
Sheffield with nocturnal singers were an order of magnitude noisier
by day than other areas of the city (Biology Letters, vol 22, p
368).

It remains to be seen whether singing at night is a successful way
to combat noise pollution. It is not the only option, however.
Nightingales, when not singing at night, have opted for an approach
that seems at odds with their delicate melodies - they simply shout
louder. When Henrik Brumm at the University of St Andrews in the UK
recorded nightingales singing between 5 am and 10 am he found that
those in Berlin sang up to 14 decibels louder than their forest
counterparts, achieving volumes of up to 95 decibels - enough to
send humans reaching for ear protection. The loudness of their
vocals was proportional to the level of background noise, with
birds singing particularly loudly on weekday mornings (Journal of
Animal Ecology, vol 73, p 434).

Hitting the high notes

Changes in the timing or volume of songs are fairly obvious
solutions to the problem, but some songbirds have taken a more
subtle approach. Urban noise is particularly loud at low
frequencies - between about 1 and 3 kilohertz. By avoiding these
low notes, birds can make their songs more audible. Blackbirds,
song sparrows and house finches have all adapted in this way, but
the most well-studied practitioner is the great tit.

For the past five years, Hans Slabbekoorn of Leiden University in
the Netherlands has analysed the ways in which great tits deal with
noisy cities. He found that those inhabitating noisier parts of
Leiden sing melodies with higher minimum frequencies than those in
quieter areas of the city. When he looked at populations of great
tits in 10 European cities, including London, Paris and Amsterdam,
he found that every one of them sang higher-pitched tunes than
their forest-dwelling counterparts, raising the minimum frequency
by 200 hertz on average, to around 3500 hertz. Not only do urban
great tits sing in a higher key, but they also eschew the standard
riffs of their forest peers for more original ones (Current
Biology, vol 16, p 2326).

The ability to change one's tune is a valuable asset in the growing
urban hubbub. Unlike some birds that learn their entire repertoire
while in the nest, great tits, song sparrows and others regularly
modify their songs throughout their lifetime. They have far more
tunes than they require and select different songs depending on the
context. By monitoring which songs work best in a particular
situation, individuals can learn from experience and adapt to local
changes. Successful new singing strategies can spread as young
birds learn to sing by imitating the performances of their seasoned
neighbours. Alternatively, songs may become better adapted by
default: if younger birds cannot hear the low-frequency segments of
their tutors' songs, they may never learn tunes containing these
lower notes, which could then drop out of local repertoires
altogether.

"This plasticity provides adaptive value in natural conditions,"
says Slabbekoorn. Forests can vary greatly in how loud they are and
those birds that live near noisy areas like waterfalls and river
torrents also sing at higher frequencies, in a similar way to
urbanites. By chance, their flexible singing has put them in a good
position to cope with the artificially noisy conditions caused by
humans.

Behavioural flexibility is what distinguishes species coping with
noise pollution from those that are struggling. The relatively
recent rise of urban noise means that most of the vocal strategies
used by city birds are likely to be learned responses rather than
the result of evolution. In the long term, however, genetic changes
are likely to occur because of the role that songs play in survival
and reproduction.

Songs are primarily sexual traits that influence the mates females
choose and so the success of males. If females come to see the
ability to avoid acoustic masking as an indicator of mate quality,
they will prefer to mate with males that can do this and the trait
will be boosted by sexual selection. In addition, individuals whose
hearing is attuned to picking out the songs of other birds amid
urban noise are also at a selective advantage, which will
ultimately increase their proportion of the population.

If singing and hearing diverge enough, urban birds may be less
likely to find the vocals of rural birds attractive, or even to
recognise them as members of the same species. These changes could
serve to eventually split populations into genetically distinct
urban and rural species. Alternatively, different populations of
the same species might adopt differing strategies to cope with
urban noise, leading eventually to a species split occurring in
birds living in the same neighbourhood. "It would be absolutely
fascinating to see this kind of sympatric speciation," says Fuller.

This is not mere speculation. Some scientists believe that the
European blackbird has already diverged into separate urban and
rural subspecies, with different body shapes and life histories.
Slabbekoorn and Erwin Ripmeester, also at Leiden University, are
now investigating how urban noise is driving this separation. They
are using playback recordings to see if urban blackbirds respond
more strongly to urban songs than to rural ones. They have also
taken blood samples from urban and rural blackbirds and plan to
check for genetic divergence. "It's certain that we are seeing
parts of the process of speciation taking place," says Slabbekoorn,
"but we might not be there to see the end result."

While blackbirds, great tits and others are apparently taking
advantage of their adaptable songs, not all birds are flexible
enough to cope with the urban clamour. Amid the loud and
low-pitched noise, the biggest losers are those that rely heavily
or exclusively on low-frequency calls and are physically incapable
of switching to higher frequencies. Orioles, cuckoos, great reed
warblers and even the familiar house sparrow all fall into this
category. House sparrows were once frequent visitors to the UK's
parks and gardens, but populations are now falling, as they are in
mainland Europe. "We don't really understand why that is, but noise
may be a factor," says Slabbekoorn. "House sparrows use an
important low-frequency component in their calls."

In the Netherlands, the great reed warbler has suffered a similar
decline. Anecdotal evidence suggests that road noise may have
contributed to this: the construction of a road near a particular
reed bed reduced the number of warbler breeding pairs from around
10 to just two. When the road was closed for repairs for two years,
five more pairs moved into the area, although the subsequent return
of traffic drove them away again.

Loud noises can also have unexpected effects, including driving
otherwise faithful birds to adultery. Zebra finches, for example,
maintain monogamous relationships through a series of calls that
allow them to recognise and locate their mate. John Swaddle from
the College of William and Mary in Williamsburg, Virginia, found
that loud environmental noise prevents female finches from hearing
these calls. This erodes the otherwise strong bonds between
partners and leaves females showing no greater preference for their
chosen males than for strangers (Animal Behaviour, vol 74, p 363).

Even among species that seem to be adapting successfully to noise
pollution, there are signs that they are merely making the best of
a bad situation and that such flexibility is not without its costs.
House finches, for example, expend more energy on louder songs in
noisy cities, and their efforts are shorter as a result. Female
house finches prefer mates with longer songs, so males who
compensate for background noise could be left with fewer mating
opportunities. A great tit's natural inclination is to show off its
lower vocal register during the prime moments of the breeding
season. These notes take more effort to produce, says Slabbekoorn,
so they probably provide a good indicator of the singer's strength
and his potential as a mate. In an urban setting, however, these
dulcet tones may be lost in the hoo-ha, so urban males face a
compromise between singing a highly rated song, or simply one that
is audible (see Graph).

Noise is just one of the challenges facing urban birds. Until
recently, concern has centred on other forms of pollution,
including chemicals and light. Yet there is no doubt that noise has
already contributed to a decline in the diversity of bird species
around cities and major roads. Frank Rheindt at the University of
Würzburg, Germany, measured the diversity of different bird species
near a busy local highway and found a dearth of birds that sing at
lower frequencies.

The very fact that urban birds around the world are coming up with
strategies to deal with noise speaks volumes about the gravity of
the problem. "There are many factors that affect a bird's capacity
for breeding in cities but noise has been the most neglected one,"
says Slabbekoorn. Just how much it will change the familiar dawn
chorus remains to be seen.

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Related Articles

Birdsong goes out of fashion too
http://www.newscientist.com/article.ns?id=mg19526115.000
05 July 2007
Birds tune in to keep their songs note perfect
http://www.newscientist.com/article.ns?id=dn10112
19 September 2006
Urban songbirds raise their voice to be heard
http://www.newscientist.com/article.ns?id=dn10720
04 December 2006

Weblinks

Ed Yong's blog
http;//scienceblogs.com/notrocketscience/
Sound recordings of urban and forest great tits, made by Slabbekoorn, 
Current Biology
http://www.current-biology.com/cgi/content/full/16/23/2326/DC1/
The Royal Society for the Protection of Birds
http://www.rspb.org.uk/
Hans Slabbekoorn, Leiden University
http://www.biology.leidenuniv.nl/ibl/S8/S8peopleinfo.php?PeopleID=143

E-mail me if you have problems getting the referenced articles.