1. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
Avian Vocalization Adjustment to
Urbanization and Anthropogenic
Noise: A Review
Maxson Jarecki
Princeton University
mjarecki@princeton.edu
I. Introduction
Features of Urbanization
I
n an increasingly urbanized world, bird
species are often exposed to anthropogenic
stresses that do not match the selective
pressures from which they evolved (Ditchkoff,
Saalfeld, and Gibson 2006). Avian vocalization
change has been linked to a number of urban
features, including the abundance of impervi-
ous surfaces (Dowling, Luther, and Marra 2012;
Kight, Hinders, and Swaddle 2013; Warren et
al. 2006; Slabbekoorn, Yeh, and Hunt 2007) and
reduction of open area (Kight, Hinders, and
Swaddle 2013), but most studies have identi-
fied noise pollution from traffic and construc-
tion as the most salient. These anthropogenic
sounds can differ from natural noise in their
pitch, amplitude, acoustic structure, and dis-
tribution, and can be more continuous than
sounds produced naturally (Wood and Yez-
erinac 2006; Slabbekoorn and Ripmeester 2008;
Pijanowski et al. 2011; Oden 2013).
Vocalization Change
Birds change their vocalizations in response
to urbanization in a number of ways. The
two most dramatic alterations can be in the
frequency (pitch) and the amplitude (volume)
of their vocalizations. This includes increas-
ing their minimum frequency (Slabbekoorn
and den Boer-Visser 2006; Wood and Yez-
erinac 2006; Luther and Baptista 2010; Dowling,
Luther, and Marra 2012; Oden 2013), changing
their maximum frequency (Dowling, Luther,
and Marra 2012; Oden 2013), and raising the
mean frequency of the entire vocalization (Par-
ris and Schneider 2009; Francis, Ortega, and
Cruz 2011; Halfwerk, Bot, et al. 2011; Oden
2013). Changes of amplitude in response to
ambient noise is known as the "Lombard ef-
fect" (Brumm and Zollinger 2011). This change
can manifest either as an increase in total vo-
calization volume (Nemeth and Brumm 2010)
or in the adjustment of specific call compo-
nents (Wood and Yezerinac 2006; Nemeth and
Brumm 2010). Finally, changes in avian call
timing (Ficken et al. 1974; Fuller, Warren,
and Gaston 2007) and vocalization structure
(Brumm and Slater 2006; Mockford and Mar-
shall 2009; Patricelli and Blickley 2006) have
also been linked to urbanization.
Questions
This literature review seeks to answer three
questions:
1: Which birds change their vocalizations in
response to urbanization?
2: How strong is the evidence for their vocal-
ization change?
3: What negative effects might adjusting (or
not adjusting) have on birds?
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2. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
II. Which birds change their
vocalizations in response to
urbanization?
Inquiry into which avian species vocaliza-
tions are most affected by urbanization has
revealed a two-fold effect. First, noise and
other urban factors may selectively filter which
species live in urban environments to begin
with (Hu and Cardoso 2009; Francis 2015). Sec-
ond, vocalization adjustment of these urban
communities may be limited to certain bird
species (Nemeth and Brumm 2010; Hu and
Cardoso 2009; Francis 2015; Oden 2013).
High-Frequency Population Preference in
Urban Areas
Several factors may play a role in the selec-
tive filtration of bird species in urban environ-
ments. Francis (2015) used a comparative anal-
ysis of 308 populations consisting of 183 bird
species across 14 locations in Europe, North
America, and the Caribbean to reveal factors re-
sponsible for avian sensitivity to anthropogenic
noise. He found that anthropogenic noise can
filter communities non-randomly by interfer-
ing with birds’ abilities to "receive, respond
to, and dispatch acoustic cues and signals"
(Francis 2015). The vocal frequency, diet, and
foraging location of each bird species helped
predict their habitat use in response to urban
noise, whereas body size, nest placement, and
the type of anthropogenic noise had no sig-
nificance (Francis 2015). This may be because
vocal frequency, diet, and foraging location are
directly related to avian ability to receive and
respond to signals, as mentioned above.
Francis found that species with low-
frequency vocalizations tended to avoid noisy
urban areas, whereas those with higher fre-
quency vocalizations tended to have less trou-
ble populating such zones (2015). This may
be because low-frequency callers experience
greater spectral overlap with urban noise (Hu
and Cardoso 2009; Francis 2015). Most anthro-
pogenic noise occupies a low frequency range,
often below 2000 Hz (Dowling, Luther, and
Marra 2012; Slabbekoorn and Peet 2003). This
can mask bird vocalizations that are transmit-
ted in similarly low frequencies and disrupt
the intended signal (Slabbekoorn, Yang, and
Halfwerk 2012; Halfwerk, Bot, et al. 2011; Oden
2013; Dowling, Luther, and Marra 2012). In
fact, Hu and Cardoso (2009) suggest that high-
frequency callers are significantly pre-adapted
for life in urban environments because of their
high-frequency vocalization strategies. Species
with high frequency vocalizations are less sus-
ceptible to interference by urban noise and so
may inhabit cities more easily (Hu and Cardoso
2009). Omnivorous and carnivorous species
were more sensitive to noise than those with
plant-based diets (Francis 2015). This is likely
due to noise interference in auditory prey de-
tection (Francis 2015). Once bird populations
have been established, though, similar filtering
occurs when we examine which species change
their vocalizations in response to urbanization,
as low-frequency callers are not completely
eliminated from urban bird communities.
Low-Frequency Passerine Birds Are Most
Likely to Change Vocalization in Urban Areas
After urban noise has filtered the inhabi-
tant species, it appears to select for vocalization
adjustments in the remaining species. The re-
search indicates that passerine birds of low call-
ing frequency are most likely to change their
vocalizations in response to urbanization (Hu
and Cardoso 2009; Dowling, Luther, and Marra
2012; Oden 2013; Nemeth and Brumm 2010;
Brumm and Slater 2006; Francis, Ortega, and
Cruz 2011; Halfwerk, Bot, et al. 2011). Non-
passerine birds may lack the phenotypic plastic-
ity to adjust the frequency of their vocalizations
(Slabbekoorn and Ripmeester 2008), perhaps
because the lack of learning abilities present in
most passerines (Catchpole and Slater 2008, as
cited by Hu and Cardoso 2009). Also, many
non-passerine vocalizations are not meant to be
long-range signals (Marler 2004) which would
eliminate the need for distance-improving com-
municative strategies like changes in frequency
and amplitude.
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3. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
Most Urban Birds Change Their Vocaliza-
tions When Compared to Rural Conspecifics
However, Hu and Cardoso (2009) compared
vocalizations frequencies of 529 bird species
from 103 genera across urban and rural envi-
ronments and found that urban conspecifics
had, on average, higher frequency vocaliza-
tions than their rural counterparts, regardless
of average call frequency. This leads to some
confusion regarding which kinds of species
are changing their vocalizations: are all urban
species changing their habits, or just the low-
frequency callers? Could frequency change
simply be a by-product of higher amplitude
(volume) calls? Further inquiry may help cool
the lively debate over whether frequency or am-
plitude is most important when adjusting vo-
calization in response to anthropogenic noise.
III. How strong is the evidence
for vocalization change?
In recent years, the literature surrounding
avian vocalization change due to urbanization
has blossomed. Amplitude changes in vocal-
ization due to increased environmental noise
have been studied extensively since the coining
of the term "Lombard effect" in 1911 (Brumm
and Zollinger 2011). Numerous studies have
confirmed that urban avian populations con-
sistently increase their call amplitude in re-
sponse to anthropogenic sound. The real de-
bate now, though, revolves around the role
of frequency change in enhancing the signal
strength of urban bird calls. It seems that two
camps are firmly established in the literature:
that of Nemeth and Brumm, on the one hand,
and Slabbekoorn, Yang, and Halfwerk, on the
other. The Nemeth/Brumm position questions
frequency change’s role in vocalization adapta-
tion. Slabbekoorn, Yank, and Halfwerk assert
that frequency change is an essential compo-
nent of avian urban adaptation. This section
lays the foundation of these two schools of
thought.
Frequency Change Not Necessarily an
Adaptive Trait
Nemeth and Brumm concede in their 2010
article, "Birds and Anthropogenic Noise: Are
Urban Songs Adaptive?" that there have been
many studies showing that urban bird species
do shift their vocalization frequencies higher
(Nemeth and Brumm 2010; Slabbekoorn and
Peet 2003; Wood and Yezerinac 2006; Parris and
Schneider 2009). This change exists both within
and between populations, as mentioned earlier.
However, Nemeth and Brumm suggest that
these observed shifts in urban song frequencies
may not actually be adaptive (2010). They posit
that high-pitched city songs may not be adjust-
ments to low-frequency traffic noise but rather
by-products of other adjustments (Nemeth and
Brumm 2010; Brumm and Naguib 2009).
Frequency Change is Extremely Limited
Figure 1: Frequency (dots) and amplitudes (light and
dark, 90 and 85 dB respectively) plotted
against communication distance, as evidenced
by the Nemeth and Brumm (2010) study on
great tit populations. The lowest arrow shows
hypothetical frequency shift required to match
communicative distances achieved through am-
plitude shift. A hypothetical shift from 3.3 to
4.3 kHz yields almost the same effect as an
increase in sound pressure level by 5 dB at 3.3
kHz (dashed lines) (Nemeth, Zollinger, and
Brumm 2012).
Reported frequency differences between ur-
ban calls and their rural counterparts can be
low, between 0.12 and 0.2 kHz (Nemeth and
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4. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
Brumm 2010; Slabbekoorn and den Boer-Visser
2006; Mockford and Marshall 2009; Hu and
Cardoso 2009). This small increase is especially
concerning in light of the natural frequency
variations found in other studies. For exam-
ple, the pitch of certain three-wattled bellbirds
(Procnias tricarunculata) decreased by about 2
kHz over 25 years (Kroodsma 2004, as cited
by Nemeth and Brumm 2010). Nemeth and
Brumm sought to put an end to the debate
with their 2010 study on signal transmission
in the great tit (Parus major) and the blackbird
(Turdus merula). They calculated communica-
tion distances in response to different levels
of urban noise, and also in the birds’ natural
forest habitat (2010).
They found that higher song pitches in ur-
ban blackbirds and great tits only had marginal
effects on communication efficacy (Nemeth
and Brumm 2010). Song amplitude increase, by
contrast, yields a signal approximately 5 times
more effective than that of frequency increase
(Nemeth and Brumm 2010). In order to match
the effect of amplitude change on signal trans-
mission, great tits would need to increase their
song frequency by over 1 kHz (Fig. 1, Nemeth
and Brumm 2010). The authors conclude that
the comparatively small benefits offered by fre-
quency could be adaptive–but more likely are
nonadaptive.
Frequency Change May Be an Epi-
Phenomenon of Amplitude Change or Urban
Stimulation
The high levels of anthropogenic noise in
urban environments induce a change in ampli-
tude, and frequency changes may be a passive
by-product of the volume change (Nemeth and
Brumm 2010; Nemeth, Zollinger, and Brumm
2012). The Lombard effect is usually accom-
panied by a change in pitch of the vocaliza-
tion since lower frequency vocalizations tend
to have less energy (Wood and Yezerinac 2006;
Nemeth and Brumm 2010). Because ampli-
tude has not yet been carefully measured in
the study species (great tits), this coupled rela-
tionship cannot yet be ruled out (Nemeth and
Brumm 2010). An alternate hypothesis may
be that songs at lower frequencies cannot be
produced as loudly, so they are pitch-shifted
up to increase their amplitude (Nemeth and
Brumm 2010; Nemeth, Zollinger, and Brumm
2012). Certain bird syrinx morphologies may
not be able to separate amplitude and pitch
from each other, leading to an unintentionally
coupled response (Beckers et al. 2003; Elemans
et al. 2008; as cited by Nemeth, Zollinger, and
Brumm 2012). Finally, higher pitch of urban
songs may be related to higher arousal of city
singers, or to differences in endocrinological
states (Nemeth and Brumm 2010).
Criticisms of Nemeth and Brumm (2010)
Two years after the Nemeth and Brumm
article was published, Slabbekoorn, Yang, and
Halfwerk responded with their article "Birds
and Anthropogenic Noise: Singing Higher May
Matter," also published in The American Natu-
ralist. They state their agreement with Nemeth
and Brumm’s first finding, that amplitude ad-
justments have a larger effect on urban com-
munication distance than frequency changes
(2012), as they follow the calculations and sta-
tistical analyses used to assert the claim. They
disagree, though, with Nemeth and Brumm’s
suggestion that frequency change may have
no adaptive value. They assert that shifting a
vocalization up in pitch may allow for fewer
low pitch notes to be masked by anthropogenic
noises (Oden 2013; Slabbekoorn and den Boer-
Visser 2006; Slabbekoorn, Yang, and Halfwerk
2012).
Frequency Change May Still Be Adaptive
Nemeth and Brumm’s 2010 study did not
include any any analytical statistical inquiry
into the adaptive value of frequency change
(Slabbekoorn, Yang, and Halfwerk 2012). TThe
data of Nemeth and Brumm’s study may, in
fact, be repurposed to show theoretical support
for the adaptive benefits of high-frequency call-
ing, as evidenced by the comparison of the
great tits and blackbird urban versus rural
pitches. Urban great tit songs reached larger
distances than the lower-pitched urban black-
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5. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
bird songs (Nemeth and Brumm 2010). The
two species share similar amplitudes, so the
conclusion follows that the great tit’s higher fre-
quency may be responsible for the longer sig-
naling distance (Slabbekoorn, Yang, and Halfw-
erk 2012). The frequency population averages
calculated for great tits and blackbirds may also
be underestimates, since Nemeth and Brumm
collected data from recordings from locations
of varying noise levels (Nemeth and Brumm
2010; Slabbekoorn, Yang, and Halfwerk 2012).
What is more, no data on amplitude adjust-
ment costs were incorporated into the study.
A reassessment of frequency importance in
light of re-calibrated amplitude costs may yield
more significant benefits of frequency change
(Slabbekoorn, Yang, and Halfwerk 2012).
Correlation and causation may be conflated
in studies on frequency and communicative
efficacy: though some bird species may sing
higher because they are singing louder (Nel-
son 2000, as cited by Slabbekoorn, Yang, and
Halfwerk 2012), this does not eliminate the pos-
sibility that the so-called ’epi-phenomenon’ has
adaptive value. Slabbekoorn, Yang, and Halfw-
erk reinterpret Nemeth and Brumm’s theoreti-
cal work in a different correlative context and
conclude that louder songs and higher songs
yield longer communication distances under
noisy urban conditions (2012).
Nemeth, Brumm, and Zollinger’s Rebuttal
Figure 2: Relationship between sound frequency and
amplitude of great tit song elements. Each
data point gives the mean value from (N = 5
males, depicted by different 10 renditions of
one element type ( N = 5 shading) (Nemeth,
Zollinger, and Brumm 2012).
In the same 2012 issue of The American
Naturalist, Nemeth and Brumm commented
on the Slabbekoorn response. Rebuffing
Slabbekoorn’s accusation of confusion regard-
ing causation and function, Nemeth and
Brumm reassert that their study was simply
a comparison of relative benefits of frequency
and amplitude change, in which they con-
cluded that amplitude was far more significant
(Nemeth, Zollinger, and Brumm 2012). They
use the effect sizes of frequency and amplitude
to restate the relative importance of these two
vocalization adjustments (Fig. 2). They also
criticize Slabbekoorn’s invocation of the Nel-
son study (2000) on eastern towhees (Pipilo ery-
throphthalmus), as it was incorrectly interpreted.
Nemeth and Brumm claim that Slabbekoorn et
al. misunderstood the towhee study, because
they neglected to mention that only the higher
components of the overall towhee vocalization
were amplified (2012). They address further
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6. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
criticisms brought up in the Slabbekoorn study
before concluding that they "see no reason
to withdraw any conclusions in Nemeth and
Brumm (2010)."
Conclusion
This back and forth between some of the
field’s foremost experts gives us the most up-
to-date perspectives on avian vocalization ad-
justment to urbanization. To conclude, the ev-
idence for amplitude change’s effect is much
stronger than that of frequency change. We
cannot yet rule out, however, the possibility
that frequency is also an adaptive trait in ur-
banized birds. The disagreements between the
two schools of thought are small. Both schools
are in broad agreement that amplitude change
is certainly an adaptive trait, while frequency
change, though it certainly exists, has yet to be
fully proven to be adaptive trait rather than a
correlational artifact.
IV. What negative effects might
adjusting (or not adjusting) have
on birds?
Bird vocalizations are evolutionarily tai-
lored to maximize signal efficiency (Morton
1975, Marten and Marler 1977, Marten et al.
1977, Wiley and Richards 1978; as cited by
Kight, Hinders, and Swaddle 2013). There-
fore, disruptions to their habitat can affect the
efficacy of their vocal signaling (Kight, Hin-
ders, and Swaddle 2013). Human disturbance,
specifically through urbanization, is progress-
ing at an unprecedented rate. In the next
two decades, a possible additional two billion
people will be inhabiting newly developed ur-
ban areas (Meyer and Turner 1992; World Re-
sources Institute 2004; as cited by Slabbekoorn
and Ripmeester 2008). Bird signals need to be
detectable, even in high levels of ambient noise;
this detectability is determined by the "signal-
to-noise" ratio of the vocalization and the audi-
tory detection threshold of the receiver (Marten
and Marler 1977, Brenowitz 1982, Dooling 2004;
as cited by Patricelli and Blickley 2006). An-
thropogenic background noise, therefore, has
a fundamental role in signal fidelity (Patricelli
and Blickley 2006). Birds that fail to change
their vocalizations in response to urbanization
may be at risk of having impaired reproductive
success through signal masking (Slabbekoorn
and Ripmeester 2008).
Mate Assessment
Acoustic assessment of mate quality may
be impaired in urban environments (Halfw-
erk, Holleman, et al. 2011). Mate attraction
is often guided by acoustic signals; many bird
songs are geared specifically toward finding
and attracting potential partners. Female birds
often differentiate mates of the correct species
and quality by ear (Slabbekoorn and Halfwerk
2009), and their investment decisions are thusly
affected (Holveck and Riebel 2009, as cited by
Halfwerk, Holleman, et al. 2011). High ambi-
ent noise may reduce the apparent quality of
signals, which may cause females to breed later,
allocate less energy to eggs, or provide less ma-
ternal care for chicks (Halfwerk, Holleman, et
al. 2011).
Community Filtering
The non-random urban community filter-
ing mentioned earlier may also have a neg-
ative impact on avian reproductive success.
This filtering is prompted by the continuous
low-frequency anthropogenic noise present in
urban communities, as it precludes certain
species from settling and developing popu-
lations. Some birds perceive these noisier
territories as low quality, and avoid them
(Slabbekoorn and Ripmeester 2008). Less expe-
rienced birds, for example, have been observed
settling in these noisier territories (Reijnen,
Foppen, and Veenbaas 1997). Also, birds living
in areas exposed to anthropogenic noise may
suffer reduced reproductive success, which
may ultimately lead to the exclusion of species
from otherwise suitable habitat (Slabbekoorn
and Ripmeester 2008; Slabbekoorn and Halfw-
erk 2009). This selective settlement can have
a negative impact on bird populations as it
decreases genetic diversity and increases com-
petition.
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7. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
Reduced Foraging Ability and Predator De-
tection
Failure to adapt to anthropogenic noise may
negatively impact birds through reduced forag-
ing opportunities and decreased predator sens-
ing. Prey are more difficult to detect in areas
of high ambient noise (Schaub, Ostwald, and
Siemers 2008, as cited by Halfwerk, Holleman,
et al. 2011). It may also be difficult for birds
to sense predators in such areas. Being able to
hear rustling prey or stalking predators has a
significant impact on individual success and
population viability (Slabbekoorn and Halfw-
erk 2009). However, this phenomenon can have
a mirrored benefit; urban noise may indirectly
facilitate reproductive success of individuals
nesting in noisy areas as a result of the disrup-
tion of predator-prey interactions (Francis, Or-
tega, and Cruz 2009). The negative or positive
effect of urban noise on predator/prey relation-
ships will depend on the species of bird and
its trophic level.
Degraded Signaling in Parent/Chick Inter-
action
Finally, parent/chick interactions often rely
heavily on auditory signals; failure to adapt
these signals to anthropogenic noise may sig-
nificantly reduce population fitness. Halfw-
erk, Holleman, et al.’s 2011 study revealed that
noise levels did not have a significant effect
on fledgling mass. However, high noise levels
did have a negative effect on the number of
fledglings. Though this may be related to the
reduced foraging capability mentioned above,
it does suggest that degraded signal-to-noise
ratios may affect food provisioning to chicks
(Halfwerk, Holleman, et al. 2011). Other im-
portant acoustic interactions can include beg-
ging by fledglings, food and alarm calling, and
production of contact calls that can be critical
to group cohesion (Slabbekoorn and Halfwerk
2009). Degraded parent-offspring communi-
cation may prevent chicks from receiving ad-
equate parental attention (Leonard and Horn
2008, as cited by Halfwerk, Holleman, et al.
2011).
Vocalization Change Also Has Risks
The section above outlines possible detri-
ments associated with vocalization change, but
adjustment can bring its own set of risks in
terms of individual fitness and population sta-
bility (Patricelli and Blickley 2006).
Signal Confusion
Ironically, the adjustments made to enhance
signaling may render the call less desirable.
Some bird females use vocalizations to deter-
mine mate quality and species, and male call
adjustments may confound their evaluation
process (Slabbekoorn and Peet 2003; Wood
and Yezerinac 2006). This breakdown in mate
recognition may eventually lead to reproduc-
tive isolation and speciation between urban
and rural species (Slabbekoorn and Peet 2003;
Warren et al. 2006; Wood and Yezerinac 2006).
Species in which females prefer low-frequency
vocalizations may be negatively affected in ur-
ban environments, where the males’ calls have
pitch-shifted up in an effort to broadcast louder
(Patricelli and Blickley 2006). For example, the
song sparrow’s minimum frequency is higher
in urban populations (Wood and Yezerinac
2006). Vocal adjustment may also affect male-
male competition. For example, low-frequency
calls are often used in threat displays (Morton
1977, as cited by Patricelli and Blickley 2006).
Also, nightingales self-regulate their song am-
plitude based on the volumes of conspecifics
(Brumm and Todt 2004), so increased ampli-
tude overall may hinder effective male-male
interactions.
Other types of crucial vocalizations may be
affected by adjustment, including: alarm calls
begging calls, contact calls, flight calls, and
food calls, in addition to calls between pair-
bonded mates, among offspring, and between
flock-mates (Marler 2004).
Conclusion
More research is needed to ascertain the
adaptive value of frequency and amplitude
change in response to urbanization. The re-
search indicates that there are both benefits and
disadvantages to adjustment. From a conser-
vation standpoint, understanding adjustment’s
affect on fitness will improve our ability to
predict our future impact on bird populations.
Future research may also improve our under-
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8. Spring Junior Paper • May 2015 • Ecology and Evolutionary Biology
standing of "vocal development, the plastic-
ity of vocal behavior, and how the process of
sensory drive shapes the diversity of animal
signals"(Patricelli and Blickley 2006).
V. Acknowledgments
I would like to thank librarian Andrea Baruzzi
for her help, and Zane Friedkin for his insight-
ful edits. I would also like to thank my advi-
sor David Wilcove, and Bert Harris. Finally, I
would like to thank my parents, Andrew and
Nancy, and my brother and sister for putting
up with my "shouting at your dinner date in
a high voice when in a crowded restaurant"
analogies.
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