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Do Northern Mockingbirds Change Their Songs to Compensate for Urban Noise?

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Do Northern Mockingbirds Change Their Songs to Compensate for Urban Noise?
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Journal of Undergraduate Research
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Patel, Puja
Robinson, Scott
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Gainesville, Fla.
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University of Florida
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Journal of Undergraduate Research
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Urban environments are characterized by increased anthropogenic noise. This increased low-frequency urban noise may interfere with the song transmission of resident birds. Because birds use songs for communication, territory defense, mate attraction, and predator avoidance, they must be able to adapt their songs to be heard in noisy areas to keep their territories and retain their mates. Based on previous studies of the adjustment of song in other bird species, we hypothesized that Northern Mockingbirds (Mimus polyglottos) adjust their song according to their habitat and urban mockingbirds sing at a higher minimum frequency than do non-urban mockingbirds. Singing male mockingbirds and ambient noise were recorded in urban (residential areas, parking lots) and non-urban (pastures and biological preserve) areas in and around Gainesville, FL. All recordings were digitally analyzed using the program Raven Pro, and the minimum frequency at which each male sang was calculated. The average power of low-frequency ambient noise (all sounds below 1.662 kHz, the highest minimum frequency at which any mockingbird sang) was calculated for urban and non-urban environments. We found that urban areas were significantly louder in low-frequency ambient noise than non-urban areas and that urban birds sang at a significantly higher minimum frequency than non-urban birds. In addition, there was a significant correlation between low-frequency ambient noise level and minimum frequency of mockingbird songs. Therefore, mockingbirds are able to adjust their song to more effectively communicate with each other, which should help them in territory defense, mate attraction, and mate retention. Thus, mockingbirds’ ability to compensate for increased urban noise may be a factor that allows them to successfully inhabit urban areas.

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Do Northern Mockingbirds Change Their Songs to Compensate for

Urban Noise?


Puja Patel


College of Liberal Arts and Sciences, University of Florida


Urban environments are characterized by increased anthropogenic noise. This increased low-frequency urban noise may interfere with
the song transmission of resident birds. Because birds use songs for communication, territory defense, mate attraction, and predator
avoidance, they must be able to adapt their songs to be heard in noisy areas to keep their territories and retain their mates. Based on
previous studies of the adjustment of song in other bird species, we hypothesized that Northern Mockingbirds (Mimus Ip.1. ! . 1-.m
adjust their song according to their habitat and urban mockingbirds sing at a higher minimum frequency than do non-urban
mockingbirds. Singing male mockingbirds and ambient noise were recorded in urban (residential areas, parking lots) and non-urban
(pastures and biological preserve) areas in and around Gainesville, FL. All recordings were digitally analyzed using the program
Raven Pro, and the minimum frequency at which each male sang was calculated. The average power of low-frequency ambient noise
(all sounds below 1.662 kHz, the highest minimum frequency at which any mockingbird sang) was calculated for urban and non-urban
environments. We found that urban areas were significantly louder in low-frequency ambient noise than non-urban areas and that
urban birds sang at a significantly higher minimum frequency than non-urban birds. In addition, there was a significant correlation
between low-frequency ambient noise level and minimum frequency of mockingbird songs. Therefore, mockingbirds are able to adjust
their song to more effectively communicate with each other, which should help them in territory defense, mate attraction, and mate
retention. Thus, mockingbirds' ability to compensate for increased urban noise may be a factor that allows them to successfully
inhabit urban areas.


INTRODUCTION

As humans have urbanized natural habitats, they have
increased ambient noise levels in those areas. If this
increased noise is of the same frequency as the songs of
birds, then it may interfere with song transmission.
Because song is an important component of bird life and
functions in territory defense, mate attraction, and predator
avoidance (Warren et al. 2006), the ability of birds to
compensate for background noise in their environment
could be quite important.
For species that occur in a wide range of natural
habitats, their vocal signals have been shown to differ in
ways that maximize the transmission of the signal and thus
likely represent adaptations to different habitats (e.g. Wiley
1991; Slabbekoor and Smith 2002). In urban
environments, urban great tits (Parus major) have been
shown to sing at a higher frequency to overcome low-
frequency ambient noise (Slabbekoor and Peet 2003),
while urban European Robins (Erithacus rubecula) resort
to nocturnal singing in areas that are noisy during the day
(Fuller et al. 2007). Studies in North America have shown
that in noisier locations Song Sparrows (Melospiza
melodia) sing higher frequency low notes (Wood and
Yezerinac 2006).


It is possible, however, that bird species unable to
adjust their song frequencies to overcome urban noise
avoid dwelling in urban areas, thus decreasing species
diversity and abundance there. Since birds sing to
communicate, an inability to transmit one's song can make
it difficult to hold a territory and attract and retain a mate,
which would have fitness consequences for these
individuals.

Study System
I studied the variation in song frequencies (as a
measure of pitch, in Hz) of the Northern Mockingbird
(Mimus polyglottos) in urban versus non-urban areas.
Unlike other birds that have been previously studied,
mockingbirds mimic songs of the local birds in each
territory and build a diverse repertoire based on the
surrounding bird community and noises from the
environment (e.g. mechanical sounds and sounds of non-
avian species). Individuals learn sounds throughout life and
integrate them into their existing songs to carry an
extensive vocal repertoire as adults (Derrickson and
Breitwisch, 1992). Males often display vocal repertoires
containing more than 150 different song types; however,
vocal repertoires ranging from as low as 45 to as high as
203 different song types have been estimated (Derrickson


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





PUJA PATEL


and Breitwisch 1992). Thus, mockingbirds represent a
unique opportunity to examine the effects of urban noise
on song properties, song composition, and even the song
patterns of other birds.

Hypotheses
Given previous studies (Slabbekoom and Peet 2003;
Fuller et al. 2007), we hypothesized that mockingbirds
adjust their song frequencies based on the background
noise in their environment to improve song transmission.
Since urban areas typically have more low frequency
ambient noise (Warren et al. 2006) than non-urban areas,
we hypothesized that mockingbirds in urban areas sing at a
higher minimum frequency than those in non-urban areas.

METHODS

Song Recordings
We looked for male mockingbirds in areas within and
around Gainesville, FL between April and July 2008. Our
study sites included two pastures (University of Florida
Beef Research Unit and Boston Farm Santa Fe River
Ranch Beef Unit), two parking lots (Oaks Mall and Butler
Plaza), three residential areas (Capri, Duck Pond, and
Gateway at Gainesville Apartments), and one biological
preserve (Ordway-Swisher Biological Station). We located
singing males and recorded each mockingbird's song for
15 minutes, obtaining a total of 22 mockingbird song
recordings. If the bird stopped singing part way through,
we returned to it at a later time to get a total 15-minute
recording. In order to compare the mockingbird's song to
the noise in the environment, we also recorded the ambient
noise in each mockingbird's environment for five minutes
directly following the song recording.
We recorded mockingbird songs and ambient noise
using a Fostex FR-2LE 2-Channel Compact Flash Field
Recorder and a ME62 Omni-Directional Condenser
Microphone attached to a plastic Telinga parabolic dish.
We detached the parabolic dish when recording the
ambient noise. The volume input level for ambient
recordings was standardized. For the song recordings, we
did not standardize the input volume or the distance to the
bird; therefore, we cannot compare the amplitude
(loudness) of the songs.

Song Processing and Analyses
We used the program Raven Pro v1.4 to display
spectrograms of all recordings on the computer. We
listened to each song recording and studied the respective
spectrogram to select parts of each song recording in which
only the targeted mockingbird was singing. We also
avoided parts of the song that were polluted with other
background noises, such as cows, buses, cars, and
mechanical noises.


For each song recording, we found the frequency at
which the lowest note was sung within the selections
previously made for that file. In cases where there were
multiple parts of recordings (multiple files adding up to 15
minutes of song), we took the average of all of the file
minimums for each bird. We then compared the average
minimum frequencies of urban mockingbirds to those of
non-urban mockingbirds using a t-test. All analyses were
performed in SAS version 9.2.
Because we were particularly interested in the low-
frequency ambient noise, we limited our analysis of
ambient noise to the lower range of mockingbird songs. As
an upper limit to our low frequency range, we used the
highest minimum frequency value (1.662 kHz) from all
songs. We then selected the noise in each five-minute
ambient recording from 0 to 1.662 kHz as our measure of
low-frequency ambient noise. We used Raven Pro to
calculate the average power of that selection. Power is a
measure of the loudness of the background noise in
decibels (dB). We compared the average power of low-
frequency ambient noise in urban areas to that in non-urban
areas. We also did a linear regression of minimum
frequency of all recorded males versus the average power
(loudness) of their respective ambient recording.

RESULTS

Urban environments have significantly louder low-
frequency noise than do non-urban environments (t19 -
4.75, p = 0.0001; Fig.l). Mockingbirds in urban areas sing
at a significantly higher minimum frequency than those in
non-urban areas (t19 = -3.75, p = 0.002; Fig. 2). Song
differs not only in urban versus non-urban environments,
but also as low frequency ambient noise (between 0 and
1.662 kHz) changes. As the background noise gets louder,
the birds sing at a higher frequency (F1,19 = 15.31, p
0.0009, r2 = 0.45; Fig. 3).


60
9 50
-a
S40
S30 -
20 -
20
I-J

S10
0


I


Non-urban


Urban


I Habitat

Figure 1: Average power of low-frequency ambient noise in urban
versus non-urban areas (with standard error bars).


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010





Do MOCKINGBIRDS CHANGE THEIR SONG TO COMPENSATE FOR URBAN NOISE?


Non-urban


Urban


Habitat


Figure 2: Minimum frequencies of mockingbird songs in urban versus
non-urban areas (with standard error bars).


20 30 40 50 60
Low Frequency Ambient Power (dB)
Figure 3: Relationship between the minimum frequency of mockingbird
songs and the average power of low-frequency ambient noise.

DISCUSSION

The results of our experiment support our hypotheses
that mockingbirds adjust their song according to the
ambient noise in their habitat. Mockingbirds in urban areas
sing at a higher minimum frequency than those in non-
urban areas. This shift in frequency prevents their songs
from being masked by the low-frequency urban noise,
thereby improving the transmission of their signal. Thus,
even with the loud noises characteristic of an urban
environment, mockingbirds are able to adjust their song to
more effectively communicate with each other.
Mockingbirds are highly territorial and typically pair
monogamously; therefore, the ability to alter their song
should help them in territory defense, mate attraction, and
mate retention (Derrickson and Breitwisch 1992).


Mockingbirds' ability to compensate for increased urban
noise may be a factor that allows them to successfully
inhabit urban areas.
Since mockingbirds mimic other birds, the fact that
they sing at a higher minimum frequency in urban areas
may be explained three ways. First, the bird species found
in urban areas differ substantially from those species found
in non-urban environments (Marzluff et al. 2001). It is
possible that the species in urban areas on average
naturally sing at higher frequencies than the species found
in non-urban areas. Thus, the suite of songs urban
mockingbirds are exposed to are of a higher frequency.
Second, mockingbirds may simply be choosing to mimic
higher frequency noises, including songs, from their
environment because those are the sounds they can best
hear over the urban ambient noise. Finally, because
mockingbirds mimic other birds in their environment, the
shift in frequency of mockingbird songs may reflect similar
responses in other urban birds, as these species may also be
responding to the urban noise.

Future Directions
To obtain more accurate results of the differences in
minimum frequency of urban and non-urban mockingbirds,
we should average the minimum frequencies of each
phrase within a recording rather than using the lowest note
throughout the entire recording to dictate the minimum
frequency at which the bird sings. In this case, a phrase
would be a group of repeated syllables (Gill 2007); finding
the minimum of each phrase would provide a more
accurate value for the minimum frequency at which the
bird predominately sings.
Since we now know that mockingbirds adjust their
song according to their habitat, we can explore how they
mimic avian songs. We can compare the species of birds in
each habitat that the mockingbird could mimic. To
determine if the songs differ because the species
compositions differ, we can list the species that make up
urban and non-urban bird communities. We can calculate
the minimum frequency at which each species naturally
sings and determine the average across species in urban
areas and compare that to the average across species in
non-urban areas. Alternatively, if mockingbird songs differ
because they are preferentially mimicking high frequency
songs, we can analyze the mockingbird's song recording to
figure out which species it actually mimics. We can then
compare the frequency of mimicked versus non-mimicked
songs in each habitat.
Finally, we can determine if mockingbirds mimic
songs of other birds verbatim or if they alter the frequency
of the mimicked version. For example, mockingbirds
frequently mimic the song of the Northern Cardinal in both
urban and non-urban habitats. We could record cardinal
songs in each environment and compare their average


University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010


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PUJA PATEL


frequencies to those of the mimicked versions in each
environment. A difference in frequency of the mimicked
and actual cardinal song would tell us that the cardinal
increased the pitch of its songs in urban areas. On the other
hand, cardinals may not increase their pitch, but rather the
urban mockingbirds increase the pitch of the mimicked
version.
We can also investigate how important a
mockingbird's ability to adjust his song to improve signal
transmission is for mating. For example, males in louder
territories that cannot adjust their song frequency may have
a harder time attracting and keeping their females and
retaining their territory. We can band all males and females
at each study site and follow them from year to year to
correlate their noise environment and song adjustment with
rates of mate and territory fidelity.


Gill, F. B. 2007. Ornithology, 3rd Edition. W.H. Freeman and Company, New
York, New York.

Marzluff, J.M., R. Bowman, and R. Donnelly. 2001. Avian ecology and
conservation in an urbanizing world. Kluwer Academic Publishers, Boston.

Slabbekoom, H. and M. Peet. 2003. Birds sing at a higher pitch in urban noise.
Nature 424: 267-268.

Slabbekoom, H. and T.B. Smith. 2002. Habitat-dependent song divergence in the
Little Greenbul: an analysis of environmental selection pressures on acoustic
signals. Evolution 56: 1849-1858.
Warren, P.S, M. Katti, M. Ermann, and A. Brazel. 2006. Urban bioacoustics: it's
not just noise. Animal Behavior 71: 491-502.
Wiley, R.H. 1991. Associations of song properties with habitats for territorial
oscine birds of eastern North America. American Naturalist 138: 973-993.
Wood, W.E. and S.M. Yezerinac. 2006. Song Sparrow (Melospzza melodia) song
varies with urban noise. Auk 123: 650-659.


Conclusions
Since urbanization has caused an increase in ambient
noise and birds continue to rely on vocal signaling as their
primary method of communication, it is important to assess
how song characteristics are affected by noise. Now that
we have established that mockingbirds sing at a higher
minimum frequency in urban areas, we can further tease
apart the reasons for this song adjustment. Mockingbirds
mimic avian songs and non-avian sounds found in their
environment and build a diverse song repertoire. This is
interesting because we can use their diverse repertoire to
analyze their song composition: which species they mimic,
how those species' songs are similar or different, and how
mimicking those species or background noise affects the
transmission of a mockingbird's song in its environment.
We can also begin to assess consequences of this song
adjustment on mate attraction and mate and territory
retention to gain a more complete understanding of how
urban noise affects the fitness of songbirds.

ACKNOWLEDGMENTS

University Scholars Program contributed financial
assistance for the research equipment. I thank Christine
Stracey, Jill Jankowski, and Dr. Scott Robinson for
mentoring and assisting me with this research. I thank
Aaron Spalding for constructing part of the apparatus used
for making the recordings.

REFERENCES

Derrickson, K.C. and R. Breitwisch. 1992. Northern Mockingbird (Mzmnus
polyglottos). In The Birds of North America, No. 7. A. Poole, P. Stettenheim and
F. Gill (Eds). The Academy of Natural Sciences, Philadelphia, PA; and the
American Orithologists' Union, Washington, D.C.
Fuller, R.A., P.H. Warren, and K.J. Gaston. 2007. Daytime noise predicts
nocturnal singing in urban robins. Biology Letters 3: 368-370.

University of Florida I Journal of Undergraduate Research I Volume 10, Issue 2 I Spring 2010




Full Text

PAGE 1

University of Florida | Journal of Undergraduate Research | Volume 10, Issue 2 | Spring 2010 1 Do Northern Mockingbirds C hange T heir S ongs to C ompensate for Ur ban N oise? Puja Patel College of Liberal Arts and Sciences, University of Florida Urban environments are characterized by increased anthropogenic noise. This increased low frequency urban noise may interfere with the song transmission of resident birds. Because b irds use songs for communication, territory defense, mate attraction, and predator avoidance , they must be able to adapt their songs to be heard in noisy areas to keep their territo ries and retain their mates. Based on previous studies of the adjustment of song in other bird species, we hypothesized that Northern Mockingbirds ( Mimus polyglottos) adjust their song according to their habitat and urban mockingbirds sing at a higher mini mum frequency than do non urban mockingbirds. Singing male mockingbirds and ambient noise were recorded in urban (residential areas, parking lots) and non urban (pastures and biological preserve) areas in and around Gainesville, FL. All recordings were dig itally analyzed using the program Raven Pro, and the minimum frequency at which each male sang was calculated. The average power of low frequency ambient noise (all sounds below 1.662 kHz, the highest minimum frequency at which any mockingbird sang) was ca lculated for urban and non urban environments. We found that urban areas were significantly louder in low frequency ambient noise than non urban areas and that urban birds sang at a significant ly higher minimum frequency than non urban birds. In addition, there was a significant correlation between low frequency ambient noise level and minimum frequency of mockingbird songs. Therefore, mockingbirds are able to adjust their song to more effectively communicate with each other , which should help them in terri tory defense, mate attraction, and mate retention. Thus, m ability to compensate for increased urban noise may be a factor that allows them to successfully inhabit urban areas. INTRODUCTION As humans have urbanized natural habitats, they hav e increased ambient noise levels in those areas. If this increased noise is of the same frequency as the songs of birds, then it may interfere with song transmission. Because s ong is an important component of bird life and functions in territory defense, m ate attraction, and predator avoidance (Warren et al. 2006) , t he ability of birds to compensate for background noise in their environment could be quite important. For species that occur in a wide range of natural habitats, their vocal signals have been sh own to differ in ways that maximize the transmission of the signal and thus likely represent adaptations to different habitats (e.g. Wiley 1991 ; Slabbekoorn and Smith 2002). In urban environments, urban great tits ( Parus major ) have been shown to sing at a higher frequency to overcome low frequency ambient noise (Slabbekoorn and Peet 2003), while urban European Robins ( Erithacus rubecula ) resort to nocturnal singing in areas that are noisy during the day (Fuller et al. 2007). Studies in North America have s hown that in noisier locations Song Sparrows ( Melospiza melodia ) sing higher frequency low notes (Wood and Yezerinac 2006). It is possible, however, that bird species unable to adjust their song frequencies to overcome urban noise avoid dwelling in urban a reas, thus decreasing species diversity and abundance there. Since birds sing to it difficult to hold a territory and attract and retain a mate, which would have fitness consequences for these indiv iduals. Study System I studied the variation in song frequencies (as a measure of pitch, in Hz) of the Northern Mockingbird ( Mimus polyglottos ) in urban versus non urban areas. Unlike other birds that have been previously studied, mockingbirds mimic songs of the local birds in each territory and build a diverse repertoire based on the surrounding bird community and noises from the environment (e.g. mechanical sounds and sounds of non avian species). Individuals learn sounds throughout life and integrate th em into their existing songs to carry an extensive vocal repertoire as adults (Derrickson and Breitwisch, 1992). Males often display vocal repertoires containing more than 150 different song types; however, vocal repertoires ranging from as low as 45 to as high as 203 different song types have been estimated (Derrickson

PAGE 2

P UJA P ATEL University of Florida | Journal of Undergraduate Research | Volume 10, Issue 2 | Spring 2010 2 and Breitwisch 1992). Thus, mockingbirds represent a unique opportunity to examine the e ffects of urban noise on song properties, song composition, and even the song patterns of other birds. Hypotheses Given previous studies (Slabbekoorn and Peet 2003 ; Fuller et al. 2007), we hypothesized that mockingbirds adjust their song frequencies based on the background noise in their environment to improve song transmission. Since urban areas typicall y have more low frequency ambient noise (Warren et al . 2006) than non urban areas, we hypothesized that mockingbirds in urban areas sing at a higher minimum frequency than those in non urban areas. METHODS Song Recordings We looked for male mockingbirds in areas within and around Gainesville, FL between April and July 2008. Our study sites included two pastures (University of Florida Beef Research Unit and Boston Farm Santa Fe River Ranch Beef Unit), two parking lots (Oaks Mall and Butler Plaza), three r esidential areas (Capri, Duck Pond, and Gateway at Gainesville Apartments), and one biological preserve (Ordway Swisher Biological Station). We located 15 minutes , obtain ing a total of 22 mockingbird s ong recordings. If the bird stopped singing part way through, we returned to it at a later time to get a total 15 minute the noise in the environment, we also recorded the ambient noise in each mocki directly following the song recording. We recorded mockingbird songs and ambient noise using a Fostex FR 2LE 2 Channel Compact Flash Field Recorder and a ME62 Omni Directional Condenser Microphone attached to a plastic Telinga parabolic dish. We detached the parabolic dish when recording the ambient noise. The volume input level for ambient recordings was standardized. For the song recordings , we did not standardize the input volume or the distance to the bird ; therefor e , we cannot compare the amplitude (loudness) of the songs. Song Processing and Analyses We used the program Raven Pro v1.4 to display spectrograms of all recordings on the computer. We listened to each song recording and studied the respective spectrogra m to select parts of each song recording in which only the targeted mockingbird was singing. We also avoided parts of the song that were polluted with other background noises, such as cows, buses, cars, and mechanical noises . For each song recording, we fo und the frequency at which the lowest note was sung within the selections previously made for that file. In cases where there were multiple parts of recordings (multiple files adding up to 15 minutes of song), we took the average of all of the file minimum s for each bird. We then compared the average minimum frequencies of urban mockingbirds to those of non urban mockingbirds using a t test. All analyses were performed in SAS version 9.2. Because we were particularly interested in the low frequency ambient noise, we limited our analysis of ambient noise to the lower range of mockingbird songs. As an upper limit to our low frequency range, we used the highest minimum frequency value (1.662 kHz) from all songs . We then selected the noise in each five minute am bient recording from 0 to 1.662 kHz as our measure of low frequency ambient noise. We used Raven Pro to calculate the average power of that selection. Power is a measure of the loudness of the background noise in decibels (dB). We compared the average powe r of low frequency ambient noise in urban areas to that in non urban areas. We also did a linear regression of minimum frequency of all recorded males versus the average power (loudness) of their respective ambient recording. RESULTS Urban environments h ave significantly louder low frequency noise than do non urban environments (t 19 = 4.75, p = 0.0001; Fig.1). Mockingbirds in urban areas sing at a significantly higher minimum frequency than those in non urban areas (t 19 = 3.75, p = 0.002; Fig. 2). Song differs not only in urban versus non urban environments, but also as low frequency ambient noise (between 0 and 1.662 kHz) changes. As the background noise gets louder, the birds sing at a higher frequency (F 1,19 = 15.31, p = 0.0009, r 2 = 0.45; Fig. 3). Figure 1: Average power of low frequency ambient noise in urban versus non urban areas (with standard error bars) .

PAGE 3

D O M OCKINGBIRDS C HANGE T HEIR S ONG TO C OMPENSATE FOR U RBAN N OISE ? University of Florida | Journal of Undergradu ate Research | Volume 10, Issue 2 | Spring 2010 3 Figure 2: Minimum frequencies of mockingbird songs in urban versus non urban areas (with standard error bars) . Figure 3: Relationship b etween the minimum frequency of mockingbird songs and the average power of low frequency ambient noise. DISCUSSION The results of our experiment support our hypotheses that mockingbirds adjust their song according to the ambient noise in their habitat. M ockingbirds in urban areas sing at a higher minimum frequency than those in non urban areas. This shift in frequency prevents their songs from being masked by the low frequency urban noise, thereby improving the transmission of their signal. Thus, even wit h the loud noises characteristic of an urban environment, mockingbirds are able to adjust their song to more effectively communicate with each other. Mockingbirds are highly territorial and typically pair monogamously; therefore, the ability to alter their song should help them in territory defense, mate attraction, and mate retention (Derrickson and Breitwisch 1992). noise may be a factor that allows them to successfully inhabit urban areas. Since moc kingbirds mimic other birds, the fact that they sing at a higher minimum frequency in urban areas may be explained three ways. First, the bird species found in urban areas differ substantially from those species found in non urban environments (Marzluff e t al. 2001). It is possible that the species in urban areas on average naturally sing at higher frequencies than the species found in non urban areas. Thus , the suite of songs urban mockingbirds are exposed to are of a higher frequency. Second, mockingbird s may simply be choosing to mimic higher frequency noises, including songs, from their environment because those are the sounds they can best hear over the urban ambient noise. Finally, because mockingbirds mimic other birds in their environment, the shift in frequency of mockingbird songs may reflect similar responses in other urban birds, as these species may also be responding to the urban noise. Future Directions To obtain more accurate results of the differences in minimum frequency of urban and non u rban mockingbirds, we should average the minimum frequencies of each phrase within a recording rather than using the lowest note throughout the entire recording to dictate the minimum frequency at which the bird sings. In this case, a phrase would be a gro up of repeated syllables (Gill 2007); finding the minimum of each phrase would provide a more accurate value for the minimum frequency at which the bird predominately sings. Since we now know that mockingbirds adjust their song according to their habitat, we can explore how they mimic avian songs. W e can compare the species of birds in each habitat that the mockingbird could mimic . T o determine if the songs differ because the species compositions differ , we can list the species that make up urban and non ur ban bird communities. We can calculate the minimum frequency at which each species naturally sings and determine the average across species in urban areas and compare that to the average across species in non urban areas. Alternatively, if mockingbird song s differ because they are preferentially mimicking high frequency songs, we can figure out which species it actually mimics. We can then compare the frequency of mimicked versus non mimicked songs in each habitat . Finally, we can determine if mockingbirds mimic songs of other birds verbatim or if they alter the frequency of the mimicked version . For example, mockingbirds frequently mimic the song of the Northern Cardinal in both urban and non urban habitats. We c ould record cardinal songs in each environment and compare their average

PAGE 4

P UJA P ATEL University of Florida | Journal of Undergraduate Research | Volume 10, Issue 2 | Spring 2010 4 frequencies to those of the mimicked versions in each environment. A difference in frequency of the mimicked and actual cardinal song would tell us that the cardinal increased the pit ch of its songs in urban areas. On the other hand, cardinals may not increase their pitch, but rather the urban mockingbirds increase the pitch of the mimicked version. We can also investigate how important a rove signal transmission is for mating. For example, males in louder territories that cannot adjust their song frequency may have a harder time attracting and keeping their females and retaining their territory. We can band all males and females at each st udy site and follow them from year to year to correlate their noise environment and song adjustment with rates of mate and territory fidelity. Conclusion s Since urbanization has caused an increase in ambient noise and birds continue to rely on vocal signa ling as their primary method of communication, it is important to assess how song characteristics are affected by noise. Now that we have established that mockingbirds sing at a higher minimum frequency in urban areas, we can further tease apart the reason s for this song adjustment. Mockingbirds mimic avian songs and non avian sounds found in their environment and build a diverse song repertoire. This is interesting because we can use their diverse repertoire to analyze their song composition: which species they mimic, are similar or different, and how mimicking those species or background noise affect s the transmission of a in its environment . We can also begin to assess consequences of this song adjustment on ma te attraction and mate and territory retention to gain a more complete understanding of how urban noise affects the fitness of song birds. ACKNOWLEDG MENTS University Scholars Program contributed financial assista nce for the research equipment. I thank Chr istine Stracey, Jill Jankowski, and Dr. Scott Robinson for mentoring and assisting me with this research. I thank Aaron Spalding for constructing part of the apparatus used for making the recordings. REFERENCES Derrickson, K.C. and R. Breitwisch. 1992. N orthern Mockingbird ( Mimus polyglottos ). In The Birds of North America, No. 7. A. Poole, P. Stettenheim and F. Gill (Eds). The Academy of Natural Sciences, Philadelphia, PA; and the American Ornithologists' Union, Washington, D.C. Fuller, R.A., P.H. Warre n, and K.J. Gaston. 2007. Daytime noise predicts nocturnal singing in urban robins. Biology Letters 3: 368 370. Gill, F. B. 2007. Ornithology, 3rd Edition. W.H. Freeman and Company, New York, New York. Marzluff, J.M. , R. Bowman, and R. Donnelly . 2001. Av ian ecology and conservation in an urbanizing world. Kluwer Academic Publishers, Boston. Slabbekoorn, H. and M. Peet. 2003. Birds sing at a higher pitch in urban noise. Nature 424: 267 268 . Slabbekoorn, H. and T.B. Smith. 2002. Habitat dependent song div ergence in the Little Greenbul: an analysis of environmental selection pressures on acoustic signals. Evolution 56: 1849 1858. not just noise. Animal Behavior 71: 491 502. Wi ley, R.H. 1991. Associations of song properties with habitats for territorial oscine birds of eastern North America. American Naturalist 138: 973 99 3. Wood, W.E. and S.M. Yezerinac. 2006. Song Sparrow ( Melospiza melodia ) song varies with urban noise. Auk 123: 650 659.