effects of age, dominance, and mating system on vocal
TRANSCRIPT
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EFFECTS OF AGE, DOMINANCE, AND MATING SYSTEM
ON VOCAL CONSISTENCY IN MOCKINGBIRDS (AVES:
MIMIDAE)
Honors Thesis
Presented to the College of Agriculture and Life Sciences, Animal Science
of Cornell University
in Partial Fulfillment of the Requirements for the
Research Honors Program
by
Rachel J. Rossman
May 2007
Professor Vehrencamp
Department of Neurobiology and Behavior
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Firstly, I would like to thank Carlos A. Botero. His countless hours of guidance and
patience made this thesis possible. I want to thank Professor Sandra Vehrencamp, my
thesis advisor, for lending me her experience and support. I also wish to thank Professor
Susan Quirk for helping me with the specifics of an animal science thesis. A special
thanks to Professor Bruce Currie, my academic advisor for his continual advice and
support throughout my Cornell experience. Lastly, I would like to thank my friends and
family for their moral support and tireless patience throughout this process.
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Table of Contents
Abstract 4
Intro 5
Literature Review 7
Methods 12
Results 14
Discussion 19
Literature Cited 23
Appendices 29
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Abstract- While many studies have looked at selection for large repertoire sizes in birds,
few have explored the potential for selection on the ability to sing each type in the
repertoire consistently. I explored this issue by studying the consistency with which two
closely related species of mockingbirds, i.e. the northern (Mimus polyglottos) and the
tropical mockingbird (M. gilvus), repeated each syllable type. I hypothesized that if there
is selection for singing ability and not just for an increase in repertoire size, it could be
expected that syllable consistency should be (1) higher in older, more experienced
individuals, (2) higher in species with higher potential for sexual selection via female
choice (i.e. higher in the northern versus the tropical mockingbird) and (3) higher in
males expected to be of higher quality (i.e. in dominant versus subordinate birds). I found
that song consistency could be improved with practice because tropical mockingbirds
became more consistent with age. When comparing across male types, I found that
consistency was higher in northern and dominant tropical mockingbirds than in
subordinate tropical mockingbirds (although the data suggests that dominant tropical
mockingbirds that compete with subordinates for breeding may not be much better
singers than these subordinates). I suggest that the lack of significant differences between
northern mockingbirds and dominant tropical mockingbirds could be a product of a trade-
off between repertoire size and singing consistency because as the number of types in a
repertoire increases, the opportunities to practice each type are reduced.
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Introduction-
Repertoire size is thought to be a sexually selected trait in male songbirds
(Macdougal-Shackleton, 1997) and large repertoire sizes have proven to be advantageous
for both inter- and intra-sexual communication (Vehrencamp, 2000). Evidence linking
repertoire size with male quality or fitness (Catchpole, 1996; Hasselquist et al, 1996;
Forstmeier and Leiser, 2004), suggests that there is directional selection towards an
increase in vocal versatility in these systems. Male songbirds acquire their song
repertoires through learning, either by copying from external sources or by improvising
new songs and learning how to sing them in a consistent fashion (Kroodsma et al, 1998;
Zollinger and Suthers, 2004). If higher vocal versatility were the only force driving the
selection on repertoire size, it would be expected that males should develop better skills
at improvising new sounds and rely less on repeating sounds they have learned in the
past. I ask whether sexual selection in two species of mockingbirds (Aves: Mimidae), a
group of songbirds with very large repertoires, could be selecting for better ability to sing
each song or syllable type, rather than simply for an increase in the total number of types
in the repertoire.
Mockingbirds (Aves: Mimidae) are a group of New World songbirds known for
their superb singing abilities and large repertoire sizes (Brewer & MacKay, 2001). Two
of the best known species in this group are the northern (Mimus polyglottos) and the
tropical mockingbird (M. gilvus). The northern mockingbird is a socially monogamous,
temperate-zone species that breeds multiple times within a single breeding season. In
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spite of being year-round residents, males and females of this species often break their
pair bonds during the winter in highly seasonal areas and, thus, it is common for males to
have to re-attract a mate every spring (Derrickson and Breitwisch, 1992). Male northern
mockingbirds can sing more than 150 different syllable types, many of which are exact
copies of the songs or calls of other bird species (Derrickson and Breitwisch, 1992). The
tropical mockingbird is the northern mockingbird’s closest relative (Brewer & MacKay,
2001) but lives in a very different social environment. This species is a cooperative
breeder that lives in stable, year-round territories where co-males (i.e., male members of
the same social group) engage in joint territorial defense (Botero et al, 2007). In some
social groups, known as competitive groups, beta males court the alpha breeding female
and attempt to breed with her. In non-competitive groups, only the dominant male courts
her and gets to breed. Tropical mockingbirds can sing at least 130 different syllable
types, which they combine to create a seemingly unlimited number of different song
types (Botero et al, 2007). In contrast to the northern mockingbird, there is no evidence
that the tropical mockingbird engages in heterospecific mimicry. Given the differences
between these two sister species, it would be expected that male northern mockingbirds
face stronger sexual selection via female choice than male tropical mockingbirds.
I quantified the ability to sing each syllable type consistently by evaluating the
mean similarity in spectral cross-correlation between multiple renditions of the same
type. A syllable type is defined as being the smallest unit of song, which is separated
from other vocalizations by at least a span of time of 0.04 seconds. I predicted that if
there is selection for syllable consistency in mockingbirds: (1) it could increase with
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experience; (2) it should be higher in the species with higher potential for sexual selection
via female choice, i.e. the northern mockingbird; (3) it should be higher in tropical
mockingbirds with better access to reproductive females, i.e. dominant birds.
Literature Review-
Mate choice in song-
Bird song is known to provide information about a singer both to potential mates
and to competing rivals (Vehrencamp, 2000). Females can select a mate based on many
different song parameters including song complexity (Buchanan and Catchpole, 1997;
Price and Lanyon, 2002; Forstmeier, et al, 2006), song repertoire (Catchpole 1996,
Hasselquist et al, 1996; Lampe and Espmark 1994, 2003: Forstmeier and Leiser, 2004)
and fine-detail acoustic structure (Forstmeier et al, 2002; Christie et al, 2003; Ballentine
et al, 2004; Mennill and Rogers, 2006)
Song can provide information on male quality because it is affected by the
nutritional status of individuals during key developmental stages (i.e. nestling phase) and
by an individual’s genetic predisposition for learning (Nowicki et al, 2002). For
example, male swamp sparrows raised under feeding regimes with poor nutrition tend to
develop low quality songs as adults (Nowicki et al, 2002). Song can also provide
information on age and experience (Catchpole, 1996; Hasselquist, et al, 1996; Forstmeier
et al, 2006). In nightingales, male song repertoire size increases greatly between the first
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and the second year of life (Kiefer, 2006). Older and more experienced males are more
attractive to the females because they are of better quality and, in some cases, are likely
to provide better parental care (Forslund and Part, 1995).
Repertoire size-
Song repertoire size has been studied extensively in birds and is thought to be an
honest signal of quality or learning ability (Osiejuk et al, 2003). Larger repertoire sizes
have been correlated with age (Forstmeier et al 2006; Keifer et al, 2006), body condition
(Lampe and Espmark, 1994), parasite load (Buchanan et al, 1999), survival rate of
offspring (Hasselquist et al, 1996), male quality (Catchpole, 1996), and territory quality
(Lampe and Espmark, 1994). Song repertoire size can be attractive to females in many
species and may be used for mate selection (Macdougall-Shackleton, 1997).
Fine structure of song-
Other variable parameters of bird song that might encode information about male
quality and condition are bandwidth (Christie et al, 2003; Mennill and Rogers, 2006),
song rate (Hofstad et al, 2002; Forstmeier and Birkhead, 2004) and trill rate (Ballentine et
al, 2004; Illes et al, 2006). Female house finches (Carpodacus mexicanus) showed a
preference for longer songs and higher song rates and these two parameters were
correlated with the energy reserves of the male singers (Nolan and Hill, 2003) as well as
with nesting date and clutch size (Mennill et al, 2006).
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One reason why females choose males based on features of song production is
that some songs are limited by physical constraints (Podos, 1996; Forstmeier et al, 2002).
A male’s ability to produce challenging songs is limited by physiological constraints and
subject to variation among males due to genetics (Byers, 2007). An example of this can
be found in the swamp sparrow (Melospiza georgiana), where trills are limited by the
coordination of syringeal and respiratory activity, thus creating a performance trade off
between trill rate and frequency range (Ballentine et al, 2004).
Song consistency is another feature that may signal male quality. In chestnut-
sided warblers (Dendroica pensylvanica), the best predictor of extra-pair mating is the
amount of variation across songs and males that sing with less variability experience less
cuckoldry (Byers, 2007). In the zebra finch (Taeniopygia guttata), males sing with more
consistency when in the presence of a female than when alone or accompanied by other
males (Kao and Brainard, 2006), suggesting that females of that species may also pay
attention to this feature.
Song learning and mimicry-
Of the two main kinds of learning strategies in songbirds, i.e., ‘age-limited’
learning and ‘open-ended’ learning, the former is thought to be more common among
songbirds (Nordby et al, 2002). ‘Age-limited’ song learners can only learn songs within
their first year of life (Nordby et al, 2002) whereas ‘open-ended’ learners, e.g.
nightingales (Keifer, et al, 2006) or mockingbirds (Derrickson 1987), continue to learn
new songs as adults (Nordby et al, 2002). Lincoln’s sparrows (Melospiza lincolnii) are
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thought to be capable of ‘open-ended improvisation’ because of their ability to change
the composition of their song types as adult birds (Cicero and Benowitz-Fredericks,
2000).
Another aspect of song learning has to do with the selection, of tutors during the
learning process. Most species learn exclusively the songs of conspecifics whereas a few
mimic those of heterospecifics or even environmental sounds. In the northern
mockingbird, a well-known song mimic, individuals achieve accurate copies of the songs
of their tutors by copying the muscular patterns with which they are produced (Zollinger
and Suthers, 2004; Suthers and Zollinger, 2004). The purpose of heterospecific song
mimicry is not fully understood and studies have failed to provide convincing evidence
that mimicry is used for inter-specific communication (Howard, 1974; Baylis, 1982). A
recent study on the greater racket-tailed drongo (Dicrurus paradiseus), however, suggests
that this species uses mimicry to manipulate the behavior of other species in the mixed-
species flocks with which it usually travels (Goodale and Kotagamat, 2006). Another
possibility is that song mimicry is a measure of individual quality because of the
difficulties implied in reproducing the special acoustic features of heterospecific songs
(Suthers and Zollinger, 2004).
Male Status-
A female can learn about potential mates from the way in which males use songs
to interact with each other. Males may live in situations where they are competing with
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other males for breeding, and their competitive songs can affect female choice (Hurd,
1997). For example, brown-headed cowbirds (Molothrus ater) that are exposed to more
competition with other males achieve more copulations later on compared to males that
competed less with other males (King et al, 2003). Great bitterns (Botaurus stellaris) are
more likely to produce a longer boom train, a sequence of ‘booming’ vocalizations, when
there are more males nearby, than when they are alone and this is thought to be a way of
competing for female attention (Poulin and Lefebvre, 2003).
When multiple males share a single territory, a dominance hierarchy is usually
needed to minimize aggression between them. A dominance hierarchy is usually
established based on differences in the individuals such as size, experience, and age
(Lemel and Wallin, 1993; Christie et al, 2003). Tropical mockingbirds often live in
cooperative breeding situations with clear dominance hierarchies among males and joint
territorial defense (Botero et al., 2007). In Panama, subordinate tropical mockingbirds
may even provide help with caring for young (Morton et al, 2004). Tropical
mockingbirds in Colombia live in two different types of groups. In competitive groups,
both the dominant and subordinate males actively court a female and build a nest while in
non-competitive groups, only one male courts and builds a nest, either because he is the
only male in the group or because the subordinate males fail to do so (Botero et al. 2007).
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Methods-
Focal individuals and song recordings
I used recordings of 20 male northern mockingbirds from the Macaulay Library at
the Cornell Lab of Ornithology. The song samples were recorded by different people in
different locations from 1950 to 1998 throughout the United States and Mexico
(Appendix 1, www.animalbehaviorarchive.org/loginPublic.do).
Tropical mockingbird recordings came from a population of color banded
individuals in Villa de Leyva, Colombia (5°36’34”N, 73°32’47”W) and were collected
between August 2004 and April 2006 as part of a long-term behavioral study (Botero et
al, 2007). Individuals were recorded digitally in wave file format at 48 kHz and 16 bits
per sample using Marantz PMD690 portable PC card recorder and a Sennheiser ME67
directional microphone. Recordings selected were produced at a close range with good
signal-to-noise ratio, no overlap of foreign sounds and little to no detectable
reverberation. My sample includes songs obtained during a single breeding season from
9 dominant males from noncompetitive groups, 11 dominant males from competitive
groups and 15 subordinate males from competitive groups (years vary among individuals
depending on availability). Nine males were also sampled during an additional breeding
season to evaluate the effects of age on song consistency. This was possible because the
study birds were all color banded so that individuals could be identified for records made
over a period of time. The time difference between the two samples for a single male
was at least one year in 4 birds and less than one year in 5 birds (mean ± SE = 10.91 ±
1.46 months).
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Consistency of syllable types-
I used SyrinxPC 2.1 sound analysis software (John Burt, www.syrinxpc.com) to
select 20 syllable types per individual and ten exemplars per syllable (Appendix 2).
Syllable types were selected at random from the most common types in the recordings.
Given that mockingbirds are thought to be open-ended learners (Howard 1974;
Derrickson 1987) and, thus, that they might improve song consistency over time, I tried
to obtain all samples for a given individual in recordings produced in a single day (mean
+ SE = 3.41+ 1.68 days). I define vocal consistency as the level of similarity between
renditions of the same syllable type as measured through spectrogram cross-correlation.
Spectrogram cross-correlations were performed in Matlab 7.1 following Cortopassi and
Bradbury (2000), and generate a measurement value between 0 and 1. To minimize the
effects of background noise in the results, I applied a band-pass filter to all the exemplars
prior to cross-correlating them against other sounds. Exemplars were filtered with
SyrinxPC and the filter bounds were tailored to the frequency range of each individual.
Statistical Analyses-
Carlos A. Botero conducted the bulk of the statistical analyses. To explore the
differences in song consistency among male categories (i.e., northern mockingbird,
dominant tropical mockingbird from non-competitive group, dominant tropical
mockingbird from competitive group, and subordinate tropical mockingbird from
competitive group), the mean correlation value for each syllable type (i.e. the mean
correlation between an exemplar and all the other exemplars of its type) was computed,
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followed by computing the average within-type correlation for each individual. The effect
of male type on these individual averages was evaluated with a one-way ANOVA.
Results-
Song consistency was significantly different among male categories (GLM: F3,52
= 4.90, P = 0.005, Fig. 1). A posteriori comparisons showed that subordinate tropical
mockingbirds in competitive groups were significantly less consistent than northern
mockingbirds (Tukey test: t = -3.502, P = 0.005) and dominant tropical mockingbirds in
non-competitive groups (Tukey test: t = -2.899, P = 0.027). Song consistency was lower,
though not significantly different, in dominant tropical mockingbirds from competitive
groups than in dominant tropical mockingbirds from non-competitive groups (Tukey test:
t = 1.407, P = 0.501) or northern mockingbirds (Tukey test: t = 1.627, P = 0.373). No
significant differences were observed between subordinate and dominant tropical
mockingbirds from competitive groups (Tukey test: t = -1.554, P = 0.413) or between
dominant tropical mockingbirds from noncompetitive groups and northern mockingbirds
(Tukey test: t = -0.065, P = 0.999).
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Figure 1. Differences in song consistency between northern mockingbirds (NM),
dominant tropical mockingbirds from non-competitive groups (DNC), dominant tropical
mockingbirds from competitive groups (DC), and subordinate tropical mockingbirds
from competitive groups (SC). Error bars represent one standard error from the mean and
letters indicate statistically significant differences (IE. the difference between SC and
both NM and DNC).
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Male tropical mockingbirds became significantly more consistent as they aged
(Paired t-test: N = 9; t = -4.43, P = 0.002, Fig. 2). Only one bird in our sample showed a
slight decrease in consistency (0.3% decrease). This individual was a dominant male
from a non-competitive group for which his first and second samples were only four
months apart (i.e., one of the shortest time intervals in this analysis). A correlation of
time between samples and change in consistency yielded marginally significant results
(Pearson’s correlation: r7=0.651, P=0.058, Fig. 3).
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Figure 2. Changes in song consistency across breeding seasons. Each line represents a
single bird’s change in song consistency between two breeding seasons. There is a
varying amount of time elapsed between selected breeding seasons of the birds.
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Figure 3. Scatterplot of delta consistency verses months between breeding seasons of
birds.
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Discussion-
The comparison of consistency levels in breeding seasons from different years
indicates that male tropical mockingbirds improve with experience and, thus, that syllable
consistency has the potential to be a signal of age. Females often prefer older males
because they are likely to provide better parental care, to be healthier and to have a better
territory (Kiefer et al, 2006; Lampe and Espmark, 2003). Previous studies have observed
that a number of different features of song, such as repertoire size (Hasselquist et al,
1996) and frequency matching (Trainer and McDonald, 1995), can signal age and
experience in birds. These studies found that older males had larger repertoires or better
frequency matching, and that these males were more attractive to females. In male zebra
finches, songs are sung significantly more consistently when in the presence of females,
than when alone or with other males (Kao and Brainard, 2006) suggesting that males
practice when alone, and then attempt to perform their best song when a female is
around. Since our results show a positive relationship between song consistency and age,
it is likely that tropical mockingbirds improve their consistency through practice.
Additionally, there is evidence of a correlation between time elapsed and the change in
consistency, which suggests that the more time a bird is allowed to practice, the more
they can improve their consistency. While the correlation was only marginally
significant, the data suggest a pattern and it is likely that a larger sample could yield a
significant relationship.
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The lack of significant differences in song consistency between northern and
dominant tropical mockingbirds suggests that despite the fact that tropical mockingbirds
do not mimic or need to attract a mate every breeding season, they are also under strong
pressure of selection for singing each syllable type consistently. It is also possible that
this similarity in song consistency is a reflection of a tradeoff between repertoire size and
vocal consistency. Given that mockingbirds appear to improve syllable consistency
through practice, it is possible that northern mockingbirds are not more consistent than
dominant tropical mockingbirds because they have more syllable types in their repertoire
and this means they have less time for practicing and improving the performance of each
one. Another possibility is that the similarity in song consistency is a reflection of similar
levels of selection for this trait in both species. Northern mockingbirds are often year-
round residents (Derrickson and Breitwisch, 1992), especially in the geographical regions
where most of our samples came from (see Appendix 1). Given that resident northern
mockingbirds are likely to spend the entire year as a pair (Derrickson and Breitwisch,
1992) it is likely that they do not need to attract a female every spring and, thus, that they
are not much more consistent than tropical mockingbirds because they are not truly
exposed to a sufficiently different selection regime. Furthermore, it is possible that
selection pressures on male tropical mockingbirds are quite high in spite of their year-
round residency and long-term pair bonds because they often live in social groups in
which males must compete for the access to females (Botero et al. 2007).
The expected link between male quality and singing ability (Hurd 1997;
Vehrencamp, 2000) provides a potential explanation as to why dominant males from
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competitive groups showed an intermediate level of consistency between dominant males
from non-competitive groups and subordinate males from competitive groups. Although
the differences in consistency between dominant males from non-competitive and
competitive groups were not statistically significant, a possible interpretation of our data
is that the latter may in fact be of slighter lower quality. Given that competitive beta
males become part of a social group by challenging the alpha male and avoiding eviction
(Botero et al. 2007), it would be expected that they are more likely to colonize groups in
which the alpha male is of similar quality to them. Thus, dominant males from
competitive groups could have intermediate consistency levels because alpha males of
higher quality and, thus, higher song consistency, are more successful at evicting
potential competitors and tend to live in non-competitive groups.
This study serves as a strong starting point in the study of selection on the ability
to sing consistently and raises interesting questions that can be addressed in future
studies. For example, a larger sample of dominant males from competitive and non-
competitive groups is needed to confirm whether these male types truly differ in syllable
consistency. Also, a characterization of spectral parameters in the different syllable types
could help us determine if certain syllable types are harder to sing in a consistent fashion
than others, and if older males or males of higher quality tend to include more of these
hard types in their repertoires. Another interesting future direction is to study the effect of
song consistency on female choice, for example by correlating male paternity levels with
consistency. This latter analysis would be especially interesting when comparing among
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males with a similar repertoire size, since it could lend some support to the idea that
features other than large repertoire sizes are also being selected in these birds.
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Appendix 1: Macaulay Library’s catalog information of northern mockingbird
recordings.
Catalog number Recordist Date Location
10280 Kellogg, P. P. 3/2/1950 Miami, FL
10284 Kellogg, P. P. 3/6/1950 Richmond, FL
10286 Kellogg, P. P. 5/29/1960 Harlingen, TX
10290 Allen, Arthur A. 4/5/1959 Fullerton, CA
10296 Davis, L. I. 5/21/1962 Puebla, Mexico
105414 Keller, G. A. 6/1/1994 Charleston, SC
11107 Jones, J. P. 6/11/1965 Fairmont, WV
11116 Glase, J. C. 6/19/1966 Reading, PA
11123 Zane, N. 1967 Stockton, CA
22931 Fish, W. R. 6/9/1951 S. Fork Valley, CA
22932 Fish, W. R. 6/22/1953 Day Ranch, CA
23402 Zimmerman, D. A. 5/21/1980 Grant, NM
28037 Davis, L. I. 7/6/1957 San Louis Potosí,
Mexico
42737 Hewitt, Oliver H. 6/30/1987 Charlotte Harbor, CA
45003 Parker, T. A. III. 5/23/1988 Devils River, TX
56847 Keller, G. A. 5/9/1986 Green Valley, AZ
61574 Gunn, W. W. H. 5/21/1981 Paradise Turnoff, AZ
85192 Hershberger, W. L. 7/4/1997 Adamstown, MD
87477 Medler, M. 4/24/1998 Cameron, TX
94374 Hershberger, W. L. 5/29/1998 Adams, PA
Appendix 2: Sample sound selections of a northern mockingbird made with SyrinxPC 2.1
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.