1

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.