Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

Differences in duet coordination influence territorial response

on a year-round territorial bird species

Luis Sandoval1, 2*; https://orcid.org/0000-0002-0793-6747

Roselvy Juárez1; https://orcid.org/0000-0002-3850-527X

Katherine Bonilla-Badilla1; https://orcid.org/0000-0002-1717-6526

Brendan Graham3; https://orcid.org/0000-0002-0839-1232

1. Laboratorio de Ecología Urbana y Comunicación Animal, Universidad de Costa Rica, Montes de Oca, San José, Costa

Rica, CP 11501- 2060; biosandoval@hotmail.com (*Corresponding), kathybonilla26@hotmail.com;

roselvy.juarez@gmail.com

2. Colección de Bioacústica, Museo de Zoología, CIBET, Universidad de Costa Rica, San José, Costa Rica;

3. Department of Biological Sciences, University of Lethbridge, 4401 University Dr W, Lethbridge Alberta, Canada, T1K

3M4; b.graham001@gmail.com

Received 28-VIII-2024. Corrected 10-I-2025. Accepted 04-III-2025.

ABSTRACT

Introduction: Twelve functions have been assigned to avian vocal duets (e.g., maintaining contact, mate guard-

ing, signaling quality, or resource defense). To separate between functions of duets it is necessary to take into

account who is the receiver, the information coded by the sender, and if there is a conflict between pair members.

Duets used for resource defense (e.g., territory) are a more aggressive signal than solo songs because they act as

a joint defense signal that encode a pairs’ strength or time together in the coordination of both individuals’ song.

Therefore, interacting pairs may use duet coordination to respond according to rival information.

Objective: Our main objective in this study was to test whether the coordination in time and frequency of White-

eared Ground-Sparrow (Melozone leucotis) pair duets influences the territorial response of conspecific pairs.

Methods: We recorded 2-5 duets from 31 territorial pairs and measured duet coordination by dividing each

ground-sparrow duet into three sections (include sections here) according to each individual’s contribution. In

each section we measured frequency range and duration and the difference in frequency range and duration

between the introductory and middle section, and the middle and terminal section of each duet. We then used

a playback experiments to test pair response to duet coordination. Each pair were exposed to two types of duets:

highly coordinated duets and a poorly coordinated duets.

Results: We exposed 31 pairs to these two treatments during duet playbacks and measured their territorial

response according to the simulated intruders’ duet coordination. We found that pairs that produced highly

coordinated duets approached faster and spent more time closer to all playbacks. By comparison, territorial

pairs spent more time closer to the poorly coordinated duet stimulus. Total number of vocalization produced in

response to duet stimuli were similar between stimuli and independent of the duet coordination of the territorial

pairs.

Conclusions: Our study indicates that, duet coordination in territorial pairs of White-eared Ground-sparrows is

a good predictor of the strength in territorial defense and suggests that pairs used duet coordination to perceive

the level of threat from an intruder.

Key words: acoustic communication, duets, duet coordination, Passerellidae, sparrows.

https://doi.org/10.15517/rev.biol.trop..v73iS2.64525

SUPPLEMENT

SECTION: ACOUSTICS

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

INTRODUCTION

Across taxa a wide diversity of behaviours

is used to defend territories, including chemi-

cal signals (mainly in mammals and insects),

vocalizations (mainly birds and amphibians),

or visual displays (mainly birds and fishes;

López-Sepulcre & Kokko, 2005; Nice, 1941;

Stamps & Buechne, 1985). The purpose of these

behaviours is to reduce probability of physical

encounters, since they are energetically expen-

sive and participants risk injury or even death

(López-Sepulcre & Kokko, 2005; Stamps &

Buechne, 1985). Territorial behavior can occur

year-round or only during specific time periods

(e.g., breeding season or food resource peak;

Brown, 1963; Duca & Marini, 2014; Holland

et al., 2017; Woltmann & Sherry 2011); but in

all cases, territory defense is used to defend

resources, including mates, food, nesting sites,

or roosting sites. Importantly, these behaviours

are not restricted to defending territories from

intraspecific individuals (Grether, 2011; Ord et

al., 2011). Some species display this behavior

against heterospecific individuals and respond

as intensely as they do towards conspecifics

(Martin & Martin, 2001; Qvarnström et al.,

2006; Sandoval et al., 2013).

Vocal duets are used by some tropical

bird species to defend resources, including

food, nesting sites, pair members, or territories

(Hall, 2004, 2009; Kovach et al., 2014). Duets

are defined as the coordination of vocaliza-

tions from two individuals (Hall, 2004; Hall,

2009), but not all duets are produced using solo

songs. For example, in cranes and geese duets

are produced by coordinating calls (Volodin

et al., 2015); and in a small group of Neotropi-

cal bird species (Passerellidae sparrows), duets

are produced using vocalizations exclusive for

RESUMEN

Las diferencias en la coordinación de dúos influyen en la respuesta territorial

de una especie de ave territorial durante todo el año

Introducción: Se han asignado doce funciones a los duetos vocales de las aves (por ejemplo, mantener el contacto,

proteger a la pareja, o defensa de recursos). Para separar las funciones de los duetos es necesario tener en cuenta

quién es el receptor, la información codificada por el emisor y si existe algún conflicto entre los miembros de la

pareja. Los duetos utilizados para la defensa de recursos (ej.: territorio) son una señal más agresiva que los cantos

en solitario porque actúan como una señal de defensa conjunta que codifica la fuerza de una pareja o el tiempo

que pasan juntos en la coordinación del canto de ambos individuos. Por lo tanto, las parejas pueden utilizar car-

acterísticas de los duetos para responder de acuerdo con la información de una pareja rival.

Objetivo: Nuestro principal objetivo en este estudio fue probar cómo la coordinación en tiempo y frecuencia de

los duetos de las parejas de Melozone leucotis influencian la respuesta territorial de parejas conespecíficas.

Métodos: Registramos de 2 a 5 duetos de 31 parejas territoriales y medimos la coordinación del dueto utilizando

un método descrito anteriormente que dividió cada dueto de M. leucotis en tres secciones según la contribución

de cada individuo. En cada sección medimos el rango de frecuencia, la duración y la diferencia en el rango de

frecuencia y la duración entre la sección introductoria y media, y la sección media y terminal de cada dueto. Luego

utilizamos experimentos de playback para probar la respuesta de la pareja a la coordinación del dueto. Cada pareja

estuvo expuesta a dos tipos de dueto: duetos altamente coordinados y mal coordinados.

Resultados: Expusimos a 31 parejas a estos dos tratamientos durante las reproducciones a dueto y medimos su

respuesta territorial de acuerdo con la coordinación del dueto de los intrusos simulados. Descubrimos que las

parejas que producían duetos altamente coordinados se acercaban más rápido y pasaban más tiempo cerca de

todos los estímulos. En comparación, las parejas pasaron más tiempo más cerca del estímulo del dueto mal coor-

dinado. El número total de vocalizaciones producidas en respuesta a estímulos de dueto fueron similares entre

estímulos e independientes de la coordinación de dueto de las parejas territoriales.

Conclusiones: Nuestro estudio indica que la coordinación del dueto en parejas territoriales de M. leucotis es un

buen predictor de la defensa territorial y sugiere que las parejas utilizan la coordinación del dueto para percibir

el nivel de amenaza de un intruso.

Palabras clave: comunicación acústica, duetos, coordinación de duetos, Passerellidae, gorriones.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

duetting, which are spectrotemporally different

from solo songs (Benedict & McEntee, 2009;

Sandoval & Mennill, 2014; Sandoval et al.,

2016; Trejos-Araya & Barrantes, 2014). Overall,

the duets produced using calls or vocalization

exclusive for duetting are less studied than

duets produced using coordinated solo songs

(Farabaugh, 1982; Hall, 2004; Hall, 2009).

Temporal and frequency coordination in

duets indicate the commitment of both indi-

viduals of the pair to act collectively (Dahlin

& Benedict, 2014; Hall & Magrath, 2007; Hall,

2009; Kovach et al., 2014), with respect to

defending or acquiring an existing resource.

For example, in Canebrake Wrens (Cantorchi-

lus zeledoni) highly coordinated duets occur in

pairs that have formed longer pair bonds in

comparison to birds with shorter or recently

formed pair bonds (Rivera-Cáceres et al., 2016).

By comparison, Magpie-lark (Grallina cyano-

leuca) males and pairs respond more aggres-

sively during territorial defense to acoustic and

visually coordinated duets than poorly coor-

dinated duets of intruders (Hall & Magrath,

2007; Ręk & Magrath, 2022). Therefore, it is

also expected that duet coordination may be

used as a signal to gain information about the

threat that a rival pair poses during interac-

tions, including the strength and aggressiveness

of rival pairs (Dahlin & Benedict, 2014; Kovach

et al., 2014; Méndez & Sandoval, 2017; Rivera-

Cáceres et al., 2016).

It is possible to classify duets into two

types, based on how they are produced (Hall,

2004; Hall, 2009). First, antiphonal duets that

are produced by alternating vocalizations as

observed for Cabanis’s Wren (Cantorchilus

modestus), Plain-tailed wrens (Thryothorus

euophrys), Spotted Morning-Thrush (Cichladu-

sa guttata), and Slate-coloured Boubou (Lania-

rius funebris) (Cuthbert & Mennill, 2007; Mann

et al., 2006; Sonnenschein & Reyer, 1983; Todt

& Fiebelkorn, 1980). Antiphonal duets need

high levels of coordination for both members of

the pair to produce a duet, because individuals

must avoid overlapping each other’s vocaliza-

tions (Kovach et al., 2014; Rivera-Cáceres et al.,

2016; Thorpe & North, 1965; Thorpe, 1972).

Polyphonal duets, however, are produced by

overlapping the time and/or frequency of the

song produced by the other individual of a pair,

as observed in Barred Antshrike (Thamnophilus

doliatus), Rufous-and-White Wren (Thryophi-

lus rufalbus), Large-footed Finch (Pezopetes

capitalis), Canyon Towhee (Melozone crissalis),

and White-eared Ground-sparrow (M. leucotis;

Benedict & McEntee, 2009; Koloff & Mennill,

2013; Mennill & Vehrencamp, 2005; Sandoval

et al., 2016; Trejos-Araya & Barrantes, 2014).

Given the overlap in polyphonal duets, pair

members also coordinate their songs to pro-

duce duets (Kovach et al., 2014; Trejos-Araya &

Barrantes, 2018). Therefore, in both antiphonal

and polyphonal duets, coordination apparently

plays an important role within and between

pair interactions. Consequently, experimental

studies examining the relationship between

form and function in duet studies in the context

of duet coordination are highly encouraged

(Dahlin & Benedict, 2014).

We used White-eared Ground-sparrows

as a model species to analyze the territorial

response to duet coordination (i.e., tempo-

ral and frequency coordination; Farabaugh,

1982; Hall, 2009) because pairs are year-round

territorial and use duets as the main vocal-

ization to claim and defend territories after

pair formation (Méndez & Sandoval, 2021;

Sandoval et al., 2016). This species is socially

monogamous and pair members stay together

1-3 years (LS unpub. data). Duets are produced

with vocalizations exclusively used for duetting

(Fig. 1), which are different from male solo

songs (vocalization used to attract females) and

male and female contact calls (see Sandoval et

al., 2014; Sandoval et al., 2016 for sonograms

of solo songs and calls). Duets are produced in

two contexts: contact and territorial response

(Méndez & Sandoval, 2021; Sandoval et al.,

2016). Contact duets are produced when both

members of the pair are foraging far apart and

move closer to one another. In this scenario

one individual (male or female) starts the duet

and the second responds and fly closer to the

position of the individual that started the duet

after the duet is completed. These duets are

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

also produced when one member of the pair

approaches the other individual after they have

been apart (Sandoval et al., 2016). Territorial

duets are produced in response to a neighbor-

ing pairs duets, male solo songs, or conspecific

and interspecific intruders’ (Sandoval et al.,

2013; Sandoval et al., 2016). This duet type is

produced by both pair members when they are

together and is less coordinated than contact

duets (Méndez & Sandoval, 2021).

Our main objective in this study is to test

the territorial defense hypothesis by examining

how pair duet coordination influences ter-

ritorial responses in year-round White-eared

Ground-sparrow pairs. More specifically, we

tested (1) how within pairs’ duet coordination

influences the territory defense and (2) how ter-

ritorial pairs respond to intruders with different

levels of coordination (i.e., high coordinated

duets vs. poor coordinated duets). We expect

that if the within pairs duet coordination is a

signal of the degree of commitment between

paired individuals, as has been suggested by

(Hall, 2004; Hall, 2009), highly coordinated

pairs will be more committed to defend terri-

tories together. Therefore, we predicted a more

aggressive behaviour against all stimuli in pairs

with highly coordinated duets. We also expect-

ed that if duet coordination is a signal that indi-

cates motivation to usurp a territory or greater

aggression as suggested by Logue & Gammon

(2004) and Méndez & Sandoval (2021), terri-

torial pairs will be more committed to defend

territories against intruder with highly coor-

dinated duets. Therefore, we predict that pairs

will respond more aggressively against simu-

lated highly coordinated duets than simulated

poorly coordinated duets independently of its

own duet coordination. But, if duet coordina-

tion is a signal of pair time length as suggested

by Rivera-Cáceres (2015), territorial pairs will

be more committed to defend territories against

intruder with poor coordinate duets. Therefore,

we predict that territorial pairs will respond

more aggressively against poorly coordinated

duets than coordinated duets. The reason being

that poorly coordinated duets are indicative of

recently formed pairs that are searching for a

new territory and are a greater threat to usurp

the resident pair (et al., 2022; Méndez & Sando-

val, 2021; Sandoval et al., 2018).

MATERIALS AND METHODS

Recording: We recorded 31 territorial

mated pairs with a unique colour-banded code

from three populations that belong to a long

term study (Heredia: n = 9, Universidad de

Costa Rica: n = 13, and Jardín Botánico Lank-

ester: n = 9) from April 1 to May 31 2017. The

three colour-banded populations of White-

eared Ground-sparrows occur within the Cen-

tral Valley of Costa Rica (Heredia: 10°01’ N

& 84°05’ W, altitude: 1350 m; Universidad de

Costa Rica: 09°56’ N & 84°05’ W, altitude: 1200

m; and Jardín Botánico Lankester: 09°50’ N &

83°53’ W, altitude: 1400 m). We carried out the

study during the breeding season of the species

(Sandoval & Mennill, 2012), when pairs actively

defend territories against other pairs (Juárez et

al., 2020; Sandoval et al., 2013). We recorded

duet pairs using the focal method (Sandoval

et al., 2024), which, in our case, consisted of

observing a focal pair for 1.05 hours per day,

from 04:55 to 06:00 h when this species is most

vocally active. Each pair was recorded during

a single recording session, but for pairs that

did not produce duets during the first record-

ing session, were recorded during a second

session. We collected all recordings using a

Marantz solid state recorder PMD661 with a

shotgun microphone Sennheiser ME66/K6 in

wav format, with a 44.1 kHz sampling rate, and

24 bits accuracy.

Coordination measurement: We mea-

sured duet coordination after conduct the play-

back experiment. Consequently, we were blind

to the duet coordination status of the pair dur-

ing the experiment, ensuring that our measure-

ments of behavioral responses to the playback

stimuli were unbiased. From all recorded duets,

we were only able to measure 2-5 duets per pair

(mean = 3.7 duets/pair), because for the study

focus we required only contact context duets

(produced spontaneously during recording

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

period) with a high signal-to-noise ratio (>30

dB difference between background and signal),

and no overlap from other sounds. We used

only contact context duets for the analysis

because we required that duet coordination

had not the influence of other pairs as occurs

in territorial duets (Méndez & Sandoval, 2021).

Therefore, differences in duet coordination

within pairs will be only influenced by both

individuals’ commitment, allowing us to test

the relationship between pair duet coordina-

tion and territorial response in the White-eared

Ground-sparrows.

To measure duet coordination in White-

eared Ground-sparrows, we did not use the

traditional method that measure separately

the duration of each individual duet contribu-

tion and the duration of silence between each

individual contribution elements (Farabaugh,

1982; Hall & Magraht, 2007; Logue et al.,

2008; Rivera-Cáceres, 2015) because it is nearly

impossible to separate the elements, and so

the contribution of each bird to the duet, as it

occurs in a related group of Neotropical spar-

rows (Sandoval & Mennill, 2014; Sandoval et

al., 2016; Trejos-Araya & Barrantes, 2014).

In these sparrows, duets elements are highly

overlapped, as described in detail by (Sando-

val et al., 2016; Sandoval, 2018) and Méndez

and Sandoval (2021). Therefore, we used the

approach employed by Méndez and Sandoval

(2021), which is a modification of the approach

used by Hall and MaGrath (2007) and Hall and

Peters (2008) to examine duet coordination

for antiphonal duets. This approach divides

each duet into three sections (Fig. 1): (1) the

introductory section, which is the section pro-

duced to start the duet and is produced by

a single individual (male or female); (2) the

middle section, which is the section where

both individuals of the pair overlap in time and

frequency the duet elements (this section starts

at the moment the second individual begins to

vocalize, overlapping with the introductory ele-

ments of the first individual and finishes when

the first individual stops vocalizing, which is

detected because the maximum frequency of

the elements produced by the second individual

is lower than the frequency when the two indi-

viduals contribute to the duet); and (3) the ter-

minal section, which is the section produced to

finish the duet and is produced only by the sec-

ond individual that participates in the duet. In

each section we measured the duration (s) and

Fig. 1. White–eared Ground–sparrow duet showing the three sections and ten measurements (of duration and frequency)

used to estimate duet coordination. A) Introduction section frequency range. B) Frequency differences between the

introduction–middle section. C) Middle section frequency range. D) Frequency differences middle–terminal section. E)

Terminal section frequency range. F) and G) Distance between vertical point lines are the difference in time between the end

of the introduction and the beginning of the middle section and between the end of middle section and the beginning of

the terminal section, respectively. This distance may be overlapped as in this example or not. Black lines on top of the duet

represent duration of each section. Bottom black segments represent the duet contribution of the first individual. Bottom grey

segments represent the duet contribution of the second individual. We indicate each individual duet element in each duet

contribution, because this duet was one of the duets created for the playbacks using a single individual contribution, and we

used this as a reference example because is clearer how measurements were obtained.

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

frequency bandwidth (kHz), as well as the dif-

ferences in frequencies (kHz) and duration (s)

between sections as follow: (1) the difference

between the minimum frequency of the intro-

ductory and middle section, (2) the difference

between the maximum frequency of the middle

section and final section, (3) the difference in

duration between the end of introductory sec-

tion and the beginning of the middle section,

and (4) the difference in duration between the

end of middle section and the beginning of the

final section (Fig. 1). We distinguish sections

using no overlap between elements (introduc-

tory and terminal section) and the differences

in frequency, because the second individual

always produce its duet contribution a lower

frequency than the first one (Sandoval et al.,

2016), allowing us to observe a step down in

the maximum frequency when the final sec-

tion begins (Fig. 1). With this approach, a pair

with highly coordinate duets in frequency and

duration will have little variation in the dura-

tion and frequency bandwidth of each duet

section (Méndez & Sandoval, 2021). We used a

combination of spectrogram window (to visu-

ally identify the duets), waveform window (to

measure the duration), and power spectrum

window (to measure the frequency) as has been

recommended previously for other authors

(Méndez & Sandoval, 2021; Podos, 2001) to

obtain the measurements of duet coordination.

We used the sound analysis software Raven Pro

1.6 (Cornell Lab of Ornithology, Ithaca, NY,

U.S.A.) with the following settings: frequency

resolution of 188 Hz and a temporal resolu-

tion of 5.8 ms in a Hann window with a hop

size of 256 kHz samples and 50% overlap. We

estimated a coefficient of variation (SD/mean

X 100) for both measurements in each section

(six coefficients of variation) and the two mea-

surements between sections (four coefficients

of variation). Finally, we used the average of all

10 coefficient of variations as our coordination

measurement. A pair with a lower average of

the coefficient of variation is considered more

coordinated than a pair with a higher average

of the coefficient of variation.

Playback treatments: We created playback

treatments using male and female duet contri-

butions (Fig. 2) recorded previously from the

study populations using a solid-state digital

recorder (Marantz PMD661; sampling rate: 44.1

kHz; accuracy: 24-bit; file format: WAV) and a

shotgun microphone (Sennheiser ME66/K6).

These duet contributions came from occasions

when the second individual did not respond to

the duet vocalization (Sandoval et al., 2016),

and we identified (unique band color) the sex

of the individual that vocalized. We used four

males (two from Heredia and one from Uni-

versidad de Costa Rica and Lankester) and four

females (one from Heredia and Universidad

de Costa Rica and two from Lankester) duet

contributions to create eight pairs of highly

coordinated and poorly coordinated duets, with

a high signal-to-noise ratio and were not over-

lapped by other sounds (Fig. 2). Each territorial

pair was exposed to the same male and female

contribution, but in the poorly coordinated

duets they varied in the time when each con-

tribution started to create the duet. Using these

type of playbacks, we were confident that the

response was influenced by coordination (or

lack of it) and not by each duet contribution

information. Prior to creating each playback,

we filtered out all the background noise below 5

kHz and above 12.5 kHz of each duet contribu-

tion using the FFT function in Adobe Audition

1.0 (Adobe Systems, San Jose, CA, USA). Then,

we uploaded each file to Adobe Audition 1.0

software and placed the two duet contributions

in separate channels. To create the coordi-

nated duets, we matched the male and female

duet contribution in time following the species

range for contact duets (0.1-0.18 s; Méndez &

Sandoval, 2021) and repeated duets at a rate

of 4 duets min-1 (Fig. 3). To create the poorly

coordinated duets, we varied the start time of

male and female contributions within the spe-

cies range for territorial defense duets (0.05-0.3

s; Méndez & Sandoval, 2021) in each of the

eight duets used to create a rate of 4 duets min-1

(Fig. 3). We used eight Cabanis’s Wren duets

recorded in 2016 from the same experiment

locations as controls (two duets per studied

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

population). This wren species shares the habi-

tat with White-eared Ground-sparrows but do

not compete for food, breeding sites, or mates

(Sandoval et al., 2014). Each control has one

wren duet repeated at a rate of 4 duets min-1

(Fig. 3). We normalized all playback treatments

to -1 dB using the normalize function of Adobe

Audition 1.0.

Experiment design: We conducted the

playback experiments from June 3 – 23, 2017.

Each pair was exposed to three playback treat-

ments in a single 21 min trial between 0600

and 1 000 h during the same day to simulate

territory intrusions. This approach that consists

of presenting stimuli consecutively has been

used previously in several playback experiment

Fig. 2. White-eared Gound-sparrow duet contribution of female (A: waveform and B: sonogram) and male (C: wave form

and D: sonogram) used to create the stimuli presented in the figure 3 (see below), with female contribution being the first

individual and male being the second individual in the duet.

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

studies (e.g., Bolton, 2007; Geberzahn et al.,

2009; Ripmeester et al., 2010; Sandoval et al.,

2013; Sosa-López et al., 2016). Our three treat-

ments were (Fig. 3): a) coordinated duets and

b) poorly coordinated duets from White-eared

Ground-sparrows, and c) a duet from Caba-

nis’s Wren (Cantorchilus modestus), which

served as our control treatment. We varied the

presentation order of each playback treatment,

following a random design (Coordinated duets

were presented: 13 in first, 12 in second, and

6 in third position). Poorly coordinated duets

were presented: 12 in first, 7 in second, and 12

in third position. Control duets were presented:

6 in first, 12 in second, and 13 in third posi-

tion). Each playback treatment lasted 2 min

Fig. 3. Duet examples used in our playback stimuli. A–C) Artificially created White–eared Ground–sparrow duet with the

same two individual contributions represented by the bottom black (first individual) and grey segments (second individual).

D) Plain Wren control duet, where male contribution is the bottom black segments and female contribution is the bottom

grey segment. Panels A and B show the appearance of the first and second duets in the highly coordinated duet stimulus.

Panels B and C show the appearance of the first and second duets in the poorly coordinated duet stimulus.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

followed by 5 min of silence. We recorded a

bird’s response behavior during the 2 min of

playback and the first 3 min of silence. The

remaining 2 min of silence were used to allow

a territorial pair to recover from the playback

stimulus and to return to the activities con-

ducted prior to the previous playback. In the

field we confirmed that all focal pairs left the

playback area (see below for the area) before the

end of the silence recovery period, which indi-

cated that we allowed an appropriate amount of

time for recovery.

We broadcasted playback treatments by

positioning a loudspeaker (Anchor Audio;

AN Mini, frequency response: 100-12 000 Hz)

inside each pair’s territory, 5-10 m from the

edge. The loudspeaker was connected to a por-

table audio player (iPod nano, Apple Cupertino,

CA), and mounted on a pole at 1.5 m height

to simulate the average height used by White-

eared Ground-sparrows to produce duets. We

hung flagging tape at 3 m from each side of

the speaker to use as a reference during the

playback trials. We broadcasted each playback

at a constant volume of 80 dB SPL, measured

at 1 m in front of the speaker with a Sper Sci-

entific sound level meter (Model 850014, using

A weight and fast response). This volume value

has been used previously in playback studies

of White-eared Ground-sparrows (Méndez &

Sandoval, 2017; Sandoval et al., 2013), and

emulates the natural volume level used by this

species in the wild. The observer was located at

8 m from the loudspeaker.

Response measurements: We measured

four behavioural responses for each playback

treatment: (1) latency time to approach at 3 m

from the speaker, in seconds (if pair did not

approach we assigned a value of 301 s); (2) time

inside 3 m radius from the speaker, in seconds

(if pair did not approach we assigned a value of

0 s); (3) latency time of the first vocalization, in

seconds (if pair did not vocalize we assigned a

value of 301 s); and (4) total number of vocal-

izations produced (if pair did not vocalize we

assigned a value of 0).

Statistical analysis: First, we tested that the

average of all 10 coefficient of variations (our

coordination measurement) was not affected by

the number of duets analyzed in each pair (2-5

duets), using a one-way analysis of variance

and the number of duets analyzed per pair as a

grouping variable (four levels) and the average

of coefficient of variation as our response vari-

able. Second, contrary to the majority of studies

that combine the behavioural responses into a

single multivariate response using a principal

component analysis, we conducted four gener-

alized linear mixed-effects models with nega-

tive binomial distribution and log link function

to analyze the relationship between treatments

and duet coordination with each behavioural

response (i.e., latency time to approach at 3

m from the speaker, latency time of the first

vocalization, time inside 3 m radius from the

speaker, and total number of vocalizations)

using lme4 library (Bates et al., 2015). We used

a negative binomial distribution to control for

the overdispersion of the data caused primar-

ily by the absence of response from territorial

birds to the control. We used a Type 3 analysis

of variance in R with the car library (Fox &

Weisberg, 2019). In our analysis we included

playback treatments (three levels), duet coor-

dination (continuous variable), and playback

treatment*duet coordination as the indepen-

dent factors. The pair identity and population

were used as crossed random factors, and

playback used in each pair as random factors

to account for the multiple tests per pair. We

reported means ± SE.

RESULTS

We found that the mean coefficient of

variation was not influenced by the number of

duets analyzed per pair (CV mean ± SE: 2 duets

= 34.58 ± 4.37%, 3 duets = 32.93 ± 3.09%, 4

duets = 28.78 ± 3.57%, 5 duets = 38.60 ± 3.09%;

F = 1.5, df = 3,22, P = 0.24). Of the ten measure-

ments of duet coordination that we collected,

eight had an average coefficient of variation

lower than 50% (Table 1), reflecting high levels

of duet coordination within most pairs.

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

Table 1

Coefficient of variation for the frequency range and duration of each duet section in White–eared Ground–sparrow duets.

Section Measurement Mean ± SE Min Max 25th percentile 75th percentile

Start Duration 32.56 ± 3.41 3.16 74.86 19.94 42.86

Freq. Bandwidth 20.44 ± 2.28 1.86 59.63 15.15 23.95

Difference Duration 29.2 ± 3.53 4.30 89.11 17.7 34.69

Start–Middle Freq. Bandwidth 75.21 ± 8.36 16.51 185.97 44.18 101.25

Middle Duration 17.41 ± 2.06 0.57 43.20 8.95 23.38

Freq. Bandwidth 12.12 ± 1.45 1.37 38.09 7 14.95

Difference Duration 40.74 ± 3.58 8.76 80.42 27.98 55.16

Middle–Final Freq. Bandwidth 58.58 ± 7.76 7.64 128.69 30.33 99.74

Final Duration 35.95 ± 2.70 15.22 62.72 25.39 47.98

Freq. Bandwidth 17.5 ± 2.07 3.49 38.51 8.49 25.99

Table 2

Results of the generalized linear mixed-effects models of the relationship between treatments and duet coordination with

each behavioral response (i.e., latency time to approach at 3 m from the speaker, latency time of the first vocalization, time

inside 3 m radius from the speaker, and total number of vocalizations) using lme4 library (Bates et al., 2015).

Latency of first vocalization Estimate Std. Error z-value P-value

Intercept 3.938 0.562 7.01 <0.001

TreatmentControl 1.598 0.767 2.09 0.04

TreatmentCoordinated 0.678 0.77 0.88 0.38

Coefficient of Variation 0.026 0.015 1.68 0.09

TreatmentControl : Coefficient of Variation -0.022 0.021 -1.05 0.29

TreatmentCoordination : Coefficient of Variation -0.007 0.021 -0.35 0.72

Latency time to approach Estimate Std. Error z-value P-value

Intercept 4.097 0.495 8.28 <0.001

TreatmentControl 1.61 0.682 2.36 0.02

TreatmentCoordinated -0.127 0.724 -0.18 0.86

Coefficient of Variation 0.034 0.014 2.48 0.01

TreatmentControl : Coefficient of Variation -0.034 0.019 -1.8 0.07

TreatmentCoordination : Coefficient of Variation 0.006 0.02 0.3 0.76

Time inside Estimate Std. Error z-value P-value

Intercept 6.719 0.573 11.73 <0.001

TreatmentControl -4.423 0.79 -5.6 <0.001

TreatmentCoordinated -1.846 0.756 -2.44 0.01

Coefficient of Variation -0.082 0.016 -5.23 <0.001

TreatmentControl : Coefficient of Variation 0.082 0.022 3.83 <0.001

TreatmentCoordination : Coefficient of Variation 0.027 0.021 1.3 0.19

Number of Vocalizations Estimate Std. Error z-value P-value

Intercept 2.74 0.17 16.13 <0.001

TreatmentControl -0.383 0.247 -1.55 0.12

TreatmentCoordinated 0.125 0.254 0.49 0.62

Coefficient of Variation -0.004 0.005 -0.78 0.43

TreatmentControl : Coefficient of Variation 0.003 0.007 0.4 0.69

TreatmentCoordination : Coefficient of Variation -0.005 0.007 -0.77 0.44

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

We found that territorial pairs vocalize

faster (i.e., latency of the first vocalization;

GLMM: Χ2 = 7.99, df = 2, P = 0.02; Fig. 4; Table

2) and spent more time inside the 3 m radius of

the speaker (Χ2 = 31.43, df = 2, P < 0.001; Fig.

5; Table 2) when we played poorly coordinated

duet stimuli, compared to coordinated duet

stimuli, and control duet stimuli (Figs. 4 and 5).

The total number of vocalizations (Χ2 = 3.54,

df = 2, P = 0.17; Fig. 4; Table 2), and latency of

approach to the 3 m radius of the speaker (Χ2

= 4.42, df = 2, P = 0.11; Fig. 5; Table 2) did not

vary in response to the type of stimuli used.

We found that less well-coordinated pairs

showed longer approach latencies (GLMM:

Χ2 = 6.15, df = 1, P = 0.01; Fig. 5; Table 2) and

spent less time close to the speaker (Χ2 = 27.33,

df = 1, P < 0.001, Fig. 5; Table 2) than pairs with

Fig. 4. Responses of White-eared Ground-sparrow territorial pairs (mean + SE) to different playback types and duet

coordination levels: (A) and (B) show the time to the first vocalization and the total number of vocalizations produced in

response to playback types simulating territory intrusion. (C) and (D) show the relationship between duet coordination

(lower mean coefficient of variation indicates higher coordination) and the time to the first vocalization and the total number

of vocalizations. (E) and (F) show the interaction between playback types and duet coordination on the time to the first

vocalization and the total number of vocalizations.

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

high coordinated duets. However, duet coordi-

nation within pairs did not affect the latency

time of the first vocalization (Χ2 = 2.81, df = 1,

P = 0.09; Fig. 4; Table 2), and total number of

vocalizations (Χ2 = 0.61, df = 1, P = 0.43; Fig.

4; Table 2).

We found a significant interaction between

treatment and coefficient of variation values for

the time inside 3 m radius from the speaker,

where pairs with lower duet coordination spent

less time inside the 3 m radius from the speaker

when responding to poorly-coordinate duets

compared to the responses to coordinated duets

(Χ2 = 15.10, df = 2, P < 0.001; Fig. 5). Finally,

latency time to approach at 3 m from the speak-

er (Χ2 = 5.13, df = 2, P = 0.08; Fig. 5), latency

Fig. 5. Responses of White-eared Ground-sparrow territorial pairs (mean + SE) to different playback types and duet

coordination levels: (A) and (B) show the approaching time and time inside the stimuli area produced in response to playback

types simulating territory intrusion. (C) and (D) show the relationship between duet coordination (lower mean coefficient

of variation indicates higher coordination) and the approaching time and time inside the stimuli area. (E) and (F) show the

interaction between playback types and duet coordination on the approaching time and time inside the stimuli area.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73 (S2): e64525, mayo 2025 (Publicado May. 15, 2025)

time of the first vocalization (Χ2 = 1.17, df = 2,

P = 0.56; Fig. 4; Table 2) and total number of

vocalizations (Χ2 = 1.10, df = 2, P = 0.58; Fig. 4;

Table 2) did not show a significant relationship

between the interaction of treatment and aver-

age coefficient of variation values.

DISCUSSION

Duet coordination has been used as an

indicator of pair commitment for resource

defense (e.g., territory, food, nesting place,

or mate), pair bond duration, and the qual-

ity of the paired individuals (Dahlin & Bene-

dict, 2014; Hall & Magrath, 2007; Kovach et

al., 2014). In our case, White-eared Ground-

sparrow pairs with highly coordinated duets

responded more strongly towards simulated

intruders (approached faster and spent more

time closer), supporting the prediction that

highly coordinated pairs are more committed

to defend their territory together. Therefore,

duet coordination in White-eared Ground-

sparrows may act as a signal of pair stability

and quality (Brumm & Slater, 2007). Variation

in commitment may arise because duetting is

a learned behaviour (Hall, 2004; Hall, 2009),

and coordination needs time and attentive-

ness from both individuals in order to develop

(Levin et al., 1996; Rivera-Cáceres et al., 2016;

Trainer et al., 2002). Consequently, it is expect-

ed that White-eared Ground-sparrow pairs

with highly coordinated duets had formed

longer pair bonds and had occupied their ter-

ritories longer, increasing the probability to

successfully defend territories against intruders.

This pattern has been reported for Cane-brake

Wrens and Magpie-Larks (Grallina cyanoleuca)

where highly coordinated pairs have stronger

social bonds (e.g., less extra-pair copulations

and more time as a pair) and respond stronger

to intruders (Hall & Magrath, 2007; Rivera-

Cáceres et al., 2016). By comparison, California

Towhees (Melezone crissalis), a closely related

species to White-eared Ground-sparrows, do

not exhibit highly coordinated duets in pairs

with longer pair bonds (Benedict, 2010), sug-

gesting that pair coordination levels are not

equally important for all species even when

they are closely related.

Previous studies have reported that duet

coordination provides information to the

receiver about the signalers condition (e.g.,

time that pair has been together or pair iden-

tity) or threat during interactions (Kovach

et al., 2014; Méndez & Sandoval, 2021). This

probably occurs with the duet coordination

of White-eared Ground-sparrows, because we

found that stimuli coordination influenced the

approach response displayed by the receiver

pairs; territorial pairs of this ground-sparrow

species approached faster and spent more time

closer to the speaker than when they were

exposed to poorly coordinated duet stimuli.

Our results agree with our third prediction,

which proposed that duet coordination is a sig-

nal of the pair bond length and strength (Hall,

2004, Hall, 2009; Rivera-Cáceres et al., 2016).

Therefore, poorly coordinated duets may be

indicative of recently established pairs that

have not establish territory boundaries or pairs

without territories. As a result, territorial pairs

respond more aggressively to poorly coordi-

nated duets than to coordinated duets, because

newly formed pairs are more likely to invade

neighboring territories and usurp part of their

territory (Hamzaj et al., 2022).

We found that White-eared Ground-spar-

rows did vary in the vocal behavior (latency

time of the first vocalization and total number

of vocalizations produced) according to the

stimuli. This was not surprising, because it was

previously reported that this species used duets

as a primary territorial signal against intruders

(conspecifics and interspecific; Sandoval et al.,

2013). White-eared Ground-sparrows inhabit

very dense vegetation (Juárez et al., 2020; San-

doval & Mennill, 2012; Sandoval et al., 2016),

which reduces long distance visibility and pre-

vents visual identification of intruders before

responding to a vocal stimulus. Consequently,

if a territorial pair did not respond to intruders

vocally when the intrusion occurs, the prob-

ability to lose part of the territory or resources

may increase.

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73 (S1): e64525, mayo 2025 (Publicado May. 15, 2025)

In conclusion, duet coordination provides

information for territorial pairs to grade the

aggressiveness of the intruders and respond

accordingly as has been reported in Plain

Wrens, Rufous-and-white Wrens, and Magpie-

larks (Hall & Magrath, 2007; Kovach et al.,

2014). Our data suggest that poorly coordi-

nated duets are more threatening for territo-

rial White-eared Ground-sparrows and elicit

a stronger response from territorial pairs. Our

findings provide new insight on the relevance

that variation in duet coordination may have

during territorial interactions, because territo-

rial pairs respond more aggressively to poorly

coordinate duets than well-coordinated duets.

Ethical statement: the authors declare that

they all agree with this publication and made

significant contributions; that there is no con-

flict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

ACKNOWLEDGEMENT

We thank the Lankester Botanical Gar-

den and Universidad de Costa Rica to allow

us to work in the installations. We thank you

to Vicerrectoría de Investigación, Universidad

de Costa Rica for the funding support to the

investigation under the project numbers B9123,

C2705, and C3025 to LS.

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