Anim Cogn (2015) 18:1113–1123
DOI 10.1007/s10071-015-0883-z
ORIGINAL PAPER
Song plasticity over time and vocal learning in clay-colored
thrushes
Luis E. Vargas-Castro1,2 • Natalie V. Sánchez1 • Gilbert Barrantes1
Received: 24 September 2014 / Revised: 11 May 2015 / Accepted: 8 June 2015 / Published online: 27 June 2015
 Springer-Verlag Berlin Heidelberg 2015
Abstract Songbirds have been traditionally classified could be explained by open-ended learning, but another
into close-ended or open-ended learning species according mechanism, extended memory and re-expression, could
to the length of the sensitive period during which birds are also explain long-term plasticity. Experimental studies
able to memorize new vocalizations. Closed-ended learners controlling the acoustic environment are needed to deter-
are generally not capable of changing their song after the mine which mechanism is responsible for such a high level
first year of life, while open-ended learners show song of song plasticity.
plasticity as adults. A few Turdus species have been sug-
gested to be open-ended learners, but no long-term study Keywords Open-ended learning  Syllable repertoire 
has been conducted to investigate their song plasticity over Behavioral plasticity  Repertoire size 
time. We analyzed the songs of clay-colored thrushes, T. Song re-expression  Turdus
grayi, over four successive breeding seasons to assess song
plasticity in their syllable repertoires within and between
breeding seasons. A total of 16,262 syllables were classi- Introduction
fied through visual inspection of spectrograms and multi-
dimensional scaling analysis based on spectrogram The capacity to learn songs or other vocalizations from
correlations. On average, 563 ± 153 (SD) syllables per social interactions has been observed in three orders of
male per breeding season were analyzed. Male repertoire birds: Psittaciformes (parrots), Apodiformes (humming-
size was 9–20 syllable types. Males were capable of birds), and Passeriformes (perching birds) (Baptista and
modifying their syllable repertoire between the initial and Schuchmann 1990; Gaunt et al. 1994; Farabaugh and
final periods of the breeding season. Song plasticity within Dooling 1996; Araya-Salas and Wright 2013). The order
breeding seasons may be associated with imitation between Passeriformes contains the suboscines and oscines. In
neighboring males, suggesting song learning in males that general, the former do not show vocal learning, although
were C2 years old. This short-term plasticity is not enough, exceptions such as the three-wattled bird Procnias tri-
however, to explain the high proportion of change carunculata have been suggested (Saranathan et al. 2007).
(mean = 65 % syllable types) in repertoire composition The oscines, or songbirds, learn their songs through imi-
between breeding seasons in adult males. Song plasticity tation of adult conspecifics (Marler 1970). Usually, only
resulting from annual changes in repertoire composition males sing, with the purpose of attracting mates and
defending their territories during the breeding season
(Krebs et al. 1978; McGregor and Krebs 1982; Catchpole
& Luis E. Vargas-Castro and Slater 2008).
luis@bio.miami.edu
The song-learning process in oscines involves two main
1 Escuela de Biologı́a, Universidad de Costa Rica, phases: memorization and rehearsal (Kroodsma 1982;
2060 San Pedro, San José, Costa Rica Brenowitz and Beecher 2005; Sung and Park 2005; Brai-
2 Present Address: Department of Biology, University of nard and Doupe 2013). The memorization phase constitutes
Miami, Coral Gables, FL 33146, USA a sensitive period for learning conspecific songs (Marler
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and Peters 1982; Beecher and Burt 2004), while during the these individuals did not begin to sing until theywere sexually
rehearsal phase, birds adjust their singing output through mature, he showed that American robins are capable of
auditory feedback in order to match the songs that were learning song components during the first breeding season. In
previously memorized (Brenowitz and Beecher 2005; Sung the field, Johnson (2006) also observed that two males grad-
and Park 2005; Brainard and Doupe 2013). Interspecific ually transformed the acoustic structure of one element of
variation in the length of these phases during the song- their repertoires during a breeding season. In one of these
learning process has been used to classify songbirds into cases, this modification made the element more similar to one
closed-ended and open-ended learning species. Songs are found in the repertoires of three other males from the same
highly variable in acoustic structure during the rehearsal population. Repertoire changes during the first breeding
phase, but this variability decreases over time until the bird season have been reported in other songbirds, usually leading
only sings stereotyped songs, a process termed crystal- to a higher proportion of repertoire sharing with territorial
lization (Marler 1970). Crystallization usually occurs dur- neighbors (DeWolfe et al. 1989; Payne and Payne 1997;
ing the first year of life, and closed learning species are not Nordby et al. 2001; Kipper and Kiefer 2010). These changes
able to learn new songs after this point (Marler and Peters typically result from dropping or adding repertoire compo-
1982; Catchpole and Slater 2008). For this reason, closed- nents, but gradual transformation of song components is
ended learners are also called age-limited learners. In seldom reported, perhaps because it is rare in nature or dif-
contrast, song learning continues after singing crystallized ficult to detect. Johnson (2006) was not able to distinguish
song in open-ended learners (Brenowitz and Beecher whether or not this capacity was limited to the first breeding
2005). Some oscines have been considered open-ended season inwildAmerican robins, since their agewas unknown.
learners because their songs change over the years (Esp- These studies suggest there could be song plasticity and
mark and Lampe 1993; Mountjoy and Lemon 1995; Not- open-ended learning in Turdus, but we lack long-term
tebohm 2004), and adult song plasticity is compatible with investigations following specific individuals between
open-ended learning. Nonetheless, this classification rep- breeding seasons to determine to what extent this capacity
resents two extremes of song-learning strategies, and it has is limited by age. In this study, we analyzed the song
been suggested that the difference between closed-ended plasticity of clay-colored thrushes T. grayi over time. The
and open-ended learning species can be seen as a quanti- objectives were to determine if repertoire size and com-
tative difference in the amount of song plasticity rather position change: (a) between successive breeding seasons
than a qualitative difference regarding the presence or in 1–4 year old males and (b) within a breeding season in
absence of this plasticity (Brenowitz and Beecher 2005). 2-year-old males. We also discuss the implications of song
Repertoires, which are the set of different song types or plasticity in relation to vocal learning focusing on the
syllable types that a bird sings, are influenced by social songbirds (suborder Passeri).
interactions since different repertoire components may be
originated, dispersed, extinguished, or modified within Clay-colored thrushes and their songs
populations in a very dynamic way over time (Lynch 1996;
Payne 1996). Thus, males of some species change their The clay-colored thrush is an open-habitat songbird,
repertoire composition within a breeding season (Espmark common in gardens, pastures, and urban areas, distributed
and Lampe 1993; Nicholson et al. 2007) or between from southern Texas, USA, to northern Colombia (Stiles
breeding seasons (Sorjonen 1987; Leitner et al. 2001), in and Skutch 1989; Clement 2001). Clay-colored thrushes,
some cases, adding new song components over time like other member of the Turdidae family, are well known
(Espmark and Lampe 1993). for their melodious and skillful vocalizations. During the
Common blackbirds Turdus merula and American robins breeding season, males sing frequently throughout the day
T. migratorius change their song repertoire as adults, at least to defend a small territory where females build a nest, in
during their first breeding season (Rassmussen and Dabel- trees or on human-made structures.
steen 2002; Johnson 2006). Rassmussen and Dabelsteen The songs of clay-colored thrushes are sequences com-
(2002) observed changes in the slopes of cumulative curves of posed of from one up to more than 100 syllables (Fig. 1),
new motif types in common blackbirds and suggested that and successive songs have different sets of syllable types.
new types were learnt during the study. However, these Males may share a small proportion of their syllable
changes occurred within a short sampling period (1–9 days) repertoires with other males of the same locality (Vargas-
and could have been the result of natural patterns of how Castro et al. 2012), but whole song sharing has not been
different motif types are used (Rassmussen and Dabelsteen observed; thus, syllables appear to be the unit of imitation
2002). Johnson (2006) conducted hand-rearing experiments during vocal learning. Therefore, it is more appropriate to
with American robins and observed that some males imitated classify syllable types instead of whole songs in order to
song elements of other males from the experiment. Since measure syllable repertoire size. This approach has been
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Anim Cogn (2015) 18:1113–1123 1115
Fig. 1 Spectrograms of the
songs of two male clay-colored
thrushes during two successive
breeding seasons (2009–2010).
The upper two spectrograms
correspond to one individual,
while the lower two correspond
to another male. These songs
are composed of 15, 11, 11, and
8 syllables, respectively.
Syllables labeled with the same
letter (within males) represent
syllable types that were present
in the repertoires of both years
(color figure online)
used in other species with a similar singing style (Gil and Materials and methods
Slater 2000). Hereafter, repertoire refers to the set of dif-
ferent syllable types that constitute the songs of a given The study was conducted at the main campus of the
male in a given period of time. University of Costa Rica (UCR), in San Pedro de Montes
Two syllable categories are found in the songs of Turdus de Oca district, San Jose Province, Costa Rica
thrushes: broadcast syllables (whistles) and soft syllables (095601100N, 840205700W; 1200 m asl). The campus is
(Grabowski 1979; Rassmussen and Dabelsteen 2002; composed of various buildings located among gardens,
Johnson 2006; Vargas-Castro 2015). In general, broadcast open fields, scattered vegetation, and two small patches of
syllables have higher amplitude and simple acoustic struc- secondary forest. The clay-colored thrush breeding season
ture compared with soft syllables. During spontaneous song extends from March to June at this site. We conducted the
bouts, clay-colored thrushes predominantly use broadcast study during four breeding seasons: 2008–2011. We ana-
syllables (97 % of total syllables) and only a few soft syl- lyzed data of eight color banded males. Individuals were
lables may be embedded in these songs (Vargas-Castro banded by LEV or NVS specifically for this investigation;
et al. 2012). In other songbird species, soft songs are mainly more than 70 individuals (both sexes) were banded at the
used in courtship displays or aggressive signaling, or both, beginning of the study, but most of them were not seen
during close range interactions (Dabelsteen et al. 1998; again in the area. As the study site contains one small
Searcy and Beecher 2009). Clay-colored thrushes also creek, it is possible that multiple nonresident individuals
produce soft song in courtship displays and even during were captured and marked as they were just passing by to
copulation (LEV personal observation), but their role in visit this creek. Male territories were separated by variable
aggressive interactions remains to be investigated. Since distances, from 30 to 500 m, and thus, not all males were
broadcast syllables are more frequent in spontaneous songs, immediate neighbors.
have higher amplitude and a relatively simple acoustic Since male age was unknown, we assigned a minimum
structure, facilitating both their sampling and classification, age of 1 year to the adult males captured during 2008. This
we only analyzed broadcast syllables in the present study. minimum age method has been used in other longitudinal
Henceforth, ‘syllables’ refers to broadcast syllables. field studies (Gil et al. 2001; Nicholson et al. 2007). We
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analyzed the songs of three males in four successive years,
two males in three successive years, and two other males
during two consecutive years. Fluctuation in these numbers
is presumably due to predation. One additional male was
recorded only during 2009 and was used in the analysis of
song plasticity within the breeding season (see below). This
research complies with the current laws of Costa Rica.
Song plasticity between successive breeding seasons
We recorded C360 song syllables from each male between Fig. 2 Examples of cumulative curves of repertoire size according to
March 21 and May 11 in each year. This syllable sample the number of syllables recorded in the songs of clay-colored thrushes
size is comparable with approximately 16 min of effective during the initial period of the breeding season in 2009. In these four
males, syllable additions were observed during the final period of the
recording time. The recordings from 2008 used here have
same breeding season (see ‘‘Results’’)
also been included in a previous study investigating local
repertoire sharing among males (Vargas-Castro et al.
2012). On average, repertoires are composed of 13 (range plasticity analysis within the 2009 season included 3318
10–17) syllable types, and even though most syllable types additional syllables that were recorded during the final
are unique to particular males, 28 ± 15 % (mean ± SD) period of the breeding season, averaging 553 ± 122 (SD)
syllable types of their repertoires may be shared with other syllables per male. All songs analyzed correspond to
males. spontaneous singing activity. Songs were recorded mainly
Clay-colored thrushes sing with immediate variety between 0400 and 0600 hours, but we also used recordings
(successive syllables are different) (Fig. 1). Cumulative from other time periods. There were no relevant daily
curves of new syllable types showed that full syllable differences in repertoire composition.
repertoires are detected after classifying 350 syllables In 2008, we used a Sony M-635VK tape recorder and
(Vargas-Castro et al. 2012). For one male in 2008, we only later digitized the recordings using Adobe Audition 1.0
recorded 272 syllables, but since the cumulative curve (Adobe Systems, Inc., San Jose, CA, USA) to produce
quickly approached an asymptote for this male, we were uncompressed audio files. From 2009 to 2011, we used
confident that we detected his full repertoire. Olympus LS-10 or Marantz PMD661 digital recorders. In
all years, we used a Sennheiser ME66/K6 shotgun micro-
Song plasticity within a breeding season phone to record the songs. All recordings were stored as
wav files (44.1 kHz sample rate, 16 bit resolution).
In 2009, the songs of six adult males (C2 years old) were
studied during a longer time period to examine if there was Syllable classification and analysis
repertoire plasticity throughout the breeding season. For
this analysis, we used the first recordings available, starting We analyzed spectrograms of the recordings using Raven
on March 21, to classify[400 syllables and assess reper- Pro 1.4 (Cornell Lab of Ornithology, Ithaca, NY, USA).
toire composition for each male (Fig. 2). Then, we also We generated spectrograms with Hann window type, 700
analyzed songs recorded after April 26 to classify [400 samples and a discrete Fourier transform (DFT) size of
additional syllables and re-assess repertoire composition. 2048 samples with a temporal overlap of 75 %. A band-
Thus, the time interval of the recordings used during this limited energy detector (Charif et al. 2010) was used to
breeding season extended from March 21 to May 31 and automatically detect clay-colored thrush song in the
was divided into two periods of approximately 5 weeks recordings.
each. We examined repertoire composition in both periods Spectrograms were visually inspected to identify the
to register possible modifications such as addition or syllable types of each male’s repertoire. Each syllable type
deletion of syllable types during the breeding season. is observed as a figure with a specific pattern in the fre-
quency-time domains of the spectrogram (Fig. 1). A sim-
Song recordings ilar procedure has been used in other studies to classify bird
vocalizations (Molles and Vehrencamp 1999; Rassmussen
We analyzed a total of 16,263 syllables, 12,945 of which and Dabelsteen 2002; Nicholson et al. 2007; Vargas-Castro
were used in the analysis of song plasticity between et al. 2012). All syllables were inspected by two indepen-
breeding seasons. On average, 563 ± 153 (SD) syllables dent observers to determine which syllables were present in
per male per breeding season were classified. The song more than one breeding season. Inter-observer agreement
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was almost perfect, reaching 99.7 % of all possible com- used to obtain a balanced model. In addition, we conducted
parisons (20/6091). The few cases of disagreement were a Pearson correlation test between the number of breeding
reevaluated until a general consensus was reached. The seasons that males were sampled and their accumulated
validity of our visual classification was re-examined using repertoire size to determine if there was an association
multidimensional scaling analysis (MDS) (see below) between the two variables. All tests were two-tailed and
(Nowicki and Nelson 1990). conducted using R version 2.14.0 (R Development Core
Song plasticity between breeding seasons was measured Team 2012).
as the proportion of change in the repertoire of each male
between years using the Sørensen–Dice index: 1 - (2Ns/
(R1 ? R2)); where Ns is the number of syllable types pre- Results
sent in the repertoires of two successive breeding seasons,
while R1 and R2 constitute repertoire size in each of these Figure 3 shows broad agreement between the visual classi-
breeding seasons. The negative part of this formula has fication of syllable types and the grouping of syllables
been used to calculate the proportion of shared song according to the MDS. For example, syllables visually clas-
components between two males (McGregor and Krebs sified as type ‘o’ due to their similar acoustic structure were
1982; Kipper et al. 2004; Eriksen et al. 2011). This index grouped together in the same area of the bi-dimensional
gives values between 0 and 1, with 0 indicating identical geometric space produced by the MDS analysis (see Fig. 3,
repertoires and 1 indicating full replacement of the syllable upper diagram bottom center). Syllable examples from dif-
repertoire. ferent breeding seasons that were visually classified as the
same syllable type were also grouped together in the MDS
Spectrogram correlation and multidimensional analysis diagrams (see syllables ‘e’ from 2010 to 2011, top
scaling analysis (MDS) right corner of lower diagram in Fig. 3). In a few cases,
however, examples of syllables visually classified as different
We selected three examples of each syllable type, with types were not completely separated in the diagram. In these
high recording quality, from the repertoires of each male cases, most of the energy of the syllables was concentrated in
and calculated spectrogram correlation values between all the same frequency band, which resulted in high spectrogram
the sound files using Raven Pro 1.4. Spectrogram correla- correlation values although their frequency-temporal pattern
tions measure the maximum pixel by pixel similarity was clearly different. For example, see syllable p grouping
between two spectrograms (Cramer 2013). We used spec- with syllable j in Fig. 3 (upper diagram, top center).
trograms with Hann window type, 700 samples, and a DFT
size of 1024 samples with a temporal overlap of 50 %. The Song plasticity between successive breeding seasons
‘normalize’ and ‘linear power’ options were selected, and a
1000–4500 Hz band-pass filter was applied to eliminate In four breeding seasons, 12,945 syllables were classified
background noise interference outside of the songs’ fre- into 219 syllable types. Average male repertoire size was
quency range. The correlation square matrix, of dimensions 13 (range 9–20) syllable types. Although there were small
equivalent to the number of sound files, was imported into variations in male repertoire size as age increased, reper-
R version 2.14.0 (R Development Core Team 2012). The toire size was similar over the years (Friedman test,
value of each cell was subtracted from 1 to convert it into a X2(2) = 0.88, P = 0.64) (see Fig. 4). However, repertoire
dissimilarity matrix. A nonmetric MDS was conducted composition did change with an average of 65 % (range
using the MASS package in R, solving for two dimensions. 41–95 %, N = 7 males) of the syllable repertoire changing
This technique allowed us to evaluate the grouping or between 1 year and the next. Figure 5 shows that 73 % of
dispersion of the syllables in a bi-dimensional geometric the syllables composing the repertoires changed, on aver-
space according to their acoustic structure differences. The age, between the first and second year. Then, an average of
procedure was conducted independently for each male in 62 % of the repertoires changed between the second and
each year, and to reevaluate the few cases where there was third year. Finally, between years three and four, 49 % of
disagreement between observers while classifying syllable the syllable repertories changed, showing that even males
types. that were C4 years old showed considerable inter-annual
changes in repertoire composition. However, this pattern of
Statistical analysis decreasing proportion in repertoire change between suc-
cessive breeding seasons was not statistically significant
We analyzed repertoire size variation within males (Kruskal–Wallis test, X2(2) = 1.26, P = 0.53).
according to age using a Friedman test (Zar 1996). Data on These annual changes resulted from the inclusion of new
five males during three breeding seasons (2008–2010) were syllable types that replaced other ones. By adding up each
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Fig. 3 MDS analysis diagrams (left) of the combined repertoires of syllable types that were present in both years. One example of each
one male in two successive breeding seasons. Each letter denotes a syllable type is shown in the spectrograms (right) of the repertoires of
syllable type in 2010 (black) and 2011 (red). The upper diagram each year (color figure online)
shows full repertoires, while the lower diagram only includes the five
Fig. 5 Proportion of change in the syllable repertoire composition of
Fig. 4 Syllable repertoire size according to minimum age in the
clay-colored thrushes according to change in minimum age during the
songs of clay-colored thrushes during the 2008–2011 breeding
2008–2011 breeding seasons. Means ? SD are shown
seasons. Each line corresponds to one male
Few syllable types remained in the repertoires during
new syllable type from all the breeding seasons each male three or four consecutive breeding seasons. On average, the
was sampled, we were able to detect an accumulated syllables that remained for three seasons, or four seasons,
repertoire through successive breeding seasons. Figure 6 constituted 10 % (range 0–23 %, n = 5) and 5 % (range
shows that the more breeding seasons a male was sampled, 0–10 %, n = 3) of the accumulated repertoires, respec-
the larger its accumulated repertoire size was (r = 0.74, tively. Also, two syllable types of one male were used
N = 8, P = 0.036). intermittently; after being present in the repertoire of a
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Fig. 7 New syllable added to the repertoire of one male (C2 years
old) that may have resulted from the gradual transformation of one
syllable type into another. The variation in acoustic structure occurred
Fig. 6 Mean accumulated repertoire size of syllable types in male between the initial (left) and final (right) periods of the breeding
clay-colored thrushes over four breeding seasons (2008–2011) season
given year, they were absent the following year and reap- breeding season. This accumulated (or lifetime) repertoire
peared again in the repertoire 2 or 3 years later. could result from two different mechanisms: (a) extended
memory and re-expression or (b) open-ended learning
Song plasticity within a breeding season (Nottebohm 2004; Hough et al. 2000; Franco and Slabbe-
koorn 2009).
We observed modifications in the repertoire composition of In the first case, extended memory and re-expression, an
four males in which 1–4 syllable types were added between individual memorizes more song components than those
the initial and final periods of the breeding season. The that are used during the first breeding season. The sensitive
proportion of use of these additions was relatively low; learning phase in which songs are memorized is variable
each added syllable represented less than 2 % of the total within songbird species, but in a number of species is
syllables used during the final period by a given male. One limited to the first year of life (Brenowitz and Beecher
exception was a syllable addition that reached 8 % of the 2005). Song components that were learnt during the sen-
syllables sung in the final period. For comparison, the mean sitive phase but that are not part of the first breeding season
proportion of use of syllable types also was 8 % of the total repertoire may be stored in the memory and re-expressed in
syllables used by one male in a given period. subsequent breeding seasons (Hough et al. 2000). Playback
Moreover, it is possible that one of these additions experiments have shown that it is possible to reactivate
resulted from the gradual modification of one syllable type song components that are not spontaneously sung but still
into another (see Fig. 7). As shown in Fig. 8 for this case, remain in the memory (common starlings Sturnus vulgaris
the new version of the syllable sung by one male in 2009 in Chaiken et al. 1994; common nightingales Luscinia
showed high acoustic similarity to syllables present in the megarhynchos in Geberzahn et al. 2002; great tits Parus
repertoires of other three males during the same or previous major in Franco and Slabbekoorn 2009). It is possible that
breeding seasons. In 2010, the same male used a similar clay-colored thrushes learn a number of syllables during
syllable again, but we are not certain if the 2010 version the memorization phase and later sing only a subset of the
originated from the one added in 2009, although it seems total repertoire that was learnt. Thus, the extended memory
possible. and re-expression mechanism may explain the song plas-
ticity observed between breeding seasons and implies that
the size of their lifetime repertoire (accumulated) is pre-
Discussion determined during the memorization phase, and no subse-
quent learning is required.
Song plasticity between successive breeding seasons Using a subset of syllables from a larger repertoire is
comparable with the selective attrition process described in
Although repertoire size in adult males did not change over white-crowned sparrows Zonotrichia leucophrys (DeWolfe
the breeding seasons, substantial plasticity in repertoire et al. 1989) and other sparrows (swamp sparrows Melos-
composition was observed in clay-colored thrush males piza georgiana in Marler and Peters 1981; field sparrows
that were C4 years old. The use and replacement of syl- Spizella pusilla in Nelson 1992). White-crowned male
lable types were very dynamic between breeding seasons. sparrows sing the same single song type every year, but at
Inter-annual changes in repertoire composition showed that the beginning of the breeding season, up to 4 years old,
over the course of their lifetime, each male will express temporarily re-express song types that had been dropped
more syllable types than those it uses during a single during the rehearsal phase of the first year (Hough et al.
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Fig. 8 Versions of a similar
syllable type in four clay-
colored thrushes. Males rar and
pan used this syllable in their
repertoires in 2008. Male eco
used a similar syllable in 2009.
Male bbo acquired a similar
syllable during the final period
of the 2009 breeding season,
possibly by gradually
transforming the pattern of
another syllable type (see
Fig. 7). One syllable used by the
later male in 2010 seems to be
even more similar to the
versions of the other males of
the local population
2000). Clay-colored thrush males may be re-expressing 2011), and sedge warblers Acrocephalus schoenobaenus
syllable types that were previously memorized, but in (Nicholson et al. 2007) have been considered open-ended
contrast with white-crowned sparrows, syllable re-expres- learners mainly because changes in their repertoire com-
sion could be associated with the capacity to change the position have been observed between breeding seasons.
repertoire composition in successive breeding seasons. However, it is necessary to control the acoustic environ-
If there is an annual syllable selection from a predeter- ment since birth to determine if the observed song plas-
mined repertoire, is it random? Common nightingales ticity is due to open-ended learning in adults instead of the
modify the repertoire composition between their first and extended memory and re-expression mechanism mentioned
second year to match those songs that are shared by most above. The present study places clay-colored thrushes in
individuals within the population (Kiefer et al. 2009; the group of songbird species with song plasticity in adult
Kipper and Kiefer 2010). A similar process could take repertoire composition. The observed song plasticity in
place in clay-colored thrushes, but more studies are needed individuals up to 4 years old or more suggests that this
to determine if syllable selection is arbitrary or not. We species possesses open-ended learning, but re-expression of
also observed that some syllables remained in the reper- syllables learnt during a previous age-limited sensitive
toires during four successive breeding seasons. At this phase remains as a possibility.
point, it is unknown if the permanence of these syllables in Accumulation of new syllable types over time opens the
the repertoires was favored by certain acoustic structural possibility of having a larger repertoire size in upcoming
traits or social interactions. breeding seasons. Males with larger repertoires may be
On the other hand, open-ended learning is another preferred by females (Searcy 1992; Lampe and Saetre
mechanism that could explain song plasticity over the years 1995; Buchanan and Catchpole 1997), whether directly or
in the clay-colored thrush. This mechanism requires that by association with other traits such as age or territory
the memorization phase remains active during adulthood quality (Howard 1974; Searcy and Andersson 1986). Since
after singing crystallized song (Brenowitz and Beecher repertoire size did not increase as age increased, we sug-
2005). Chaiken et al. (1994) determined that common gest that the main function of repertoire size is not female
starlings have open-ended learning by showing that males attraction in this species. Also, it appears that neural
of this species are capable of learning new song compo- mechanisms or traits, such as the volume of the high vocal
nents up to 20 months old. Other species such as canaries center (HVC) in the brain (Devoogd et al. 1993; Pfaff et al.
Serinus canarius (Nottebohm et al. 1986), red-winged 2007), may be limiting repertoire size during each breeding
blackbirds Agelaius phoeniceus (Marler et al. 1972), season. However, Gil and Gahr (2002) reported that there
common nightingales (Todt and Geberzahn 2003), Euro- is no convincing evidence about neuronal costs constrain-
pean pied flycatchers Ficedula hypoleuca (Eriksen et al. ing repertoire size.
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Song plasticity within a breeding season Ethical standard All applicable international, national, and/or
institutional guidelines for the care and use of animals were followed.
We also observed song plasticity during the breeding Conflict of interest The authors declare that they have no conflict
season. Adult males that were C2 years old had the of interest.
capacity to modify their repertoire composition over the
course of the breeding season. This short-term plasticity
consisted of the addition of one or a few syllable types References
that were used with low frequency during the final period.
It is possible that the low proportion of use of these syl- Araya-Salas M, Wright T (2013) Open ended song learning in a
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