Vargas-Castro et al • SONG OF THE CLAY-COLORED THRUSH 
451 
TABLE 2. Mean spectrogram correlations lor shared 
syllable types and one set of random syllables from the 
songs of eight male Turdus grayi by male pair. Each value 
represents die average of 100 spectrogram correlations, 
except for male pairs A-F and D-H, which are based on 1(1 
and 70 correlations, respectively. 
Shared type 
Male pair 
First 
Second 
Third 
Random 
A-D 
0.75 
0.70 
— 
0.33 
A-E 
0.66 
— 
— 
0.18 
A-F 
0.68 
— 
— 
0.34 
A-H 
0.70 
— 
— 
0.29 
C-D 
0.86 
— 
— 
0.31 
C-E 
0.64 
— 
— 
0.10 
C-F 
0.64 
0.76 
— 
0.27 
C-G 
0.60 
— 
— 
0.27 
C-H 
0.62 
0.55 
0.65 
0.35 
D-E 
0.69 
— 
— 
0.21 
D-F 
0.55 
— 
— 
0.30 
D-H 
0.63 
— 
— 
0.29 
E-F 
0.73 
— 
— 
0.16 
(Rufous-backed Thrush [7'. rufopalliatus] in Gra- 
bowski 1979, American Robin |7'. migratorm\ 
in Johnson 2006. Japanese Thrush IT", cardial in 
Tooru 2006). Common Blackbirds (T. me rid a) in 
contrast have a larger repertoire of motifs 
(Rasmussen and Dabelstcen 2002) than the syllable 
repertoires of Clay-colored Thrushes, bid this 
species has a different song structure. 
The repertoire of the Clay-colored Thrush is 
composed of a set of individually unique syllabic 
types and a smaller number of syllable types lhai 
arc shared with neighbors. A larger fraction of 
individual syllables comprising the repertoires 
probably facilitates individual recognition (Falls 
1982, Beecher et al. 1994), although unique 
re 
>» 
</) 
C 
c 
o 
t: 
& 
o 
CL 
FIG. 4. Proportion of individual (white) and shared 
(gray) syllables sung by each male Clay-colored Thrush 
during 2008 in San Jose, Costa Rica. Total number of 
recorded syllables is at the base of each bar. 
syllables or song types may not be necessary to 
accomplish individual recognition (Weary and 
Krebs 1992). Individual syllables could be 
invented, improvised or learned elsewhere (Mar- 
ler and Peters 1982, Payne 1996, Kroodsma et al. 
1999). Johnson (2006) observed that most of the 
elements (75-82%) in the repertoires of hand- 
reared American Robins were acquired by inven¬ 
tion. The high and similar proportion of individual 
syllables that we found in the repertoires of Clay- 
colored Tlmtshes suggests that at least part of 
these syllables are invented as well. 
Shared syllables in mrn reflect social learning 
among neighboring males (Payne 1981. Baptista 
and Peirinovich 1984. Beecher and Burt 2004). 
Only single syllables were shared among males, 
suggesting that syllables arc the unit of imitation. 
Shared song components are maintained through 
time in a local population if vocal learning occurs 
early in life (e.g.. as nestlings) followed by short 
natal dispersal or if song learning occurs after 
young males disperse and interact with local 
neighbors in a new area (Kroodsma 1974, Lynch 
1996, Payne 1996), Generally, the extent of re¬ 
pertoire sharing decreases with increasing dis¬ 
tance between territorial males (Bertram 1970. 
McGregor and Krebs 1982). However, we did not 
find a relationship between repertoire sharing and 
song perch distance. Such a relationship would 
not be expected if Clay-colored Thrushes disperse 
relatively long distances before setting up their 
First territory. Unfortunately, natal dispersal dis¬ 
tance of Clay-colored Thrushes is only known 
for one male that we found defending a territory 
about 130 m distant from where it was banded 
as a nestling. Alternatively, Clay-colored Thrush¬ 
es could follow a different pattern of repertoire 
sharing with increasing distance, similar to that of 
Common Blackbirds and Common Nightingales 
(Luscinia megarhynchos). which are more likely 
to share components of their repertoires with other 
males at intermediate distances (>100 m) rather 
than with closer neighbors (Hultsch and Todt 
1981). 
Local repertoire sharing has an important role 
in territory possession (Beecher et al. 2000) and 
song discrimination by females (Searcy 1990. 
Searcy et al. 2002). The relative importance of 
song sharing in Clay-colored Thrushes for differ¬ 
ent social functions needs further investigation. 
Future research on population dynamics, espe¬ 
cially natal dispersal, as well as on the critical 
period for song learning is required to better 
