Pieplow and Francis • SONG VARIATION OF YELLOW-EYED JUNCOS 
469 
FIG. 4. DFA results of song variation of Yellow-eyed 
Juncos from Arizona. USA (A), Baja California Sur, 
Mexico (B), and Oaxaca. Mexico (O). Song features 
included as variables in the DFA included all features 
(Table 1) except number of notes per phrase. Discriminant 
score I explains 97.5% of the variance in the data and 
discriminant score II explains 2.5%. Baja California songs 
are easily distinguishable based on discriminant score I. 
Arizona and Oaxaca songs are not easily distinguishable 
from each other by either score. 
mainland Yellow-eyed Junco subspecies ( J. p. 
palliatus and J. p. phaeonotus). The number of 
unique syllables per song and the number of 
repeated syllables are diagnostic because there is 
no overlap in these features between ./. p. bairdi 
and mainland subspecies. In contrast, there were 
fewer differences between songs of the two 
mainland subspecies and they were not easily 
distinguishable from one another through discrim¬ 
inant function analysis. 
Vocal features, like other phenotypic traits, arc 
expected to diverge among species and subspecies 
over time for a variety of reasons (Irwin 2000). 
One potential explanation for the difference in 
song is that signal features have diverged in 
response to unique habitat features (acoustic 
adaptation hypothesis', e.g.. Morton 1975, Bon- 
coraglio and Saino 2007). However, because both 
mainland and peninsular populations inhabit 
structurally similar pine-oak forests and arid 
woodlands (Sullivan 1999). it is unlikely that 
habitat features would differ sufficiently to 
account for significant differences in song fea¬ 
tures. 
A more probable explanation for the distinctly 
different J. p. bairdi song may be that sexual 
selection pressures have differed for J. p. bairdi 
relative to mainland populations (e.g., Irwin et al. 
2001). It is not clear whether the difference in 
TABLE 2. Percent variance explained and loadings of 
each song feature for discriminant axis I (DA I) and DA II. 
Variance explained/sung feature 
DAI 
DA II 
Percent variance explained 
97.50 
2.50 
Song length (sec) 
1.23“ 
-2.03“ 
Syllables per song 
-0.04 
-0.37 
Number of trills 
-0.90 
1.09“ 
Number of syllables per trill 
-0.71 
0.69 
Number of unique notes per trill 
-0.43 
0.04 
Number of unique syllables 
0.43 
0.47 
Repeats per syllable 
0.58 
-0.33 
Number of phrases 
1.48“ 
-0.39 
Peak frequency (Hz) 
0.00 
0.00 
Lowest frequency (Hz) 
0.00 
0.00 
Highest frequency (Hz) 
0.00 
0.00 
Song bandwidth (Hz) 
0.00 
0.00 
“ Denote song features with strong loadings for that discriminant axis. 
song would constitute a behavioral barrier to gene 
flow upon secondary contact among these cur¬ 
rently allopatric populations. Song can be used in 
combination with other tools to identify taxonom¬ 
ic boundaries (Irwin et al. 2001, Piickert et al. 
2004, Toews and Irwin 2008), and our results 
provide initial support for re-evaluating the 
taxonomic status of J. p. bairdi. However, several 
key pieces of information are still needed to 
clarify taxonomic limits. A phylogenetic analysis 
of the genus Junco by Mila et al. (2007) 
concluded the genus represents a case of extreme¬ 
ly rapid diversification following a postglacial 
range expansion. Unfortunately, individuals from 
Baja California Sur ( J . p. bairdi ) were not 
included in their analysis, and knowledge of this 
population's phylogenetic relationship with the 
rest of the genus is lacking. 
The largest differences in song were between J. 
p. bairdi and the two mainland subspecies, but our 
results also show some variation within the songs 
of mainland Yellow-eyed Juncos, Oaxaca birds 
sing slightly shorter songs with fewer repeated 
syllables and shorter trills (fewer syllables per 
trill) than birds from Arizona, Overall, there 
appears to be a gradual trend towards increased 
song complexity (e.g., an increased number of 
unique syllables and a decrease in the number of 
repeated syllables) with decreasing latitude 
among populations included in this study and 
throughout the ranges of all Junco laxa in western 
North America (NDP. pers. obs.). Most taxa 
currently included in the Dark-eyed Junco (./ 
hyemalis) complex sing simple songs consisting 
