Croston el al. • FLANGE COLOR DIFFERENCES OF COWBIRDS 
143 
the host-specific begging call matching by cuckoos 
of Australian songbirds (Langmore et al. 2008). 
The difference in chromatic reflectance of 
flange colors (PC U) across cowbird nestlings 
from the two hosts may also be a developmental 
result of carotenoid concentration based on the 
diets of our two host species. Carotenoids in birds 
are derived entirely from diet, and are known to 
modulate variation in nestling mouth color by 
blocking the reflectance of short-wavelength 
Iblue/grecn) light and cause the mouthparls to 
appear orange, yellow or reddish (Thorogood 
et al. 2008). Thus, nestlings with carotenoid-rich 
mouthpares are favored in species where parents 
exhibit a feeding preference based on nestling 
mouth coloration (Dugas 2009). Nestling flanges 
are significantly brighter, more UV reflective, and 
more chroma rich in the part of the flange visible 
during begging than hidden pails (the sides), 
lending support for the signaling hypothesis in 
the function of flange coloration (Dugas 2010). 
However, measurements in our study were made 
on the sides of the flange, and would he less visible 
during begging. Differences in the coloration of 
these two regions may be the combined result of 
signaling during begging and decreasing conspic- 
uousness when not, resulting in decreased UV 
reflectance on sides compared to insides (Dugas 
-010). The inside and side flange colors were 
weakly correlated (Dugas 2010), but we might 
expect to find even better color matching based on 
the inner part of the flanges. Future studies would 
benefit from comparing these two regions using 
1 V -sensitive spectrophotometric measures as op¬ 
posed to the human-visible only color measure- 
ments available to us from digital photographs. 
We did not measure ambient light at the nest. 
Thus, our results of species-specific differences in 
'he color variables contributing to PC I and host- 
specific differences in the relative amount ot blue 
reflectance of cowbird nestlings (PC ID may be an 
adaptive result of nestlings optimizing their own 
detection in the specific ambient light environ¬ 
ments of the different host species’ nests (Ficken 
1965). Nestlings in dark nests increase conspicu- 
ousijcss through the relative color and size ol the 
n «nge (Kilner and Davies 1998), and open- 
nesting species show higher achromatic contrast 
w 'th the nest than cavity-nesting species (Aviles 
et al. 2008). Both Yellow Warblers and Song 
Sparrows nest in open cups, albeit typically at 
different heights and in different plant substrates 
•Tonra et al. 2009), and differences in nestling 
coloration based on detection under ambient light 
conditions are an unlikely causal explanation for 
the pattern of differences in cowbird flange color 
diversity. 
Our results support predictions of the host- 
parasite flange color matching hypothesis for 
Brown-headed Cowbirds and their hosts. Further 
experimentation is necessary to examine the role 
of flange color in the host-brood parasite revo¬ 
lutionary amis race, including nestling mimicry 
and discrimination (Fraga 1998. Lichtenstein 
2001). These studies have the potential to shed 
light on the evolution of host use strategies in 
cowbirds, revealing the extent to which special¬ 
ization may exist in individual cowbird females. 
This information would have important and 
broadly applicable implications for both coevolu¬ 
tionary theory and conservation. 
ACKNOWLEDGMENTS 
We are grateful to the University of California. Berkeley. 
Miller Institute for Basie Research in Science. Marin Rod 
and Gun Club Scholarship for Graduate Research in 
Wildlife. Humboldt Stale University Foundation. Mono 
Basin Bird Chautauqua Bird Research Grant. Human 
Frontier Science Program, and the PSC-CUNY Faculty 
Grant program for funding. M. D. Johnson. M. A. Colwell, 
and T. L. George provided support towards the completion 
of this study. We thank the Mono Lake Committee for 
providing logistical support in the field. This is PRBO 
Contribution # 1841. 
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