Condition Dependence and Status Signaling Function of Structural Coloration in 
the Eastern Bluebird (Sialia sialis) 
 
by 
 
Austin Nicholas Mercadante 
 
 
 
 
A thesis submitted to the Graduate Faculty of  
Auburn University  
in partial fulfillment of the  
requirements for the Degree of  
Master of Science 
 
Auburn, Alabama 
13 December, 2010 
 
 
 
 
Keywords: Eastern Bluebird, Sialia sialis, sexual selection, condition dependence,  
status signal, structural coloration 
 
 
Copyright 2010 by Austin Nicholas Mercadante 
 
 
Approved by 
 
Geoffrey Hill, Chair, Scharnagel Professor of Biological Sciences 
Debbie Folkerts, Assistant Professor of Biological Sciences 
Robert Lishak, Associate Professor of Biological Sciences 
 
ii 
 
 
Abstract 
 
 
I examined the relationship between blue feather coloration, nutritional condition, 
and status signaling in eastern bluebirds (Sialia sialis). I quantified nutritional condition 
by measuring growth rates of tail feathers for an entire bluebird population to compare 
different age classes for both males and females. I found that males had wider growth 
bars than females and that older breeders had wider growth bars than younger breeders. 
Wider growth bars were correlated with longer tail feathers. Growth bar width was 
correlated with tail coloration for yearling males but not for males older than two years. 
Growth bar width was correlated with tail coloration in all females but there was a 
stronger correlation for second year females. Growth bar width was significantly 
correlated with number of offspring fledged the prior year as well as the number of 
offspring produced in the current year. I then compared the behavioral response from 
territorial males toward artificially brightened and darkened model conspecifics to test 
the signaling function of bright plumage. Bluebird males attacked models with brighter 
blue coloration significantly more often than models with darker blue coloration. 
Bluebird males did not attack models with brighter chestnut coloration significantly more 
often than models with darker chestnut coloration. The structurally-based blue plumage 
appears to be condition dependent and a signal of status used during male competition, 
whereas the melanin-based orange plumage does not. 
 
iii 
 
 
Table of Contents 
 
 
Abstract ............................................................................................................................... ii 
List of Figures .................................................................................................................... iv 
Chapter 1 ? Condition Dependence of Structural Coloration in the Eastern Bluebird ........1 
            Abstract ....................................................................................................................1  
            Introduction ..............................................................................................................2 
            Methods....................................................................................................................5 
            Results   ....................................................................................................................7 
 Discussion   ..............................................................................................................8 
 Literature Cited   ....................................................................................................10 
Chapter 2 ? Coloration as a Signal of Status in the Eastern Bluebird ...............................19 
            Abstract ..................................................................................................................19  
            Introduction ............................................................................................................20 
            Methods..................................................................................................................23 
            Results   ..................................................................................................................25 
 Discussion   ............................................................................................................26 
 Literature Cited   ....................................................................................................28 
 
iv 
 
 
List of Figures 
 
Chapter 1 ? Condition Dependence of Structural Coloration in the Eastern Bluebird  
Figure 1 Male tail length and growth bar width ....................................................13 
Figure 2 Female tail length and growth bar width .................................................14 
Figure 3 Male tail coloration and growth bar width ..............................................15 
Figure 4 Female tail coloration and growth bar width ...........................................16 
Figure 5 Feather growth rate and offspring fledged the current year ....................17 
Figure 6 Feather growth rate and offspring fledged the prior year ........................18 
 
Chapter 2 - Coloration as a Signal of Status in the Eastern Bluebird 
Figure 1 Spectral reflectance of blue rump coloration...........................................31 
Figure 2 Spectral reflectance of orange breast coloration .....................................32 
Figure 3 Aggressive responses toward models ......................................................33 
 
1 
 
 
CHAPTER ONE 
Condition Dependence of Structural Coloration in the Eastern Bluebird 
 
ABSTRACT 
 Elaborate plumage, whether used for intrasexual or intersexual selection, must be 
difficult to acquire or retain in order to be an honest signal of status. However, the 
relationship between structurally produced blue feather coloration and an individual?s 
nutritional condition is unclear. I examined the relationship between blue feather 
coloration and nutritional condition at time of molt in eastern bluebirds (Sialia sialis) by 
measuring growth rates of tail feathers of an entire bluebird population. I measured 
growth bars on tail feathers of males and females of different age classes and compared 
growth bar width to age, gender, tail feather chroma, and number of offspring produced. I 
found that males had wider growth bars than females and older birds had wider growth 
bars than first time breeders. Wider growth bars were correlated with longer tail feathers. 
Growth bar width was correlated with tail coloration in yearling males but not males 
older than two years. Growth bar width was correlated with tail coloration in all females, 
but there was a stronger correlation for second year females. Growth bar width was 
significantly correlated with number of offspring fledged the prior year as well as the 
number of offspring fledged the current year. The structurally-based blue plumage in 
eastern bluebirds appears dependent upon condition at time of molt. 
 
 
2 
 
 
INTRODUCTION 
Honest indicators of individual condition must be difficult to produce or maintain 
(Zahavi 1975) so that cheating does not diminish the reliability of the status signal. 
Elaborate plumage, whether used for intrasexual or intersexual selection, can serve as an 
honest indicator of individual condition if it is limited by the nutrients required to develop 
the ornament or the time needed to consume the necessary amount of nutrients. Several 
types of ornamental plumage pigments have been found to be either difficult to acquire 
(e.g. carotenoids (Hill and Montgomerie 1994)) or difficult to maintain in a social group 
(e.g. melanins (McGraw et al. 2003)) due to repeated challenges from conspecifics. 
However, whether blue structural coloration is condition dependent trait is a matter of 
ongoing debate.  
Non-iridescent structural coloration in feathers is the result of coherent scattering 
of light in a keratin-air matrix in the spongy medullary layer of the feather (Prum 2006). 
Prum et al. (2009) said that the structures in the keratin matrix creating the blue 
coloration are self-assembling and are not nutrient limited or dependent upon the 
condition of the individual. However, empirical evidence supports at least partial 
condition dependence of blue feather coloration. Shawkey et al. (2003) found that the 
number and similarity of the microstructure keratin rods correlated with the chroma and 
saturation of a feather with structural coloration. Furthermore, the thickness of the keratin 
cortex layer, which can affect the total amount of reflected light, may be affected by 
feather degrading bacteria (Shawkey et al. 2007; Gunderson et al. 2009). At least one 
study to date has attempted to indirectly link condition and coloration in eastern bluebirds 
(Sialia sialis). Bluebirds that were randomly given increased broods and therefore forced 
 
3 
 
 
to devote increased parental effort in one season had decreased coloration in following 
years (Siefferman and Hill 2005), presumably due to being in poorer condition during the 
molt at the end of the breeding season. 
I examined the relationship between blue structural coloration and nutritional 
condition at the time of molt in eastern bluebirds by measuring tail feather growth rates. 
Feathers grow at different rates during a day leaving light and dark bands (called growth 
bars) that represent corresponding periods of fast and slow growth (Michener and 
Michener 1938; Wood 1950; Brodin 1993). Feather growth rates (ptilochronology) as a 
measure of avian condition have increasingly been used in recent decades (Grubb 1989). 
Growth bar width can be used as a measure of condition at the time of natural molt or as 
a measure of current condition of a bird by forcing anabolic re-growth after plucking a 
feather (e.g. Grubb and Yosef 1994) with wider growth bars (i.e. faster feather growth) 
indicating better condition. Birds may be especially vulnerable to predation during molt 
(Swaddle and Witter 1997) and thermoregulation may be more difficult during heavy 
molt, therefore faster feather growth and shorter molt duration should be beneficial. 
Feather growth rates have thus far been compared to structural coloration in males 
of only two species. Keyser and Hill (1999) found that tail feather growth rate was 
correlated with a score of rump and breast blueness on male blue grosbeaks (Guiraca 
caerulea). Doucet (2002) found that male rump and wing covert coloration related to 
growth bar width in male blue-black grassquits (Volatinia jacarina). However, both 
studies compare growth bars on one part of the body (tail feathers) to ornamental 
coloration on another part of the body. There may be a difference of up to several weeks 
from when individual tail feathers are molted to when specific contour feathers are 
 
4 
 
 
molted and so comparing growth rates of one feather patch to coloration on another color 
patch may not accurately represent the relationship between color and condition at the 
time of molt of either. 
The eastern bluebird is a sexually dichromatic passerine with both structural- and 
melanin-based ornamental plumage. Males have vivid ultraviolet-blue heads, wings, 
backs, and tails and a chestnut-orange breast. Females resemble males in patterning but 
with duller wings, backs, and tails and a fainter breast color. In eastern bluebirds, 
coloration is related to age (Siefferman et al. 2005) and reproductive success (Siefferman 
and Hill 2003), and coloration may be a signal used in sexual selection. Although female 
bluebirds are more dully colored than males, they have bright blue coloration on their 
wing and tail feathers which may be a sexually selected trait for females as well as for 
males (Siefferman and Hill 2005). The bluebird?s ornamental tail coloration allows us to 
directly compare a structurally-based ornament with the individual?s condition at the time 
the feather was grown, rather than to another plumage patch which may have grown at a 
different time from the measured tail feather. Bluebirds do not have a pre-alternate molt 
and are non-migratory at the study location (Auburn, AL). Brighter individuals are able 
to breed earlier in the year (Siefferman and Hill 2003), which can lead to more offspring 
produced in a year (Darwin 1871; Fisher 1930). However, there has not yet been a 
bluebird study quantifying individual condition at time of molt with the brightness of the 
feathers produced. I compared growth bars on the outermost rectrices of both male and 
female eastern bluebirds to color data collected from those feathers and reproductive 
output over the breeding season to determine if condition at time of molt relates to age, 
sex, color of an ornamental feather, and reproductive success. 
 
5 
 
 
 
METHODS 
I conducted this study in fields adjacent to the Auburn University campus, in Lee 
County, Alabama (32?35?N, 82?28?W) during spring and summer of 2008, 2009, and 
2010 (March ? July). I captured breeding males and females at nest boxes using traps and 
mistnets. All adults were banded with a U.S. Fish and Wildlife Service aluminum band 
and given three plastic color bands for future identification. Captured bluebirds were 
weighed with a digital scale accurate to 0.1 g (Flipscale F2),  wing length and tail length 
were measured using a wing ruler accurate to 0.5 mm (Avinet), and bill and tarsus length 
were measured using calipers accurate to 0.02 mm. Nestlings were banded with only an 
aluminum band. The Auburn population of breeding bluebirds has been monitored since 
1999 so the age of almost all breeding adults is known.  
Juvenile eastern bluebirds have an incomplete first pre-basic molt where they 
replace their juvenile body feathers but retain many of their flight feathers through their 
second year. Eastern bluebirds have no pre-alternate molt; therefore first time breeders 
(termed ?second year? adults) will often have tail feathers that were grown as nestlings 
the prior year. At the end of the breeding season (and for all subsequent years), adults 
have a complete pre-basic molt and will have fully adult plumage the following year 
(when they will be termed ?after second year? adults). Birds of unknown age were 
included as part of the overall male and female population data but were removed for 
age-specific analyses. 
 I captured adult bluebirds and pulled the outermost rectrix (R6) from both sides. I 
then taped the tail feathers flat on non-reflective black paper (Canson Mi-Tientes, Stygian 
 
6 
 
 
Black) and recorded color data with an Ocean Optics S2000 spectrometer (Dunedin, 
Florida). Following methods described by Siefferman and Hill (2003), I recorded feather 
reflectance data from 300-700 nm using a micron fiber-optic probe held at a 90-degree 
incident angle, scanning each feather three times, lifting the probe off the feather between 
scans. I reduced reflectance data into one color variable: chroma. I define chroma as the 
proportion of reflectance from 350 to 450 nm (encompassing the entire blue peak) in the 
total reflectance from 300 to 700 nm. I only use chroma in order to reduce the number of 
correlations tested and because chroma appeared to be the color variable with the greatest 
potential to be condition dependent. Hue (the wavelength of maximum reflectance) does 
not vary greatly in the population (Shawkey et al. 2003), brightness (total reflectance 
from 300 to 700 nm) seems more likely than chroma to be influenced by external factors 
(e.g. keratinophilic bacteria degrading the cortex layer (Gunderson et al. 2009)), and 
chroma appeared to best measure the development of the structures giving the feather its 
blue coloration (Shawkey et al. 2003). Chroma was standardized to a mean of zero and 
standard deviation of one for each year to allow for comparisons over multiple years. To 
determine the growth rate of the rectrices, I counted eight growth bars in the middle third 
of the feather and pushed pins through the feather vain next to the rachis to mark the 
width of the eight growth bars. I then measured the distance between the two holes on the 
back side of the paper using digital calipers accurate to the nearest 0.02 mm and divided 
the distance measured by the number of growth bars counted (eight) to reach an average 
growth bar width for each individual. 
 I compared growth bar width of males and females using a two tailed t-test. I also 
compared second year birds to after second year birds for each gender using two tailed t-
 
7 
 
 
tests. I then compared growth bar width to chroma of the tail feathers and number of 
offspring produced. I used a Pearson correlation to compare growth bar width, tail 
coloration, and reproductive output. Data were analyzed using SAS version 9.1. 
 
RESULTS 
Male growth bars were significantly wider than female growth bars (t = 4.15, P = 
0.0004). Male growth bars averaged 3.14 +/- 0.01 mm (mean +/- SE) (n = 234) and 
female growth bars averaged 3.08 +/- 0.01 mm (n = 249). After second year birds had 
significantly wider growth bars than second year birds for both males (t = -3.55, P < 
0.001) and females (t = 4.34, P < 0.001). 
 Growth bar width was significantly positively correlated with the overall length of 
the tail feather for both males (r = 0.4, P = 0.001, n = 234, Fig 1) and females (r = 0.39, P 
< 0.0001, n = 249, Fig 2). However, there was no significant correlation between growth 
bar width and tarsus length for either males (r = 0.04, P = 0.54, n = 234) or females (r = 
0.001, P = 0.98, n = 249). 
Including all males, there was no significant correlation between growth bar width 
and tail coloration (r = 0.12, P = 0.27, n = 234, Fig 3A), however there was a significant 
correlation for second year males (r = 0.29, P = 0.05, n = 47, Fig 3B) while there was no 
significant correlation for after second year males (r = -.05, P = 0.58, n = 121, Fig 3C). 
Female tail coloration was significantly correlated with growth bar width (r = 0.30, P < 
0.0001, n = 249, Fig 4A), and second year females had a stronger correlation (r = 0.35, P 
= 0.02, n = 47, Fig 4B) than after second year females (r = 0.14, P = 0.05, n = 119, Fig 
4C). 
 
8 
 
 
 Male growth bar width was significantly correlated with number of offspring 
fledged in that year (r = 0.29, P = 0.0005, n = 155, Fig 5A) and the number of offspring 
fledged in prior years (r = 0.31, P = 0.04, n = 37, Fig 5B). Female growth bar width was 
correlated with number of offspring fledged in the current year (r = 0.17, P = 0.017, n = 
174, Fig 6A) and the prior year (r = 0.32, P = 0.049, n = 39, Fig 6B). 
 
DISCUSSION 
 For eastern bluebirds, the relationship between feather growth, coloration, and 
reproductive success depends upon an individual?s gender and age. Surprisingly, females 
exhibited a stronger relationship between tail feather chroma and tail feather growth rate 
than did males. Both males and females showed a significant correlation between growth 
bar width and tail length, indicating that there is a structural consequence to the different 
growth bar widths in the population. However, growth bar width is unrelated to tarsus 
length, so faster tail growth rate is not simply a byproduct of having a larger body. 
 For both males and females, second year birds (first-time breeders) had narrower 
growth bars than after second year birds. This likely stems from the pre-juvenal molt that 
nestlings undergo where they grow all 12 tail feathers at the same time instead of two at a 
time as is typical during the adult pre-basic molt. This rapid growth of all rectrices leaves 
nestlings and second year birds with characteristically poorer quality feathers (Pyle 1997) 
that are often ragged by the end of the following year. This pressure for fast simultaneous 
growth of all tail feathers in nestlings may explain the stronger correlation between color 
and tail feather growth rate in second year birds. Furthermore, fledglings likely have 
greater variability in body condition as well as overall poorer body condition compared to 
 
9 
 
 
adults. Selection acts strongly on juveniles and there is very low survivorship from 
fledglings to breeding adults (Bauldry et al. 1995). Condition at time of molt as a juvenile 
may be crucial in determining the chance of surviving to adulthood. 
Males and females showed a significant correlation between feather growth rate 
and the number of offspring produced, indicating that condition in the autumn of one year 
can have carry-over effects into the breeding season of the following year. Surprisingly, 
both males and females also exhibited a significant positive relationship between feather 
growth rate and the number of offspring produced in the prior year. Chicks are 
energetically costly to care for and raising a larger number of offspring could leave 
parents in poorer condition during the following molt. However, adults in better quality 
territories may be able to raise more offspring and still be in better condition at the end of 
the season (Individual Optimization Hypothesis (Pettifor et al. 1988)). For this reason, 
cross-fostering and manipulating brood sizes (e.g. Siefferman and Hill 2005) are needed 
to disentangle territory quality and individual quality from the cost of raising more 
offspring. 
 In this study, even when traits were significantly correlated, the r2 values are 
rather low. There may be a strong genetic component to growth rates that doesn?t 
fluctuate with environmental conditions (Gienapp and Meril? 2010), or environmental 
factors like feather degrading bacteria or ultraviolet light may alter feather coloration in 
the several months before I was able to catch adults in the breeding season. 
 The study of condition dependence of structural coloration has few large scale 
correlative analyses. Further population-wide comparisons of molting condition and 
 
10 
 
 
feather coloration are needed before accepting the premise that bright structural 
coloration is an honest signal of condition in natural populations. 
 
LITERATURE CITED 
Bauldry, V. M., D. M. Muschitz, L. A. Radunzel, and P. Arcese. 1995. A 27-year study 
of eastern bluebirds in Wisconsin: productivity, juvenile return rates and dispersal 
outside the study area. North American Bird Bander 20:111-119. 
Brodin, A. 1993. Radio-ptilochronology ? tracing radioactively labeled food in feathers. 
Ornis Scandinavica 24:167-173. 
Darwin, C. 1871. The descent of man and selection in relation to sex. John Murray, 
London. 
Doucet, S. M. 2002. Structural plumage coloration, male body size, and condition in the 
blue black grassquit. Condor 104:30-38. 
Fisher, R. A. 1930. The genetical theory of natural selection. Clarendon, Oxford. 
Gienapp, P., and J. Meril?. 2010. Genetic and environmental effects on a condition-
dependent trait: feather growth in Siberian jays. Journal of Evolutionary Biology 
23: 715-723. 
Grubb, T. C., Jr. 1989. Ptilochronology: Feather growth bars as indicators of nutritional 
status. Auk 106: 314-320. 
Grubb, T. C., Jr., and R. Yosef. 1994. Habitat-specific nutritional condition in loggerhead 
shrikes (Lanius ludovicianus): evidence from ptilochronology. Auk 111: 756-759. 
 
11 
 
 
Gunderson, A. R., M. H. Forsyth, and J. P. Swaddle. 2009. Evidence that plumage 
bacteria influence feather coloration and body condition of eastern bluebirds 
Sialia sialis. Journal of Avian Biology 40:440-447. 
Hill, G. E., and R. Montgomerie. 1994. Plumage colour signals nutritional condition in 
the house finch. Proccedings: Biological Sciences 258:47-52. 
Keyser, A. J., and G. E. Hill. 1999. Condition-dependent variation in the blue-ultraviolet 
coloration of a structurally based plumage ornament. Proceedings of the Royal 
Society B: Biological Sciences 266:771-777. 
McGraw, K. J., J. Dale, and E. A. Mackillop. 2003. Social environment during molt and 
the expression of melanin-based plumage pigmentation in male house sparrows 
(Passer domesticus). Behavioral Ecology and Sociobiology 53:116-122. 
Michener, H., and J. R. Michener. 1938. Bars in flight feathers. Condor 40: 149-160. 
Pettifor, R. A., C. M. Perrins, and R. H. McCleery. 1988. Individual optimization of 
clutch size in great tits. Nature 336:160-162. 
Prum, R. O. 2006. Anatomy, physics, and evolution of structural colors. In Bird 
Coloration: Mechanisms and Measurements. Ed by Hill GE and McGraw KJ. 
Harvard University Press Cambridge. 
Prum, R. O., E. R. Dufresne, T. Quinn, and K. Waters. 2009. Development of colour-
producing ?-keratin nanostructures in avian feather barbs. Journal of the Royal 
Society Interface 6:S253-S265. 
Pyle, P. 1997. Identification guide to North American birds: Part I Columbidae to 
Ploceidae. Slate Creek Press, Bolinas, CA.  
 
12 
 
 
Shawkey, M. D., A. M. Estes, L. M. Siefferman, and G. E. Hill. 2003. Nanostructure 
predicts intraspecific variation in ultraviolet-blue plumage colour. Proceedings of 
the Royal Society B: Biological Sciences 270:1455-1460. 
Shawkey, M. D., S. R. Pillai, G. E. Hill, L. M. Siefferman, and S. R. Roberts. 2007. 
Bacteria as an agent for change in structural plumage color: correlational and 
experimental evidence. American Naturalist 169:S112-S121. 
Siefferman, L., and G. E. Hill. 2003. Structural and melanin coloration indicate parental 
effort and reproductive success in male eastern bluebirds. Behavioral Ecology 14: 
855-861. 
Siefferman, L., and G. E. Hill. 2005. Male eastern bluebirds trade future ornamentation 
for current reproductive investment. Biology Letters 1: 208-211. 
Siefferman, L., G. E. Hill, and F. S. Dobson. 2005. Ornamental plumage coloration and 
condition are dependent on age in eastern bluebirds Sialia sialis. Journal of Avian 
Biology 36:428-435. 
Swaddle, J. P., and M. S. Witter. 1997. The effects of molt on the flight performance, 
body mass, and behavior of European starlings (Sturnus vulgaris): an 
experimental approach. Canadian Journal of Zoology 75:1135-1145. 
Wood, H. B. 1950. Growth bars in feathers. Auk 67:486-491. 
Zahavi, A. 1975. Mate selection ? a selection for a handicap. Journal of Theoretical 
Biology 53:205-214. 
 
 
 
 
13 
 
 
 
 
Figure 1. Comparison of tail length and growth bar width in eastern bluebird males. 
 
14 
 
 
 
 
Figure 2. Comparison of tail length and growth bar width in eastern bluebird females. 
 
15 
 
 
 
 
 
Figure 3. Eastern bluebird tail chroma compared to growth bar width for all males (3A), 
second year males (3B), and after second year males (3C). Solid lines indicate 
significant correlation at ? = 0.05 significance level, dashed lines indicate non-
significant correlation.
 
16 
 
 
 
 
 
 
Figure 4. Eastern bluebird tail chroma compared to growth bar width for all females (4A), 
second year females (4B), and after second year females (4C). 
 
17 
 
 
 
 
 
Figure 5. Eastern bluebird feather growth rate compared to the number of offspring 
fledged in the same summer for adult males (5A) and females (5B). 
 
 
18 
 
 
 
 
 
Figure 6. Eastern bluebird feather growth rate compared to the number of offspring 
fledged in the previous summer for adult males (6A) and females (6B). 
 
19 
 
 
CHAPTER TWO 
Coloration as a Signal of Status in the Eastern Bluebird 
 
ABSTRACT 
Ornamental plumage can function for sexual selection through either attracting 
mates or threatening rivals. Elaborate ornamentation can be used in male competition as a 
signal of status if the ornaments are difficult to obtain, even if not useful as weaponry. 
The eastern bluebird (Sialia sialis) is a sexually dichromatic species with males that have 
bright structurally-based blue plumage as well as melanin-based chestnut plumage. 
Elaborate male coloration in the eastern bluebird may function as a signal of status during 
male-male competition for ownership of nestboxes. I tested the signaling function of both 
ornamental plumage colors by presenting breeding adults with artificially brightened and 
darkened bluebird models on their territories and recording the attack rate on each model. 
Bluebird males attacked models with brighter blue coloration significantly more often 
than models with darker blue coloration. Bluebird males did not attack models with 
brighter chestnut coloration significantly more often than models with darker chestnut 
coloration. The structurally-based blue coloration on bluebird wings, back, and tail 
appears to function as a signal during male competition, whereas the chestnut breast 
coloration does not appear to influence the amount of aggression from conspecifics.
 
20 
 
 
INTRODUCTION 
Sexual selection is the differential ability to secure mates (Darwin 1871) and can 
occur through either intersexual or intrasexual selection. Intersexual selection (female 
choice of male traits) potentially leads to elaborately ornamented males while intrasexual 
selection (male competition) often leads to armaments or weaponry (Andersson and 
Iwasa 1996). However, male competition can also select for traits that signal fighting 
ability or dominant status without actually serving as weaponry (Rohwer 1975). 
Signaling status can benefit the top males by reducing the number of fights that 
they must conduct, saving energy and reducing the possibility of injury (Rohwer 1975). 
Lower-quality males can also obtain some benefit from honestly signaling their low 
status because high quality males may not expend the effort to fight with low-quality 
males that are not a direct threat to their territory (Lyon and Montgomerie 1986). For 
signaling to be honest, lower-quality males cannot cheat and create elaborate ornaments 
similar to high quality males without the associated costs. Cheating is therefore prevented 
either by the high cost of constructing or maintaining an ornament (Owens and Hartley 
1991) or by the high cost associated with losing competitive battles with high quality 
males (Rohwer 1977). 
 Elaborate plumage coloration is the most common type of avian ornamentation, 
and can come from pigmentation, structural coloration, or a combination of the two. The 
most common pigments are carotenoids, which provide many of the yellow, orange, and 
red feather colors (McGraw 2006a), and melanins, which provide brown and black 
feather colors (McGraw 2006b). Structural coloration is produced from the reflection and 
 
21 
 
 
coherent scattering of light at specific wavelengths based on the microstructure of the 
feathers (Prum 2006) creating various blues, greens, and iridescent colors. 
Pigments and feather microstructures have their own synthesis or developmental 
pathways and may signal different aspects of a bird?s lifestyle. Carotenoids cannot be 
synthesized by vertebrates, so they have to be ingested from food sources (Brockmann 
and V?lker 1934).  For that reason, carotenoid-based coloration is thought to reflect the 
nutritional condition of a bird and its foraging ability (McGraw 2006a). Melanins are 
synthesized in vertebrates (McGraw 2006b) and do not appear to be limited by nutrition 
or parasite load (Hill 2006). Thus, melanin-based coloration often reflects the social 
status of the male and the signal honesty is maintained through frequent testing from 
rivals (Senar 2006). Coloration and dominance is a topic that has been studied in several 
species, but most of those species exhibit carotenoid- or melanin-based coloration 
(reviewed by Senar 2006). Fewer studies have examined male dominance interactions in 
a species with structural coloration as the status signal. 
Eastern bluebirds (Sialia sialis) are sexually dichromatic passerines, with males 
having brighter blue wings and back and a darker chestnut breast than females (Gowaty 
and Plissner 1998). The chestnut coloration is produced by melanin pigments, but the 
blue wing and body feathers get their coloration from nanostructures in the wings 
reflecting light over an ultraviolet-blue range (Shawkey et al. 2003). Sexual dimorphism 
(in this case dichromatism) can indicate some level of sexual selection with the gender 
under strongest selection being more elaborately colored (Owens and Hartley 1998). 
Eastern bluebirds are a secondary cavity nesting species that readily occupy man-made 
nest boxes. However, in natural settings, the number of available cavities would be a 
 
22 
 
 
limiting factor on bluebird breeding, so competition for the few available cavities would 
be fierce (Newton 1994). 
There has been extensive testing of blue coloration as an indicator of male quality 
in bluebirds, with color being correlated with earlier breeding and greater number of 
offspring fledged (Siefferman and Hill 2003; Siefferman and Hill 2005). Although the 
brightness of male blue coloration increases with age, chestnut coloration becomes duller 
with age (Siefferman et al. 2005). Eastern bluebirds have been shown to be able to 
discriminate between adult and juvenile chest coloration and divert aggression toward 
conspecifics with adult chest coloration (Ligon and Hill 2009), however there has been 
no study on whether the variation in orange chest coloration among adults influences 
aggression from conspecifics. 
 Although males grow brighter plumage as they age (Siefferman et al. 2005), and 
presumably gain experience and competitive ability, experimental tests have shown a 
lack of female choice for the male with brighter blue plumage when females are 
presented with two males differing in plumage coloration (Liu et al. 2007). One possible 
explanation is that bright male coloration serves as a signal of dominance when male 
birds are making first assessments of a rival?s competitive ability. Although this has been 
proposed for other thrushes (Udvardy 1977), it has not yet been directly tested. 
Dominance has been correlated with coloration in other species (summarized by 
Senar 2006), however it is often unclear whether the brightness of the color itself is the 
signal or if body condition or other traits are the signal and color is simply correlated with 
condition. The relative influence of multiple traits is difficult to distinguish in the field 
and an experimental manipulation of body condition or plumage coloration is required to 
 
23 
 
 
separate their relative importance in dominance competitions. Although a previous study 
assumed brighter males were dominant due to their earlier breeding when nestboxes were 
limited (Siefferman and Hill 2005), there has not yet been a direct test of color, 
dominance, and signal detection in eastern bluebirds. 
In this study I manipulated coloration on taxidermic models of eastern bluebird 
males and then presented the models to resident bluebirds during the breeding season. 
Bluebirds display threatening postures including bill wiping, wing flapping, and singing 
(Krieg 1969) toward taxidermic mounts in their territory and will violently attack a model 
bluebird that remains in the territory. By using models, I reduced the problems of 
interactive behavior between live models and resident pairs that can override any 
potential signal function of coloration. Furthermore, by artificially manipulating the blue 
and chestnut coloration of eastern bluebirds, I can test for the importance of each color 
ornament independent of the age or body condition of the bird. 
 
METHODS 
I studied eastern bluebirds at a long-term monitored breeding population in 
Auburn, Alabama (32?35?N, 82?28?W) from February to August, 2009 - 2010. Male 
eastern bluebirds defend a territory surrounding a nest box and will attack conspecifics 
that encroach upon their territory. I conducted behavioral experiments at times when 
male aggression is high, during egg-laying and chick raising (Gowaty and Wagner 1988), 
to maximize the behavioral response to intruders. I used taxidermic mounts of eastern 
bluebirds as models on which I artificially manipulated the brightness of the plumage to 
test whether the brightness of male bluebird coloration affects aggressive behaviors in 
 
24 
 
 
territorial males. I made taxidermic models from male eastern bluebirds that had been 
collected in prior years. I mounted the models in upright postures and attached each 
model to a dowel and small PVC fitting. I scanned each model on the breast, head, wing, 
and rump with an Ocean Optics Optics S2000 spectrometer (Dunedin, Florida) using a 90 
degree incident and reflective angle. Each feather patch was scanned three times, lifting 
the probe off the feathers between scans.  
In 2009, I brightened the structurally-based blue coloration on a male bluebird 
model?s head, wings, back, and tail with a violet marker (Prismacolor Violet Mist) and 
darkened the blue coloration on other models with a black permanent marker (Sharpie 
Black) (Liu et al. 2007) (Fig.1). These manipulations altered the brightness (total 
reflectance from 300 to 700 nm) and chroma (proportion of reflectance from 350 to 450 
nm) within the range of variation observed in the population without altering hue 
(wavelength of maximum reflectance). I matched models of similar size, posture, and 
chest coloration and used six pairs of models to reduce the effect of any one model upon 
eliciting aggression from territorial males. 
In 2010, I increased the chroma and brightness of the melanin-based chestnut 
coloration on a male bluebird model?s breast using a brown marker (Prismacolor Walnut) 
and decreased the coloration using hydrogen peroxide (White et al. 1980) to bleach the 
feathers (Fig. 2). I again used six different pairs of models to minimize the influence of a 
single model on the overall results. 
During both years I followed the same protocol: I drove two 1.2 meter rebar poles 
into the ground five meters away from a bluebird nestbox and five meters apart from each 
other. Atop each rebar I placed a taxidermic bluebird model attached to a 1.2 meter 
 
25 
 
 
wooden dowel (diameter 1cm) placed perpendicular to the rebar. The dowel gave the 
resident bluebirds a place to perch near the model and display before attacking. One of 
the models was experimentally darkened and the other model was experimentally 
brightened. Models were covered with a paper bag and then simultaneously exposed by 
pulling the bags off the models using thin fishing line attached to the bags. I continuously 
played a 30 second audio recording of eastern bluebird songs (A Field Guide to Songs: 
Eastern and Central North America (Peterson Field Guides)) from a speaker placed 
between the models for five minutes before exposing the models and continued to play 
the eastern bluebird song throughout the trial to ensure that the resident bluebirds were 
searching for an intruding adult male before the models were presented and were 
attentive to the models when they were exposed.  Trials ran for 10 minutes after the 
models were exposed or stopped when the resident bluebirds physically attacked a model 
to preserve model condition for later trials. Observations were made from a concealed 
location >30 meters away. 
To determine the extent that color influences the first aggressive response from 
territorial males I quantified which model received the first signs of aggression. I also 
determined which model received the most aggression and the amount of time until the 
models were attacked. I analyzed the number of trials that each model was attacked using 
a Chi-Square goodness-of-fit test.  
 
RESULTS 
 In 2009, I conducted 66 trials with blue brightened and blue darkened models. Of 
the 66 trials, seven males did not express quantifiable aggression toward either of the 
 
26 
 
 
models. Of the remaining 59 trials, resident males first attacked the blue brightened 
model significantly more often than the blue darkened model (?2 = 4.90, N = 59, P = 
0.03, Fig. 3).  The model that was attacked first was also the model that was attacked 
most in 48 of the trials (81%). Of the 11 trials where the model receiving the most 
aggression differed from the model receiving the first aggression, six trials (55%) were a 
switch from attacking the dark model first to attacking the bright model the most and five 
trials (45%) were a switch from attacking the bright model first to attacking the dark 
model the most (Fisher?s exact test, P = 0.99). 
 In 2010, resident males did not differ in the number of times that they first 
attacked the chestnut brightened versus chestnut darkened model (?2 = 1.78 N = 36, P = 
0.18). 
 
 DISCUSSION 
There are many factors influencing the outcome of competitive interactions. 
Hormone levels, familiarity with the territory, breeding experience, and current body 
condition can all influence dominance between two individuals. Since feather coloration 
is fixed at the time of molt, plumage coloration is not an indicator of the current 
physiological state of an individual. However, most antagonistic encounters are resolved 
without an actual fight and signals of status like plumage coloration can allow visual 
shortcuts to assess the quality of a rival.  
Male eastern bluebirds responded to manipulated differences in structurally-based 
blue feather coloration. Males attacked conspecific models that were artificially 
brightened significantly more often than they attacked artificially dulled models. These 
 
27 
 
 
color differences and behavioral responses match age- and condition-related differences 
in the brightness of bluebird plumage. Older males and males in better condition are often 
brighter than younger males and high quality males are likely a greater threat for territory 
usurpation or cuckoldry. By showing more aggression toward conspecifics that are a 
greater fitness threat, bluebirds can likely maximize the benefits from aggressively 
defending a territory. 
In contrast to male responses to the structurally-based blue coloration, males did 
not appear to respond to manipulated differences in melanin-based chestnut breast 
coloration. Male breast coloration in the eastern bluebird does not become brighter with 
age; older males actually have less ornamented breast coloration than young males 
(Siefferman et al. 2005). Since the melanin pigments in the eastern bluebird breast 
feathers can be synthesized by the individual (McGraw 2006b), variation in bluebird 
breast coloration may not reflect variation in condition and therefore the breast coloration 
may not act as a signal of status in this species. 
Male competition and female choice are not mutually exclusive selective forces 
driving elaborate ornamentation, but previous studies of both aviary-based and field-
based mate choice experiments have shown a lack of female choice for bright male 
plumage in the eastern bluebird (Liu et al. 2007). Male competition can be influenced by 
male coloration, and it is possible that female bluebirds judge males based more on 
territory quality rather than male coloration. Experimental tests of male competition and 
female choice can help explain the relative signaling importance of different color 
ornaments and expand the scope of ongoing studies of sexual selection in the eastern 
bluebird. 
 
28 
 
 
LITERATURE CITED 
Andersson, M., and Y. Iwasa. 1996. Sexual Selection. Trends in Ecology and Evolution 
11: 53-58. 
Brockmann, H., and O. V?lker. 1934. Der gelbe Federfarbstoff des Kanarienvogels 
(Serinus canaria canaria [L.]) und das Vorkommen von Carotinoiden bei V?geln. 
H-S Z Physiological Chemistry 224:193?215. 
Darwin, C. 1871. The descent of man and selection in relation to sex. John Murray, 
London. 
Gowaty, P. A., and S. J. Wagner. 1988. Breeding season aggression of female and male 
eastern bluebirds (Sialia sialis) to models of potential conspecific and 
interspecific egg dumpers. Ethology 78:238-250. 
Gowaty, P. A., and J. H. Plissner. 1998. Eastern Bluebird (Sialia sialis), The Birds of 
North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; 
Retrieved from the Birds of North America Online: 
http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/381. 
Hill, G. E. 2006. Environmental Regulation of Ornamental Coloration. in Bird 
Coloration, Volume I. edited by Hill GE and McGraw KJ. Harvard University 
Press. Harvard, MA. 
Krieg, D. C. 1969. The agonistic and courtship behavior of the eastern bluebird. PhD 
Thesis. St. Bonaventure, NY. 177 pp. 
Ligon, R. A., and G. E. Hill. 2009. Do adult eastern bluebird, Sialia sialis, males 
recognize juvenile-specific traits? Animal Behaviour 77:1267-1272. 
 
29 
 
 
Liu, M., L. Siefferman, and G. E. Hill, 2007. An experimental test of female choice 
relative to male structural coloration in eastern bluebirds. Behavioral Ecology and 
Sociobiology 61:623-630. 
Lyon, B. E., and R. D. Montgomerie. 1986. Delayed plumage maturation in passerine 
birds: Reliable signaling by subordinate males? Evolution 40: 605-615. 
McGraw, K. J. 2006a. Mechanics of carotenoid-based coloration. in Bird Coloration, 
Volume I. edited by Hill GE and McGraw KJ. Harvard University Press. Harvard, 
MA. 
McGraw, K. J. 2006b. Mechanics of melanin-based coloration. in Bird Coloration, 
Volume I. edited by Hill GE and McGraw KJ. Harvard University Press. Harvard, 
MA. 
Newton, I. 1994. The role of nest sites in limiting the numbers of hole-nesting birds: a 
review. Biological Conservation 70:265-276. 
Owens, I. P. F., and I. R. Hartley. 1991. ?Trojan sparrows?: Evolutionary consequences 
of dishonest invasion for the badges-of-status model. American Naturalist 138: 
1187-1205. 
Owens, I. P. F., and I. R. Hartley. 1998. Sexual dimorphism in birds: Why are there so 
many different forms of dimorphism? Proceedings of the Royal Society of 
London B: Biological Sciences 265:397-407. 
Prum, R. O. 2006. Anatomy, Physics, and Evolution of Structural Colors. in Bird 
Coloration, Volume I. edited by Hill GE and McGraw KJ. Harvard University 
Press. Harvard, MA. 
 
30 
 
 
Rohwer, S. A. 1975. The social significance of avian winter plumage variability. 
Evolution 29: 593-610. 
Rohwer, S. A. 1977. Status signaling in Harris? Sparrows: Some experiments in 
deception. Behaviour 61: 107-129. 
Senar, J. C. 2006. Color Displays as Intrasexual Signals of Aggression and Dominance. 
in Bird Coloration, Volume II. edited by Hill GE and McGraw KJ. Harvard 
University Press. Harvard, MA. 
Shawkey, M. D., A. M. Estes, L. M. Siefferman, and G. E. Hill. 2003. Nanostructure 
predicts intraspecific variation in ultraviolet-blue plumage colour. Proceedings of 
the Royal Society B: Biological Sciences 270: 1455-1460. 
Siefferman, L., and G. E. Hill. 2003. Structural and melanin coloration indicate parental 
effort and reproductive success in male eastern bluebirds. Behavioral Ecology 
14:855-861. 
Siefferman, L., and G. E. Hill. 2005. UV-blue structural coloration and competition for 
nestboxes in male eastern bluebirds. Animal Behaviour 69: 67-72. 
Siefferman, L., G. E. Hill, and F. S. Dobson. 2005. Ornamental plumage coloration and 
condition are dependent on age in eastern bluebirds Sialia sialis. Journal of Avian 
Biology 36: 428-435. 
Udvardy, M. D. F. 1977. The Audobon Society Field Guide to North American Birds: 
Western Region. New York. Chanticleer Press. 
White, S. B., T. A. Bookhout, and E. K. Bollinger. 1980. Use of human hair bleach to 
mark blackbirds and starlings. Journal of Field Ornithology 51:6-9. 
 
31 
 
 
 
Figure 1. Spectral reflectance of eastern bluebird rump coloration including average 
reflectance curve (grey line), artificially brightened models (blue line), and 
artificially darkened models (black line). 
 
32 
 
 
 
Figure 2. Spectral reflectance of eastern bluebird breast coloration including average 
reflectance curve (grey line), artificially brightened models (tan line), and 
artificially darkened models (brown line). 
 
33 
 
 
 
 Figure 3. Aggressive responses from eastern bluebirds toward models with 
experimentally darkened or brightened blue coloration. Bars represent the 
number of eastern bluebird trials with the first aggressive response toward the 
experimentally darkened (black bar) or experimentally brightened (white bar).