J. Field Ornithol. 85(2):168–179, 2014

DOI: 10.1111/jofo.12058

Reproductive correlates of plumage coloration of female
Mountain Bluebirds
Ashley Morrison, Nancy J. Flood, and Matthew W. Reudink1
Department of Biological Sciences, Thompson Rivers University, Kamloops, British Columbia V2C 0C8, Canada
Received 24 July 2013; accepted 12 February 2014
ABSTRACT. Many studies have shown that the plumage coloration of male birds can act as an honest signal
of quality, indicating benefits that a female could gain from pairing with a specific male. In some species, females
also display ornamental plumage, but less is known about the function and potential adaptive significance of female
coloration because most research has focused on male coloration. Male Mountain Bluebirds (Sialia currucoides)
display full body, ultraviolet (UV)-blue plumage, whereas female plumage is more subdued, with blue color focused
on the rump, wing, and tail. During the 2011 and 2012 breeding seasons (May–July) near Kamloops, BC, Canada,
we examined coloration of the rump and tail of female Mountain Bluebirds to determine if their plumage could
act as an indicator of direct reproductive benefits (e.g., enhanced parental care or reproductive success) to potential
mates. We found no relationship between female plumage coloration and either provisioning rate or fledging success.
However, female coloration varied with age, with after-second-year (ASY) females having brighter, more UV-blue
tail feathers than second-year (SY) females. In addition, ASY females with brighter, more UV-blue tails had larger
clutches. We also observed positive assortative mating by tarsus length. Because previous work with other species
suggests that female body size may be a good predictor of breeding success, males could potentially benefit from
pairing with larger females. However, reproductive success did not vary with female size in our study. Although our
evidence that structural plumage coloration of female Mountain Bluebirds is a signal of direct reproductive benefits
for males (e.g., higher reproductive success) is limited, our results (i.e., ASY females with brighter tails than SY
females, and ASY females with brighter tails having larger clutches) do suggest the potential for sexual selection to
act on female coloration.
RESUMEN. Asociación entre la reproducción y la coloración en hembras de Sialia
currucoides
Muchos estudios han mostrado que la coloración del plumaje en machos puede actuar como una señal honesta
sobre la calidad, indicando los beneficios que pueden ganar las hembras al apearse con un macho especifico. En
algunas especies, las hembras también despliegan plumajes ornamentales, pero su función y la capacidad adaptativa
potencial de la coloración en las hembras es menos conocida, en gran parte porque la mayorı́a de las investigaciones se
han enfocado en la coloración de los machos. Machos de Sialia currucoides despliegan en todo el cuerpo un plumaje
azul ultravioleta (UV), mientras que el plumaje de la hembra es más suave, en donde el color azul se concentra
en la rabadilla, alas y cola. Durante la temporada reproductiva del 2011 y 2012 (May-Jul) cerca de Kamloops,
BC, Canadá, examinamos la coloración de la rabadilla y la cola en hembras de S. currucoides para determinar si su
plumaje puede servir como un indicador de beneficios directos en la reproducción (e.g., aumenta el cuidado parental
y el éxito reproductivo) a potenciales parejas. No encontramos ninguna relación entre la coloración del plumaje de
la hembra y la tasa de alimentación o el éxito reproductivo. Sin embargo, la coloración de las hembras vario con la
edad, en donde después del segundo año (DSA) las hembras tienen plumas de la cola UV-azules mas brillantes que
las hembras de segundo año (SA). Adicionalmente, DSA hembras con plumas de la cola UV-azules mas brillantes
tuvieron nidadas más grandes. También observamos una apareamiento dirigido y asociado positivamente con el
largo del tarso. Debido a que estudios previos en otras especies sugiere que el tamaño corporal de las hembras
puede ser un buen predictor de el éxito reproductivo, los macho se podrı́an beneficiar al aparearse con hembras más
grandes. Sin embargo, el éxito reproductivo no vario con el tamaño de las hembras en nuestro estudio. Aunque
nuestra evidencia muestra que la coloración estructural de las hembras de S. currucoides es una señal directa para el
beneficio de los machos (e.g., mas alto éxito reproductivo) los datos son limitados. Pero, nuestros resultados (i.e.,
DSA hembras con colas mas brillantes que SA hembras, y DSA hembras con colas mas brillantes teniendo nidadas
más grandes) sugieren que potencialmente la selección sexual podrı́a actuar sobre la coloración de las hembras.
Key words: mate choice, parental care, sexual selection, Sialia currucoides, structural coloration

Many studies have demonstrated that male
plumage coloration may reflect various aspects
of individual quality and condition (Hill 2006).
1

Corresponding author. Email: mreudink@tru.ca

Thus, male plumage traits may indicate both
direct (e.g., parental care) and indirect (e.g.,
good genes) benefits that a female would gain
from pairing with a particular male (Hill 2006).
In many species, females also exhibit ornamental


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plumage, and their color may similarly provide
information about quality to potential mates
(Amundsen 2000, Amundsen and Pärn 2006,
Drobniak et al. 2013).
The mutual selection hypothesis states that
female ornamentation may arise through mutual
mate choice, with both sexes exercising choice
(Kraaijeveld et al. 2007, Edward and Chapman
2011). Because both males and females benefit
from choosing good mates, sexual selection may
act on females (via mate choice by males) to
increase their ornamentation (Clutton-Brock
2009). Additionally, females play a role in territory defense in some species (Whittingham
et al. 1992, 1996, Bleiweiss 1997, Murphy et al.
2009, Midamegbe et al. 2011), and color may
act as a signal of status and dominance when
competing with other individuals (usually females) for limited resources; thus, this too could
lead to selection for increased ornamentation
(social selection; West-Eberhard 1979, Johnson
1988, Jones and Hunter 1999, LeBas 2006,
Rosvall 2011). In either case, the highest-quality
females are predicted to be the most colorful
and be preferred as mates. Multiple studies
have demonstrated condition-dependence of female plumage coloration (e.g., Siefferman and
Hill 2005, Doutrelant et al. 2012) and there
is also evidence of direct sexual selection on
female plumage coloration through correlative
studies, mate-choice trials, and manipulative
experiments (Hill 1993, Amundsen et al. 1997,
Griggio et al. 2005, 2009). One reason why
males may prefer colorful females is that female
color may signal reproductive benefits that potential mates would receive, such as parental care
and reproductive capacity (Siefferman and Hill
2005, Amundsen and Pärn 2006, Doutrelant
et al. 2008, Osmond et al. 2013).
Two main theories have been developed to explain the relationship between attractiveness and
parental care in species that exhibit biparental
care: the Good Parent Hypothesis and the
Differential Allocation Hypothesis. The Good
Parent Hypothesis suggests that there should be
a positive association between parental care and
individual attractiveness if plumage coloration
is indicative of individual quality and higherquality individuals provide better resources to
their offspring (Hoelzer 1989). On the other
hand, the Differential Allocation Hypothesis
states that an individual mated with a highquality partner will allocate more energy to

169

parental investment than if it had a lower quality
partner; in response, the high-quality partner
can decrease its own investment, resulting in
either a negative correlation between an individual’s attractiveness and the amount of parental
care it contributes, or a positive correlation
between the attractiveness of an individual’s
mate and the level of parental care it provides
(Burley 1988, Møller and Thornhill 1998). To
date, few investigators have applied the Good
Parent and Differential Allocation hypotheses
to females to test whether plumage coloration
is an indicator of the extent of parental care.
Among studies that have been conducted, results
are mixed and have revealed positive (Linville
et al. 1998, Jawor et al. 2004, Siefferman and
Hill 2005), negative (Badyaev and Hill 2002,
Johnsen et al. 2005, Osmond et al. 2013), and
no relationships (Rohde et al. 1999, Smiseth and
Amundsen 2000, Balenger et al. 2007) between
attractiveness and parental care.
Although most studies of the possible signaling function of plumage coloration have
focused on species that display carotenoid-based
coloration, some investigators have examined
the role of structural coloration (e.g., blue,
green, and iridescent colors) as an indicator of
individual quality. Because structural color depends on feather microstructure (Prum 2006),
nutritional health during molt may affect its
expression (Keyser and Hill 1999, McGraw
et al. 2002). Thus, structurally based plumage
coloration may also function as an honest signal
of individual quality, though the mechanisms of
condition-dependence remain unclear (Keyser
and Hill 2000, Peters et al. 2011).
Our objective was to examine the structural plumage coloration of female Mountain
Bluebirds (Sialia currucoides) to determine if
coloration in one or more body regions reflects individual quality in terms of parental
care and reproductive success. Although socially
monogamous, the rate of extra-pair paternity is
high in this species (72% of broods; Balenger
et al. 2009b), thus providing the opportunity
for sexual selection. Because both sexes contribute to offspring provisioning and territory
defense (Gibson and Moehrenschlage 2008),
mutual mate choice is expected to be beneficial (Clutton-Brock 2009). Thus, Mountain
Bluebirds are well suited for studying the
role of structural plumage in mate choice
and sexual selection. In closely related Eastern

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A. Morrison et al.

Bluebirds (Sialia sialis), female color appears
to be correlated with condition, reproductive
success, maternal care, and offspring quality
(Siefferman and Hill 2005). However, in a
Wyoming, USA, population of Mountain Bluebirds, no relationship between provisioning and
female rump coloration was detected (Balenger
et al. 2007). In the same Wyoming population,
however, coloration of male Mountain Bluebirds
did predict reproductive success (Balenger et al.
2009a). Thus, a second aim of our study was to
determine if the relationships (or lack thereof )
among plumage coloration, provisioning, and
reproduction found in Wyoming (Balenger et al.
2007, 2009a) are also found in a British
Columbia population facing different ecological and evolutionary pressures. In addition, we
examined two plumage regions (tail and rump)
that are grown under different conditions and
may contain differing signal content.
METHODS

Our study was conducted during the 2011
and 2012 breeding seasons (May–July) in the
Knutsford area, near Kamloops, BC, Canada
(803–953 m asl; 50°37 N, 120°19 W). Nest
boxes were monitored every 1–3 d to determine
first egg date, clutch size, hatch date, number
of nestlings, and fledging success (nests empty
at the putative fledge date were considered to
have successfully fledged; number of fledglings
was estimated as the number of nestlings
present during the last visit prior to fledging).
Five to 10 d after eggs hatched, adult males
(N = 54) and females (N = 60) were captured
at nests while provisioning young. Adults were
banded with a single Canadian Wildlife Service
aluminum band and a unique combination of
three color bands. We classified adults as either
second-year (SY) or after-second-year (ASY) by
examining molt limits of the primary and greater
coverts (Pyle 1997). We collected 10 rump
feathers and a single tail feather (R3) from each
individual. We measured mass, unflattened wing
chord, tail length, and tarsus length to evaluate
body size and condition.
Parental care. We measured the level of
parental care provided by males and females
during the early nestling stage (3–5 d after hatching) and a late stage (14–16 d after hatching),
following Balenger et al. (2007). Nests were
watched either by making recordings using video

J. Field Ornithol.

cameras (Handycam DCR-SX45, Sony, Tokyo,
Japan, and HD Hero2, GoPro, San Mateo, CA)
or via focal observation periods lasting 2 h at
each nest. All nest watches occurred between
06:20 and 11:15 (mean = 08:48 ± 92 [SD]
min) during both the early and late stages
(4 h/nest). When using video cameras, cameras
were placed in a plywood box (to avoid theft) on
a fencepost 5–10 m from nests. Parental care
was quantified as the number of trips to nests per
hour per nestling (Smiseth et al. 2001). Similar
to Balenger et al. (2007), we also recorded the
time per hour that females spent at nests during
the early phase, which we interpreted as time
spent brooding. (Nest visits that lasted <30 s
were not recorded as brooding.) We checked and
recorded the number of nestlings each time we
video-recorded nests. When analyzing videos,
we began the 2 h of measurements after the first
trip to a nest by either the male or female. We
avoided capturing adults at nests within 48 h
preceding measuring nestling provisioning rates.
Color analysis. We quantified plumage
coloration by measuring reflectance across the
avian visual range (300–700 nm) using a spectrometer (JAZ, Ocean Optics, Dunedin, FL)
with a xenon light source. The fiber optic probe
was housed in a non-reflective probe holder
to consistently measure feathers at a 90° angle and 5.9 mm from feathers. We mounted
rump feathers in an overlapping pattern on
ebony paper (#142, Canson, Inc., South Hadley,
MA). We took 10 readings for each plumage
region (tail [R3] and rump) at haphazard locations along the feather. We calculated three
different color variables (brightness, UV-blue
chroma, and hue) using the R-based color
analysis program RCLR v.28 (Montgomerie
2008) and averaged the value for each variable over the 10 measurements. Brightness was
measured as the percent reflectance averaged
across the avian visual spectrum (300–700 nm).
Chroma was measured as the proportion of
total reflectance within both the blue range
(400–510 nm) and the ultraviolet (UV) range
(300–400 nm) relative to total light reflected
(300–700 nm). Hue was determined as the
wavelength at maximum reflectance. Due to a
high degree of colinearity among these color
variables, the three were examined together
using principal components analysis in JMP
10 (SAS Institute 2012). Because the first
principle component (PC1) explained most

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Table 1. Results from a principle components analysis of measures of plumage coloration (brightness, hue,
and chroma) for rump and tail coloration of male and female Mountain Bluebirds. Because the first principle
component (PC1) explained most of the variation, we used PC1 to represent overall color variation.

Male tail PC1

Eigenvalue
1.99

Proportion of variance
0.66

Female tail PC1

1.97

0.66

Male rump PC1

1.5

0.50

Female rump PC1

1.87

0.62

of the variation for each plumage area (Table 1), we used variation in PC1 to represent overall color variation. PC1 scores loaded
positively on brightness and UV-blue chroma
and negatively on hue (lower hue values indicate a shift toward UV); (Table 1). No
color variables differed between years (male tail
PC1: t53 = 0.8, P = 0.41, male rump PC1:
t53 = 0.4, P = 0.67, female tail PC1: t59 = −1.7,
P = 0.09, female rump PC1: t59 = 0.2, P = 0.86)
so data for 2011 and 2012 were combined for
subsequent analyses.
Statistical analyses. Statistical analysis
was performed using JMP 10 (SAS Institute
2012). All analyses were performed separately
for males and females. Linear mixed models with
individual as a random effect (because six males
and six females were present in both years) were
used to determine relationships between color
and (1) body size and condition, (2) clutch size,
(3) number of young fledged, (4) provisioning
rates during the early nestling period (3–5 d after
hatching), and (5) provisioning rates during the
late nestling period (14–16 d after hatching).
Differences in sample sizes between early and
late stages were due to nest failures that occurred
between stages. We used analysis of covariance
with year as a fixed effect to examine whether
the time or Julian date of a nest watch predicted
provisioning rates. Because there was a relationship between early female provisioning and
nest watch date (see Results), we included date
as a covariate in subsequent analyses examining factors predicting early female provisioning.

Color variable
Brightness
UV + blue chroma
Hue
Brightness
UV + blue chroma
Hue
Brightness
UV + blue chroma
Hue
Brightness
UV + blue chroma
Hue

Factor loading
0.82
0.87
−0.75
0.62
0.87
−0.91
0.29
0.90
−0.78
0.37
0.94
−0.92

When age classes were separated for analyses
using tail PC1 (due to differences in tail PC1
between ASY and SY females), linear mixed
models were used with ASY females, whereas
linear regression was used with SY females because repeated samples with SY females were
not possible. Mixed models were also used to
determine if there were differences between ASY
and SY individuals in plumage coloration, body
size, clutch size, fledging success, and parental
care. Prior to analysis, we conducted a square
root transformation on clutch size, but nontransformed data are shown in Figure 1. Values
are presented as means ± SD.
RESULTS
Differences between age classes. ASY
females (0.26 ± 0.21) had higher tail PC1 scores
than SY females (−0.66 ± 0.33; F1,38.2 = 9.4,
P = 0.004; Fig. 2), but the rump coloration
of ASY and SY females did not differ (F1,56.5 =
0.4, P = 0.55; Fig. 2). ASY (N = 41) and SY
(N = 13) males did not differ in either rump
(PC1: F1,51.0 = 2.4, P = 0.13) or tail (PC1: F1,51.8
= 0.9, P = 0.36) coloration (Fig. 2). In addition,
ASY and SY females did not differ in size (wing
chord, tarsus, and tail length; all P  0.12), first
egg date, clutch size, number of young fledged,
early or late provisioning rates, or duration of
brooding during the early period (all P  0.22).
ASY males (N = 46; mean = 116.6 ±
2.1 mm) had longer wing chords than SY
males (N = 13; mean = 114.8 ± 3.0 mm)

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J. Field Ornithol.

Clutch Size

7
6
5
4
3

-4

-3

-2

-1
0
1
Female Tail PC1

2

3

Fig. 1. Female Mountain Bluebirds with brighter, more UV-blue tail coloration had larger clutches, but this
relationship remained significant only for ASY females after separating by age class (black circles: second-year
[SY] females, white circles: after-second-year [ASY] females). Best fit lines are shown for SY (black) and ASY
(gray) females.

(F1,46.8 = 8.9, P = 0.005), but did not differ
in tarsus or tail length (P  0.26), first egg
date, clutch size, number of young fledged, or
early provisioning rates (all P  0.16). However,
SY males provisioned at higher rates (N = 11;
mean = 3.5 ± 2.6 trips/h/chick) than ASY males
(N = 25; mean = 2.2 ± 1.4 trips/h/chick)
during the late nestling period (F1,6.4 = 8.1,
P = 0.03).
Plumage color and body size. Tail PC1
was positively associated with rump PC1 in
females (F1,57.8 = 4.4, P < 0.0001, N = 60), but
not males (F1,51.7 = 0.4, P = 0.54, N = 54). We
found a significant positive correlation between
female tail PC1 and tail length for SY females
(r16 = 0.33, P = 0.015), but not ASY females:
F1,35.5 = 0.5, P = 0.48). No other relationships
between color and body size were significant
(Table 2).
Plumage color and reproduction. ASY
females with higher tail PC1 scores had larger
clutches (F1,37.1 = 4.4, P = 0.04, N = 43), but the
relationship was marginal and non-significant
for SY females (r2 = 0.21, P = 0.07, N =
17). There was a non-significant trend toward
higher rump PC1 scores for ASY females with
larger clutches (F1,39.9 = 4.4, P = 0.06, N =
43), but no such trend existed for SY females
(r2 = 0.01, P = 0.71, N = 17). We found no
relationship between female plumage coloration
(tail or rump) and either first egg date or number

of young fledged (all P  0.11). For males (N =
54), we found no relationship between either
tail or rump coloration and clutch size, first egg
date, or number of young fledged (all P  0.24).
Parental care. We found no effect of time
of day on provisioning rates of males and females
during the early or late periods (all P  0.42).
Similarly, date had no effect on provisioning
rates of males during the early period, and both
males and females during the late period (all P 
0.35). However, during the early period, females
had higher provisioning rates later in the season
(F1,52 = 11.9, P = 0.001). Thus, for subsequent
analyses of female provisioning during the early
period, we also include models with date as a
covariate.
Provisioning rates of mated pairs were positively correlated during the early nestling stage
(male mean = 1.4 ± 0.8 trips/h/chick, female
mean = 1.3 ± 0.7 trips/h/chick; r52 = 0.41,
P = 0.002), but not the late nestling stage (male
mean = 2.6 ± 1.9 trips/h/chick, female mean =
2.8 ± 1.6 trips/h/chick; r35 = 0.11, P = 0.52).
Provisioning rates of males and females did not
differ during either the early (t52 = −0.5, P =
0.59) or late (t35 = 0.7, P = 0.52) nestling stages.
We found no relationship between fledging
success and provisioning rates for either males or
females (including model with date) during the
early nestling stage (all P  0.43) or for females
during the late nestling stage (F1,34.0 = 3.6,

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Vol. 85, No. 2

A. Female tail color

173

B. Male tail color

3
4

1

Tail PC1

Tail PC1

2

0
-1

2
0

-2
-3

-2

ASY

Age

ASY

SY

C. Female rump color

D. Male rump color

3

2

2

SY

1

1

Rump PC1

Rump PC1

Age

0
-1

0
-1
-2

-2

-3

-3

ASY

ASY

SY
Age

SY
Age

Fig. 2. (A) Older, after-second-year (ASY) female Mountain Bluebirds (N = 43) had brighter, more UV-blue
tail coloration than second-year (SY) females (N = 17), but (C) rump coloration did not differ with female
age. Neither tail (B) nor rump (D) coloration differed with male age. Brightness, chroma, and hue were
examined together using principal components analysis and, because the first principle component (PC1)
explained most of the variation, we used PC1 to represent overall color variation. Boxes illustrate median
(thick line) and quartiles (box), whiskers are defined as the upper quartile plus 1.5 times the interquartile
distance (IQD) and lower quartile minus 1.5 times the IQD, and open circles represent individuals that fell
outside this range.
Table 2. Relationships between plumage coloration and body size of male and female Mountain Bluebirds.
Correlation coefficients are shown with sample sizes in parentheses.

Male tail color
Male rump color
Female tail color
Female rump color
a

Mass
−0.07 (30)
0.03 (30)
−0.04 (30)
0.29 (30)

Wing chord
−0.04 (56)
−0.02 (56)
0.00 (60)
−0.02 (60)

Tarsus length
−0.03 (55)
−0.01 (55)
−0.04 (59)
0.10 (59)

Tail length
−0.14 (56)
−0.19 (56)
0.28 (59)a
0.16 (59)

Significant at ␣ = 0.05.

P = 0.06, N = 36). However, fledging success was negatively related to male provisioning
rate during the late nestling stage (F1,31.9 =
6.9, P = 0.01, N = 36). Provisioning rates

during early and late periods were correlated
for males (F1,26.0 = 14.9, P = 0.0007, N =
36), but not females (F1,28.3 = 1.4, P = 0.24,
N = 36).

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3

Male Rump PC1

2
1
0
-1
-2
-3
-4
-4

-3

-2

-1

0

1

2

3

Female Tail PC1

Fig. 3. Male Mountain Bluebirds with brighter, more UV-blue rump coloration mated with females with less
bright, UV-blue tail color, but, analyzing by age class (dark circles: second-year [SY] females, white circles:
after-second-year [ASY] females), this relationship remained significant only for SY females. Best fit lines are
shown for reference (black: SY females, gray: ASY females).

During the early nestling stage, there was
no relationship between the tail PC1 scores
and nestling provisioning rates of females (SY:
r16 = 0.29, P = 0.25; ASY: F1,17.1 = 1.3, P =
0.26). We found no relationship between female
tail PC1 scores and either male provisioning
rates during the early and late nestling stages or
female provisioning rates during the late nestling
stage (all P  0.17). In addition, there was no
relationship between female rump PC1 and the
provisioning rates of males and females during
either nestling stage (all P  0.08). Similarly,
we found no relationship between either male
tail or rump PC1 scores and the provisioning
rates of males and females during either nestling
stage (all P  0.38), or between time spent
brooding during the early nestling phase and
either female tail PC1 or female rump PC1 (both
P  0.37).
Assortative mating. We found a significant negative correlation between SY female
tail PC1 scores and male rump PC1 scores
(r16 = −0.61, P = 0.01), although this relationship was not observed with ASY females (ASY:
r31 = −0.29, P = 0.11; Fig. 3). There was
no evidence of assortative mating for any other
color variables, including female tail PC1 and
male tail PC1, female rump PC1 and male tail
PC1, and female rump PC1 and male rump PC1
(all P  0.08). We found a positive correlation
between male and female tarsus length (r50 =

0.52, P < 0.0001), but not mass (r23 = 0.33,
P = 0.12), wing chord (r52 = 0.13, P = 0.37),
or tail length (r51 = −0.05, P = 0.74). We also
found no evidence of assortative mating by age
(␹ 2 = 2.3, P = 0.13, N = 53).
DISCUSSION

The Mutual Selection Hypothesis predicts
that more highly ornamented females should
provide more care (e.g., Linville et al. 1998,
Siefferman and Hill 2005, but see Smiseth and
Amundsen 2000), mate assortatively with highly
ornamented males (MacDougall and Montgomerie 2003, Jawor et al. 2004, Griggio et al.
2005), and generally have higher reproductive
success (Doutrelant et al. 2008, Henderson et al.
2013). Our results were mixed with respect to
the Mutual Selection Hypothesis; we found no
relationship between female color and provisioning rate, and evidence of negative assortative
mating by SY females. Similarly, Balenger et al.
(2007) found no relationship between parental
care and plumage coloration for either male
or female Mountain Bluebirds. We did find
that ASY female Mountain Bluebirds exhibited
brighter, more UV-blue (higher PC1 scores)
tail feathers than SY females. In addition, tail
coloration of SY females was positively correlated with body size, and ASY female Mountain
Bluebirds with higher tail PC1 scores produced

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Female Mountain Bluebird Coloration

larger clutches. Thus, although our evidence that
structural plumage coloration of female Mountain Bluebirds is a signal of direct reproductive
benefits for males (e.g., higher reproductive
success) is limited, our results (i.e., ASY females
with brighter tails than SY females, tail color of
SY females correlated with size, and ASY females
with brighter tails had larger clutches) do suggest
the potential for sexual selection to act on female
coloration.
ASY female Mountain Bluebirds in our study
had brighter, more UV-blue tail coloration than
SY females, but we observed no age-specific
differences in males. Age-related differences in
female plumage have been reported in several
songbird species (Flood 2002, Morales et al.
2007, del Val et al. 2010, Winkler et al.
2011, Osmond et al. 2013). Among bluebirds, Siefferman and Hill (2005) found no
significant differences between age classes in
plumage coloration of female Eastern Bluebirds, but the UV-blue coloration of male Eastern and Western (Sialia mexicana) bluebirds
was found to increase in brightness with age
(Siefferman et al. 2005, Budden and Dickinson 2009). In addition, Balenger et al. (2007)
found that older female Mountain Bluebirds had
marginally higher rump PC1 scores (P = 0.08).
Variation among bluebird species and populations with respect to the presence or absence of
age-specific variation in plumage coloration of
males and females could be caused by a variety of
proximate, ecological (e.g., nutrient limitation
and molt constraints), or ultimate, evolutionary
(e.g., mate choice and age-related dominance)
factors. Differences in tail coloration of SY and
ASY female Mountain Bluebirds in our study
could be due to differences in how food is
obtained during molt (i.e., older birds would
have been foraging independently, whereas SY
birds would have been fed by their parents), and
the time when feathers are grown. For example,
because nestling tail feathers are grown several
months prior to adult tail feathers (Power and
Lombardo 1996, Pyle 1997), the feathers of SY
birds would likely experience more wear.
Age-related plumage variation and delayed
plumage maturation (DPM) is relatively common among male songbirds, but less so for
females (Hawkins et al. 2012). However, female Tree Swallows exhibit pronounced DPM,
whereas males exhibit more moderate changes in
plumage coloration (Bitton and Dawson 2008);

175

the duller coloration of young female swallows
appears to reduce conspecific aggression during
the breeding season (Coady and Dawson 2013).
Although female Mountain Bluebirds do not
exhibit DPM, reduced ornamentation of SY
female Mountain Bluebirds could also function
to reduce intrasexual aggression from older females. Importantly, we observed no difference
between rump PC1 scores of ASY and SY males
or females. In contrast to tail feathers, contour
feathers of SY bluebirds, such as those on the
rump, are grown at about the same time as those
of ASY bluebirds (late summer) and are thus subject to the same constraints for both age classes
(Pyle 1997). Thus, if molt timing is a critical
component of plumage expression, this could
explain why we observed age-specific differences
in tail, but not rump, coloration. Regardless of
the mechanism(s) underlying plumage differences between ASY and SY females, males that
could differentiate female age by color could potentially benefit because older, more experienced
individuals tend to have higher reproductive
success than first-time breeders in other species
(Forslund and Pärt 1995, Martin 1995, Fowler
1995), including Eastern Bluebirds (Siefferman
and Hill 2005). We did not, however, observe
age-specific variation in reproductive success in
our study. Other than tail coloration, we found
no other significant differences between female
age classes in either individual characteristics
or reproductive variables. It is possible that
age effects only manifest under more extreme
conditions, such as drought or food limitation.
In addition, the presence and use of nest boxes
may artificially increase mating opportunities
for young females, thus masking potential age
effects on reproduction (Robertson and Rendell
1990).
We found no relationship between female
plumage coloration and either first egg dates or
fledging success, but observed a positive relationship between female tail color and clutch size
(though this relationship was marginal for SY
females, P = 0.07), suggesting a potential benefit
for males that mate with more colorful females.
These results are consistent with those of previous studies, suggesting that relationships between female plumage coloration and reproductive variables, such as first egg date (Siefferman
and Hill 2005, Osmond et al. 2013), may
provide evidence of selection on female plumage
coloration (but see Amundsen and Pärn 2006).

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A. Morrison et al.

The only relationship we observed between
body size and plumage coloration was between
SY female tail color and tail length, which
could indicate that SY female coloration is
dependent on nesting conditions the previous
year and the parental care they received while
developing tail feathers. Balenger et al. (2007)
found no relationship between color and body
size of female Mountain Bluebirds, but did observe a significant positive relationship between
plumage coloration and wing length in males.
One possible explanation for the difference
between these studies could be nutrient limitation; ecological conditions in the Wyoming
population studied at low and high elevation
sites in the Bighorn Mountains may be variable
and contribute to differences in the ability of individuals to produce longer, more colorful wing
feathers. Positive relationships between body size
and plumage coloration in either sex may indicate better nutritional condition during molt
or development and condition-dependence of
plumage coloration. For example, male Eastern
Bluebird nestlings in experimentally reduced
broods were in better condition (i.e., greater
mass) and grew brighter UV-blue flight feathers
(Siefferman and Hill 2007). If the plumage of female Mountain Bluebirds is similarly conditiondependent, the opportunity exists for sexual
selection to occur.
Our provisioning rate data did not support
the Good Parent Hypothesis, which suggests
that more colorful females should provision offspring at higher rates. Balenger et al. (2007) also
found no relationship between female plumage
coloration and provisioning rates in Mountain
Bluebirds. The lack of a relationship between
plumage coloration and provisioning may be
interpreted as indicating little or no sexual selection on female color. However, we did observe a
positive relationship between ASY female color
and clutch size.
We also observed negative assortative mating
by plumage coloration (for SY females) and positive assortative mating by tarsus length. In previous studies, positive assortative mating by body
size (Delestrade 2001) and color (MacDougall
and Montgomerie 2003, Jawor et al. 2004,
Griggio et al. 2005) has been proposed as providing indirect evidence of sexual selection acting
on females, with brighter color and larger body
size potentially functioning as signals either
during female-female competition or male mate

J. Field Ornithol.

choice (Kraaijeveld et al. 2007). Surprisingly,
rather than positive assortative mating, which
could provide further evidence of positive selection on female coloration, we found evidence of
negative assortative mating, i.e., SY females with
lower PC1 scores paired with males with higher
rump PC1 scores. Few examples of negative assortative mating have been reported (Houtman
and Falls 1994, Nisbet et al. 2007) and recent
work suggests that most, if not all, cases of such
mating may be a result of Type 1 errors (Jiang
et al. 2013). Our small sample sizes and the fact
that the relationship was significant only for SY
females may support this interpretation.
We did find evidence of positive assortative
mating by tarsus length, and previous investigators have noted that assortative mating is
often only observed in one or a small subset of
body size measurements (Wagner 1999, Forero
et al. 2001, Einoder et al. 2008). Furthermore,
tarsus length, the only measure of skeletal body
size we recorded, is generally considered the
best measure of body size in birds (Rising
and Somers 1989), suggesting that male mate
choice for females may be driven primarily by
preference for larger size, rather than plumage
coloration. Previous work with other species
has revealed that female body size may be a
good predictor of breeding success (Choudhury
et al. 1996), and may thus present an advantage for males pairing with larger females.
However, we found no evidence of increased
reproductive success for larger females in our
study.
In summary, we found that brighter, more
UV-blue female tail coloration was related to
both female age and the ability to produce larger
clutches by ASY females, suggesting that males
may be able to use this information to assess
the age, and perhaps quality, of potential mates.
Surprisingly, however, we also found evidence
of negative assortative mating on the basis of
plumage color, and a weak negative relationship
between female plumage coloration and parental
care, suggesting the possible existence of nonadaptive genetic correlation between the sexes
(Lande 1987, Amundsen 2000, Amundsen and
Pärn 2006). Due to the conflicting results of the
potential signalling function of female plumage
coloration in our study and in the literature,
more studies are clearly needed, especially studies that explore the potential signalling function
of female plumage in intrasexual interactions

Vol. 85, No. 2

Female Mountain Bluebird Coloration

during both the breeding and non-breeding
seasons.
ACKNOWLEDGMENTS

This study would not have been possible without the
support from the members of the Kamloops Naturalist
Club who allowed us to use their bluebird trails. Many
thanks to S. Joly for helping us capture and band
bluebirds. Special thanks to G. Dreger for his extensive
knowledge of, and enthusiasm for, Mountain Bluebirds
and for use of his route. Thanks also to L. Dixon and P. and
J. Gray for their support and use of their bluebird route.
The Natural Sciences and Engineering Research Council
of Canada Discovery Grant (MWR), British Ecological
Society (AM), Sigma Xi (AM), the North American
Bluebird Society (AM), and the TRU Comprehensive
University Endowment Fund (AM) provided financial
support that was instrumental to completing our research.
We also thank T. Dickinson, S. McArthur, and D. Pouw
for intellectual and technical assistance on this project.

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