EFFECTS OF WEATHER AND AGE ON FEATHER COLOURATION IN MOUNTAIN
BLUEBIRDS

by

GENEVIEVE MICHELLE WARD

A THESIS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF

BACHELOR OF SCIENCE (HONS.)
in the
DEPARTMENTS OF BIOLOGICAL AND PHYSICAL SCIENCES
(Chemical Biology)

This thesis has been accepted as conforming to the required standards by:
Matthew Reudink (Ph.D.), Thesis Supervisor, Dept. Biological Sciences
Kingsley Donkor (Ph.D.), Co-supervisor, Dept. Physical Sciences
Nancy Flood (Ph.D.), External examiner, Dept. Biological Sciences

Dated this 22nd day of May, 2020, in Kamloops, British Columbia, Canada

© Genevieve Michelle Ward, 2020

ABSTRACT
Birds exhibit a vast array of colours and ornaments and while much work has focused on
understanding the function and evolution of carotenoid-based colours (red, orange, yellow),
structural colouration (blue, green, purple, iridescent) can also play a key role in sexual signaling
by acting as an indicator of individual quality. While several studies have examined how factors
such as diet and age may influence structural colour, few studies have looked at how structural
colour may be influenced by variation in weather conditions experienced during the annual moult.
In this study, we examine variation in structural colour expression in relation to age as well as
rainfall and temperature during moult for a population of Mountain Bluebirds (Sialia currucoides)
breeding in Western Canada. We looked at feather colouration over a period of nine years by using
reflectance spectrometry of rump and tail feathers from male and female Mountain Bluebirds and
examined the relationship between feather colour and temperature and rainfall from the previous
breeding season as well as age-related colour changes. At a population level, we found that feathers
from males and females were more colourful (higher brightness and chroma, hue values shifted
more towards UV) as birds aged from juveniles to adults. Within individuals, after they reached
adulthood (second breeding season and beyond), male bluebird rump and tail feathers became less
colourful. At a population level, male Mountain Bluebirds exhibited more colourful feathers
following years with higher August rainfall, while female Mountain Bluebirds expressed more
colourful tail feathers after years with higher July and August rainfall. However, these effects were
dependent on age. Young female Mountain Bluebirds were most affected by weather conditions,
exhibiting more colourful structural plumage after years with higher rainfall and higher August
temperatures. We suggest that higher rainfall may increase insect abundance and thus improve
food intake and overall condition of Mountain Bluebirds, and we suggest that males and females

ii

may be affected differently due to differences in behavior during the breeding and post breeding
periods. This is one of the first studies to show how both age and weather conditions may affect
structural colours in birds.
Thesis Supervisor: Matthew Reudink (Ph.D.), Dept. Biological Sciences
Co-Supervisor: Kingsley Donkor (Ph.D.), Dept. Physical Sciences

iii

ACKNOWLEDGEMENTS
I would like to thank Dr. Matthew Reudink for his teaching and support throughout this project,
and I would like to thank Dr. Sean Mahoney for all his help during the project. I also want to thank
Stephen Joly for his guidance in the field, and Dr. Kingsley Donkor for his role as co-supervisor.
Thank you to the Natural Sciences and Engineering Research Council of Canada for providing
funding for this project through my Undergraduate Student Research Award as well as Dr.
Reudink’s Discovery Grant. Finally, thank you to my parents, Phil and Barb, for their unwavering
help and support throughout this and every other project I attempt.

iv

TABLE OF CONTENTS

ABSTRACT ................................................................................................................................... II
LIST OF FIGURES .................................................................................................................... VI
LIST OF TABLES ..................................................................................................................... VII
INTRODUCTION......................................................................................................................... 1
MATERIALS AND METHODS ................................................................................................. 6
FIELD METHODS ..................................................................................................................... 6
FEATHER COLOUR ANALYSIS ............................................................................................. 7
WEATHER DATA ..................................................................................................................... 8
STATISTICAL ANALYSIS ....................................................................................................... 9
RESULTS .................................................................................................................................... 10
POPULATION-LEVEL CHANGES OVER TIME: SY VS. ASY .......................................... 10
WITHIN-INDIVIDUAL CHANGES OVER TIME................................................................. 11
YEARLY VARIATION ........................................................................................................... 13
WEATHER EFFECTS .............................................................................................................. 14
DISCUSSION .............................................................................................................................. 20
LITERATURE CITED .............................................................................................................. 26

v

LIST OF FIGURES
Figure 1. ASY females had greater tail PC1 values than SY females ......................................... 10
Figure 2. ASY males had greater tail PC1 values than SY males ............................................... 11
Figure 3. Within-individuals, there was no significant difference in PC1 tail values for ASY1 and
ASY2 males. ................................................................................................................................. 12
Figure 4. Within-individuals, ASY3 males had lower tail PC1 values than ASY1 males. ......... 12
Figure 5. Yearly colour variation in female Mountain Bluebirds. ............................................... 13
Figure 6. Yearly colour variation in male Mountain Bluebirds. .................................................. 13

vi

LIST OF TABLES
Table 1. Results from a principal components analysis of three measures of rump and tail plumage
colouration (brightness, hue and chroma). ...................................................................................... 8
Table 2. Tail and Rump PC1 value variation across years........................................................... 14
Table 3. Final best fit models examining the effects of weather and age (SY/ASY) on male
colouration. ................................................................................................................................... 16
Table 4. Final best fit models showing the effects of weather and age (SY/ASY) on female
colouration. ................................................................................................................................... 17
Table 5. Final best fit models showing the effects of weather and age class (SY and ASY1) on
female feather colouration. ........................................................................................................... 18
Table 6. Final best fit models showing the effects of weather and age class (ASY2, ASY3, ASY4)
on female feather colouration. ...................................................................................................... 19

vii

INTRODUCTION
Scientists and naturalists alike have long been fascinated by birds, due in part to the
incredible diversity of their colours and ornaments and many have spent decades unravelling the
evolution of such vibrant plumages. Sex-specific differences in colour are often driven by sexual
selection and are important signals for mate choice (see Hill 2006a, Robinson et al. 2012). Females
are typically the choosy sex and more colorful males tend to experience greater reproductive
success (see Hill 2006a, Siefferman et al. 2005). However, bright colours also come with costs.
Greater ornamentation is typically associated with higher risks, as it is physiologically more
challenging to produce (see Hill 2006b) and may increase the risk of predation (see Zuk and
Kolluru 1998; Promislow 1992). As a result, brighter colours tend to act as honest indicators of
health/condition and overall individual quality (see Hill 2006a). A greater degree of ornamentation
has been associated with more parental efforts in female birds (Siefferman and Hill 2005), success
in male-male competition (Robinson et al. 2012), better individual health (Siefferman et al. 2005),
and ultimately greater breeding success (Hill 2006b). While many studies focus on the impacts of
male coloration on breeding success, female ornamentations can also act as important inter- and
intrasexual signals (reviewed in Hill 2006a, Santos et al. 2011).
In birds with carotenoid-based plumage, feather colouration is closely linked to nutrient
access, as ingested carotenoids can be directly deposited into feathers or metabolically converted
into orange or red pigments and subsequently deposited into growing feathers (Hill and
Montgomerie 1994). Birds are incapable of producing carotenoid pigments themselves; instead,
they must ingest carotenoids by consuming plants or insects that have consumed carotenoidcontaining plants (reviewed in McGraw 2006, and see Eeva et al. 2010). Carotenoid-based
pigmentation is thus dependent on the individual bird’s access to nutrients (reviewed in Hill and
1

McGraw 2006). However, because carotenoids also have other important physiological roles as
antioxidants and immunostimulants birds face a trade-off between utilizing carotenoids for selfmaintenance and ornamentation (McGraw 2006). Furthermore, the metabolic conversion of
dietary carotenoids to orange/red pigments appears to incur further costs, indicating that the
display of pigments from derived carotenoids may be the best honest indicators of individual
condition (Weaver et al. 2018).
In birds with structurally based plumage (e.g., blue, purple, iridescent colours) a clear link
between diet and feather colouration is not apparent because structural colours do not come directly
from ingested nutrients (see Hill and McGraw 2006). Whereas carotenoid-based colours are
dependent on the intake of carotenoids, structural colours such as UV-blue colouration are
produced by the arrangement of melanosomes (produced in vivo) within the feather microstructure
and the scattering of light as it passes through nanoscale variations in the feather structure (Prum
2006). At the nanoscale level, feather barbules can contain a matrix of air and biological structures,
such as proteins and pigment granules, like melanin (Prum 2006). As light strikes the feather and
passes through these biological structures, it encounters different refractive indices, causing some
of the light waves to scatter in different directions (Prum 2006). Thus, the presence and spatial
distribution of miniscule biological structures, such as melanosomes, proteins, and pigment
granules, and their respective refractive indices in the feather determine how light waves are
scattered and what colour results (Prum 2006).
Despite the lack of a direct mechanism linking nutrient ingestion and structural colours,
numerous studies have found evidence that structural colours are linked to individual condition
and quality: a study by Siefferman and Hill (2005) found that female Eastern Bluebirds (Sialia
sialis) on a food-restricted diet displayed less-colourful rump feathers than females that were given

2

free access to food, and a study by Doyle and Siefferman (2014) found that male Eastern Bluebird
nestlings that were fed supplemental mealworms exhibited brighter plumage. While they did not
directly manipulate diet, a study by Jacot and Kempenaers (2007) on Blue Tits (Cyanistes
caeruleus) found that nestlings raised in larger broods developed duller feathers than males raised
in smaller broods. There is also evidence that aspects other than diet might influence structural
colour. Harper (1999) found a correlation between lower plumage brightness and higher mite load
in Blue Tits, while Doucet and Montgomerie (2003) found that brighter Male Bowerbirds
(Ptilonorhynchus uiolaceus) had fewer blood parasites.
Besides variation that may occur due to individual quality, other factors like aging may
affect feather colouration. In species that experience delayed plumage maturation, a significant
shift in colour occurs after the birds have undergone their first moult as an adult, typically during
their second year of life (see Lyon and Montgomerie 1986; Hawkins et al. 2012; Lyu et al. 2015).
Even species with a less-stark transition to adulthood show changes as they age, however, and
colour may continue to change after they reach maturity (see Siefferman et al. 2005; Budden and
Dickinson 2009; Delhey and Kempenaers 2006). A study on male Western Bluebirds (Sialia
mexicana) found that UV-blue plumage patches were larger and brighter in older males (Budden
and Dickinson 2009), while a study examining both male and female Blue Tits showed increased
structural chroma and brightness values as they aged (Delhey and Kempenaers 2006). In American
Redstarts (Setophaga ruticilla), Marini et al. (2015) found that the orange plumage exhibited by
adult males became more yellow-shifted as birds aged, which may have indicated a shift in how
carotenoids were metabolized and deposited in older male birds.
For both carotenoid- and structural-based colouration, variation can also be influenced by
geography and weather. A study by Tisdale et al. (2018) found latitudinal differences in

3

ornamentation in Golden-Winged Warblers (Vermivora chrysoptera), with more southern birds
showing less colourful crown and throat feathers, while Gamero et al. (2015) found that feather
brightness in Great Tits (Parus major) was lower in populations that lived at higher elevations.
Even minor geographic variation can have a major impact; Hill (1993) found that male House
Finches (Haemorhous mexicanus) sampled only 12 km apart showed significant variation in
average male colouration. Possible explanations for geographic variation in colouration are
diverse: Hill (1994) suggested genetic drift as possible mechanism, while Gamero et al. (2015)
pointed to higher population density in the southern Great Tit population potentially causing
stronger sexual selection to explain the population differences. Finally, another possible
explanation could be nutrient access: for birds with carotenoid-based feathers, carotenoid
availability within habitats could influence population colouration (Tisdale et al. 2018), as could
nutrient access.
Weather can also play a major role in plumage variation from year to year. Migratory birds
moult annually, usually in late summer, on or near the breeding grounds at the end of the breeding
season (Pyle 1997). Reudink et al. (2015) found that American Redstarts exhibited more colourful
carotenoid-based plumage when moult occurred during years of higher rainfall and temperatures
in July, and lower temperatures in August; they suggested that the most likely mechanism behind
this was variation in insect abundance, as higher levels of rainfall would produce a more abundant
insect population, thus increasing the redstarts’ access to nutrients (and specifically to
carotenoids). A similar mechanism could be at work in birds with structurally based colouration,
as a more abundant insect population could provide more nutrients and improve individual feather
quality during moulting by altering feather nanostructure. While the specific effects of nutrition
on microscopic feather structure are not known, changes in diet could potentially influence the

4

layout and expression of proteins and other biological structures that feather barbules are
composed of, thus changing the feather’s appearance. A study by Shawkey et al. (2003) found that
UV-violet chroma in feathers was associated with a higher number of circular keratin rods, and
suggested that at least some colour variation could be explained by feather nanostructure.
Mountain Bluebirds (Sialia currucoides) possess brilliant UV-blue structural colouration,
and there is evidence that females prefer more colourful males as social and extra-pair partners
(Balenger et al. 2009). While colour seems to be an important factor in mate selection (Balenger
et al. 2009; but see Liu et al. 2007; Liu et al. 2009), little is known about how specific factors like
aging or weather conditions during moult affect feather colouration. And, while male and female
Mountain Bluebirds undergo a significant shift in colouration as they transition from their first
breeding season (second year, SY) to subsequent breeding seasons (after second year, ASY), how
their colour changes after reaching their first season as an ASY (ASY1) remains largely unknown.
While studies have looked at possible effects of aging (Siefferman et al. 2005) and weather
conditions (Warnock 2017) on structurally based plumage colouration separately, few studies have
offered a concurrent examination of both effects. Our goal was to examine Mountain Bluebird
feathers grown over 9 seasons to determine the effects of aging and weather on the expression of
UV-blue structural colouration. We predicted that aging would be correlated with an increase in
ornamentation (higher brightness, greater UV-blue chroma, and more UV-shifted hue) and higher
temperatures and less rainfall would be associated with lower ornamentation in male and female
Mountain Bluebirds.

5

MATERIALS AND METHODS
Field methods
Field work was conducted during breeding seasons (May-July) from 2011-2019 in
Kamloops, British Columbia, Canada (885–1116 m asl; 50°40.23′ N, 120°23.86′ W). Adult male
and female Mountain Bluebirds were captured and banded at nest boxes along routes maintained
by the Kamloops Naturalist Club. Birds were banded with a Canadian Wildlife Service (CWS)
aluminum band and a unique combination of three colour bands. Adults were classified as either
second-year (SY) or after-second-year (ASY) based on moult limits. A bird was classified as
ASY1 if it was captured as an SY in the previous season or was captured for the first time as an
adult. Birds that were initially banded as ASY1 and recaptured the subsequent season were
classified as ASY2, and so on. This approach assumes that birds captured for the first time as an
ASY were actually ASY1 and thus it is likely that some of the bird classified as ASY1 were in fact
older. However, breeding site fidelity tends to be high with Mountain Bluebirds after they reach
ASY and the effects of misclassification, if any, would weaken any observed effects of age and
weather (see also Marini et al. 2015).
Between five and ten rump feathers were collected from each adult, and a single tail feather
(R3) was plucked for colour analysis. Feathers were stored in manila coin envelopes prior to
analysis. Our analysis involved a total of 363 tail feathers collected from 308 individuals, as some
birds were caught more than once. All work was approved by the Thompson Rivers University
Animal Care and Use Committee and was conducted under a Canadian Federal Master Banding
Permit (#10834) and Scientific Collection Permit.

6

Feather colour analysis
Feathers were mounted on low-reflectance black paper and scanned using a JAZ
spectrometer and a PX-2 xenon light source from Ocean Optics (Dunedin, FL). Rump feathers
were placed with an overlapping pattern, mimicking the way the feathers would lie on a bird, while
tail feathers were mounted individually. Ten readings were taken for each plumage area (rump and
tail), haphazardly across the feathers. The probe was held in a non-reflective probe holder at a 90°
angle at a set distance of 5.9 mm. Measurements between each successive tail feather or group of
rump feathers were standardized using an Ocean Optics WS-1 white standard, and a non-reflective
dark standard.
RCLR v.28, an R-based colour analysis program, was used to analyze reflectance
measurements (Montgomerie 2008). A smoothing function was performed on all curves to
eliminate noise (to reduce random effects from other sources of colour that may have been present
when scanning). Using RCLR v.28, brightness, chroma, and hue were calculated for each feather.
Brightness was calculated as the mean amount of light reflected across the mean visual spectrum
(in other words, the area under the reflectance peak). Chroma was calculated as (R300-510/R300-700),
or the area under the peak between wavelengths of 300-500 nm divided by the area under the peak
between wavelengths of 300 and 700 nm. Hue was calculated as the wavelength at maximum
reflectance. The ten readings taken for each plumage area were averaged to produce a single value
for hue, chroma, and brightness for each feather. Due to high collinearity among the three colour
variables, we used principal component analysis to collapse the three variables into a single factor,
and we used the first principal component (PC1) to represent overall plumage colour variation, as
it explained most of the variance among birds with respect to each of the plumage areas (Table 1).

7

A greater PC1 value corresponded to increased brightness and chroma, but a decrease in hue
(meaning maximum reflectance shifted more towards the UV portion of the spectrum).

Table 1. Results from a principal components analysis of three measures of rump and tail plumage
colouration (brightness, hue and chroma).

Male tail PC1

1.53

Proportion
variance
0.51

Brightness
UV + blue chroma
Hue

Factor
loading
0.61
0.57
-0.54

Female tail PC1

1.84

0.61

Brightness
UV + blue chroma
Hue

0.54
0.58
-0.60

Male rump PC1

1.88

0.63

Brightness
UV + blue chroma
Hue

0.47
0.66
-0.59

Female rump PC1

2.11

0.71

Brightness
UV + blue chroma
Hue

0.47
0.64
-0.60

Eigenvalue

of

Colour variable

Weather Data
Rainfall and temperature data were obtained from the Government of Canada website
(https://climate.weather.gc.ca/historical_data/search_historic_data_e.html), which provided data
collected at a weather station in Kamloops. Weather data for the years 2010-2018 were obtained
from Environment and Climate Change Canada for the Kamloops A station, located at the
Kamloops Airport (50.70°N, 120.44°W). In some months, weather data for the station was not
available; instead, data from the Pratt Road weather station, located approximately 20 km SE
(50.60°N, 120.20°W) was used.

8

Statistical Analysis
In our analyses of whether older individuals differed from younger individuals in plumage
coloration at a population level, we analyzed tail and rump PC1 values separately. Males and
females were also analyzed separately, due to the differences in male and female plumage
colouration. We compared SY and ASY age classes for both male and female Mountain Bluebirds
with linear mixed models that included individual as a random effect, using data standardized by
year to account for yearly variation. Data were standardized separately for males and females by
setting the mean to 0 and standard deviation to 1 within each year. This was done to account for
the significant variation among years and allowed us to ask whether different age classes showed,
on average, different plumage colouration, while controlling for environmental effects during
feather growth (Marini et al. 2015).
We next wanted to examine within-individual changes as bluebirds aged. To determine
whether individuals changed over time, we performed paired t-tests on unstandardized PC1 values
for both rump and tail colouration using birds that were caught more than once.
To assess how colour variation between sexes and among age classes was explained by
weather during moulting periods, we related rump PC1 and tail PC1 separately to rainfall and
temperature during June, July, and August from the previous year using mixed effects models. In
each model, we set rump or tail PC1 as the response variable, age, sex, rainfall and temperature as
fixed effects, and individual as a random effect. We built separate models for each month to avoid
problems associated with multiple correlation among months (Reudink et al. 2015). To help with
interpretation, we only allowed two-way interactions between all fixed effects. We then undertook
model reduction for all mixed models based on the change in Akaike information criterion (AIC
Burnham and Anderson 2003) between the full model and each reduced model. We chose our final

9

model based on no significant change in AIC. Using the final model, we then assessed the
relationship between weather (either rainfall or temperature) and colour (rump or tail PC1) using
the R package emmeans, an R-based software package that can generate least-squares means for
linear and mixed models (Lenth 2019). Analyses were performed in R v.3.6.3 (R Development
Core Team 2014).
RESULTS
Population-level changes over time: SY vs. ASY
At a population level, when we examined all individuals captured from 2011-2019 using a
linear mixed model with individual as a random effect, SY birds had tail feathers with significantly
lower tail PC1 values than ASY birds for both males and females (male: F1,173 = 22.07, p < 0.0001,
Figure 2; female: F1,183.4 = 11.20, p = 0.001, Figure 1). However, neither males nor females differed
in rump PC1 between SY and ASY age classes (male: F1,175.1 = 2.5864, p = 0.1096; female: F1,182.2
= 0.2192, p = 0.6402).

Figure 1. ASY females had greater tail PC1 values than SY females

10

Figure 2. ASY males had greater tail PC1 values than SY males

Within-individual changes over time
When we examined within-individual changes in tail PC1 values, we found no difference
between individuals in year ASY1 and year ASY2 and this pattern held true examining both
standardized (female: t1,16 = -0.36, p = 0.72; male: t1,14 = -0.41, p = 0.69) and non-standardized
colour values (female: t1,16 = 0.14, p = 0.89; male: t1,14 = -0.27, p = 0.79, Figure 3). There were too
few recaptures of individuals first captured as an SY and subsequently recaptured as an ASY to
perform statistical analyses within individuals (female n = 3, male n = 5).

11

Figure 3. Within-individuals, there was no significant difference in PC1 tail values for ASY1 and
ASY2 males.
Sample sizes were too small (n = 4 males, 4 females) to examine changes from ASY2 to
ASY3. However, when we examined changes from ASY 1 to ASY 3 using non-standardized
values, we found differences in tail colours in males (t1,7 = 2.53, p = 0.04) and in rump colour
values (t1,7 = -3.18, p = 0.02). We found no difference when we examined standardized values for
rump colour in males (t1,7 = -1.42, p = 0.20), however, we still found a difference between the tails
of ASY1 and ASY3 birds using standardized values for colour in males (t1,7 = -2.60, p = 0.04,
Figure 4). Sample size was too small to examine changes from ASY1 to ASY3 in females (n = 5).

Figure 4. Within-individuals, ASY3 males had lower tail PC1 values than ASY1 males.
12

Yearly Variation
When we examined yearly variation in plumage colouration (unstandardized colour
values), all age and sex classes varied significantly across years (Table 2, Figures 5 and 6). In
Table 2, df stands for degrees of freedom, F is the F-statistic, and p is the probability value.

Figure 5. Yearly colour variation in female Mountain Bluebirds.

Figure 6. Yearly colour variation in male Mountain Bluebirds.

13

Table 2. Tail and Rump PC1 value variation across years.
Tail PC1

Rump PC1

Age/Sex Class

df

F

p

df

F

p

ASY Female

8, 114.5

3.14

0.003

8, 126.4

8.27

<0.0001

SY Female

8, 41

5.75

<0.0001

8, 41

5.6

<0.0001

ASY Male

8, 101.8

3.57

0.001

8, 101.2

6.12

<0.0001

SY Male

8, 511

2.93

0.009

8,51

4.69

0.0002

Weather effects
To test the combined effects of weather and age on colouration, we conducted models
separately for males and females and conducted one analysis in which birds were classified only
as SY/ASY and a second analysis in which birds were classified by age class (e.g., ASY1, ASY2).
For our first analysis using only SY/ASY classification, we found different effects for both males
and females and significant interactions between weather and age. For males, more colourful tail
feathers were correlated with higher levels of rainfall in August, but only for ASY males (Table
3). For females, higher rainfall in July was associated with more colourful rump feathers for both
SY and ASY females, but the strength of this effect was much stronger in SY females (Table 4).
For our second analysis that included all age classes (ASY1, ASY2, etc.), we observed no
associations between age class or feather colour in males. In females, higher rainfall in July
positively influenced tail feather colour for both ASY1 and SY birds, but not older age classes
(Table 5, Table 6). Higher July temperature and rainfall were also associated with brighter rump
plumage in SY females (Table 5). Higher August temperature was associated with brighter tail
plumage for both SY and ASY3 females (Table 5, Table 6, although the small sample size for
14

ASY3 females (n=5) should be noted. In all tables, SE stands for standard error, t is the t-statistic,
and p is the p-value (probability value).

15

Table 3. Final best fit models examining the effects of weather and age (SY/ASY) on male colouration.
Patch

Month
June

Rump
PC1

July
August*
June

R3 PC1 July
August

Final Model

Variable
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Age*AuRn+(1|Band.Nu Temp
mber)
Rain

Slope

SY
SE

ASY
t

p

Slope

SE

t

p

0.002

0.007

0.27

0.8

0.02

0.007

3.4

0.0009

16

Table 4. Final best fit models showing the effects of weather and age (SY/ASY) on female colouration.
Patch

Month

Final Model

June
Rump
PC1

July
August
June

R3 PC1

July
August

Age*JlRn+(1|Band.
Number)

Variable
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain

Slope

SE

SY
t

0.032

0.009

3.71

p

Slope

0.0003

0.02

SE

ASY
t

p

0.006

2.62

0.01

17

Table 5. Final best fit models showing the effects of weather and age class (SY and ASY1) on female feather colouration.
Females
Patch Month

Final Model

June
Rump
July
PC1

Age Class*JlRn+(1|Band.Number)

Aug
June
R3
PC1

July

Age
Class*JlTp+Age*JlRn+(1|Band.Number)

Aug

Age Class*AuTp+(1|Band.Number)

SY
Variable Slope
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain
Temp
Rain

ASY 1

SE

t

p

Slope

SE

t

p

0.01

3.7

0.0003

0.02

0.008

2.3

0.03

0.76 0.24
0.031 0.01
0.6 0.21

3.2
2.8
2.9

0.002
0.006
0.005

0.002
0.007
0.18

0.13 0.01
0.008 0.85
0.14 1.34

0.99
0.4
0.18

0.04

18

Table 6. Final best fit models showing the effects of weather and age class (ASY2, ASY3, ASY4) on female feather colouration.
Females
Patch Month
Rump
June
PC1

Final Model

ASY2
Variable Slope SE

ASY3
t

p

Slope SE

t

ASY4
p

Slope SE

t

p

Temp
Rain

July

R3
PC1

Age
Class*JlRn+(1|Band.Number)

Temp

Aug

Rain
Temp
Rain

June

Temp

-0.02 0.02 -0.9 0.35 0.02 0.03 0.63 0.53

-0.11 0.07 -1.67 0.09

-0.07 0.21 -0.33 0.74

NA

NA

NA

NA

-0.02 0.014 -1.13 0.27 -0.06 0.02 -2.85 0.007 NA
-0.17 0.29 -0.61 0.55 10.8 2.9 3.7 0.0009 5.02

NA
3.8

NA
1.3

NA
0.2

Rain
Age
July Class*JlTp+Age*JlRn+(1|Band. Temp
Number)
Rain
Temp
Rain

-2.2

1.4 -1.6

0.13

19

DISCUSSION
Plumage ornamentation serves a variety of functions in birds, but the mechanisms that lead
to variation in structural colours are not well understood. In this study, we found that for both male
and female Mountain Bluebirds, SY tail feathers, but not rump feathers, were less colourful than
those of ASY birds. We also found some evidence of colour change among age classes of ASY
birds, with ASY birds appearing to become less colourful in later years. Finally, weather conditions
during moult were associated with differences in colour for both males and females, but these
effects were dependent on age.
At a population level, both male and female SY birds had less colourful tail feathers (i.e.,
feathers with lower brightness, with hue shifted towards longer wavelengths and lower chroma
values) than ASY birds. Siefferman et al. (2005) found similar results in Eastern Bluebirds, with
older males expressing more colourful structural plumage for both rump and tail feathers. In
contrast to what was found in Eastern Bluebirds, however, we found no differences in rump
colouration between SY and ASY birds. In Bluebirds, the duller colour of tail feathers in SY birds
may result from tail feathers being grown in the nest under different conditions than those of adults,
which are grown later during the late summer post-breeding period. Because SY feathers are grown
earlier, these birds have also maintained their tail feathers for a longer period of time. Maintaining
tail feathers longer could in turn contribute towards decreased brightness: a study by Surmacki et
al. (2011) found that male Eastern Bluebirds captured twice within a breeding season showed a
decrease in UV chroma and brightness of feathers later in the season. Similarly, Örnborg et al.
(2002) found that the UV/blue crowns in both male and female Blue Tits varied in hue, chroma,
and brightness throughout the year. In our study, the lack of a difference in rump feather

20

colouration between SY and ASY birds was likely because both juveniles and adults moult body
feathers either during the pre-supplemental (juvenile) or pre-basic (adult) moult in late summer.
Within individuals, there was no difference in plumage between ASY1 and ASY2 males
and females. Male ASY3 birds, however, exhibited less-colourful feathers than male ASY1 birds,
in both rump and tail feathers. This appears to contrast with past studies: Siefferman et al. (2005)
found that male Eastern Bluebird rump feathers increased in brightness and tail feathers increased
in hue and chroma between first year (SY) and subsequent years. Similarly, Budden and Dickinson
(2009) found that older male Western Bluebirds had brighter blue head plumage, although rump
feathers did not show a difference with aging. Budden and Dickinson (2009) did find that the size
of the blue plumage head patches increased on male Western Bluebirds with age, while the size of
rufous back patches decreased with age. Budden and Dickinson (2009) suggested that the quantity
of blue plumage, rather than the quality, could vary with age; while our study did not examine the
size of blue plumage patches, it does provide support for the idea that colour quality may not be a
good indicator of age once bluebirds have reached adulthood.
In addition to age-based variation, we also observed high variation in plumage colour
across years for all age classes, suggesting that large-scale factors are likely influencing
population-level variation in colouration. Previous work on American Redstarts demonstrated that
population-level variation may be driven by differences in temperature and rainfall across years
(Reudink et al. 2009). The authors suggested that temperature and rainfall could influence the
abundance of the carotenoid-rich insects necessary for producing colourful orange plumage.
Though structural plumage colouration would not be directly influenced by dietary pigments,
temperature and rainfall could still influence colouration directly or indirectly through exposure to
weather-induced stress or diet-mediated condition, by affecting feather nanostructure.

21

Males and females appeared to be affected differently by weather conditions. In our first
analysis, we included age in our models with individuals classified only as SY or ASY. ASY males
had more colourful tail feathers after years with higher August rainfall, while female rump colour
for both SY and ASY birds was higher after years with more July rainfall. In previous studies
examining the effects of rainfall on birds with carotenoid colours, results have been mixed:
Reudink et al. (2015) found that American redstarts expressed plumage with higher red chroma
and lower brightness after years following high July rainfall, but there were no significant effects
from August rainfall. Laczi et al. (2020) found that Great Tits developed breast feathers with lower
brightness and UV chroma and higher yellow chroma after drier and warmer August weather.
Reudink et al. (2015) suggested that rainfall could result in higher insect abundance, while Laczi
et al. (2020) suggested that weather conditions could influence insect populations by reducing the
activities of arthropods and that the birds might shelter under wetter conditions rather than
foraging, thus reducing overall food intake bye Great Tits. Laczi et al. (2020) also suggested that
wetter weather conditions may adversely affect foraging by making flight more difficult and
increasing energy expenditure for foraging birds, which provides a contrast to the presumed benefit
of increased insect populations.
In birds with structural colours, Warnock (2017) found that Eastern Bluebirds expressed
brighter and more saturated UV-blue plumage after cooler and wetter moulting seasons, and that
greater precipitation during August in particular had the strongest positive correlation with
structural plumage ornamentation. Again, insect abundance was suggested as a possible cause for
changes in structural colours. Given that higher levels of precipitation are typically associated with
greater insect abundance (Moser 1967; Boomsma and Leusink 1981), wetter summers could
potentially increase food intake and improve structural colours for Mountain Bluebirds.

22

Differences between the sexes could be due in part to breeding behaviours: in July, re-nesting
female bluebirds or those with a second clutch may be spending more time in the nest. Those
females would then be less affected by adverse effects of weather (such as increased energy
expenditure to forage in the rain), while still benefitting from increased insect abundance.
Alternatively, females may be moulting slightly earlier than males, though we currently do not
have the data to test that hypothesis.
In our second analysis examining the effects of weather on colouration, we used birds
classified with respect to different age classes (ASY1, ASY2, etc.). When comparing weather
effects across age classes, we found no effects of weather or age on male birds. We did find weather
effects on female birds, however the small sample sizes (n = 17 for ASY2, n = 5 for ASY3, and n
= 2 for ASY4) should be taken into consideration. For female birds, higher July rainfall was
correlated with more colourful rump feathers in ASY1 and SY birds, and in SY and ASY3 tail
feathers. The increased effects on female rather than male birds contrast with past studies, as most
found that UV-blue colour in males was more sensitive to environmental conditions than UV-blue
colour in females (Siefferman et al. 2005; Doyle and Siefferman 2014). Warnock (2017) did find
effects from temperature and precipitation on rump feather brightness in both male and female
Eastern Bluebirds, however, while another study by Siefferman and Hill (2005) found that female
Eastern Bluebirds on restricted diets exhibited duller structural coloration than female Eastern
Bluebirds on unrestricted diets, suggesting that condition influences structural colouration in both
male and female bluebirds.
Overall, weather effects were most pronounced in SY birds, and precipitation had a
stronger effect than temperature on structural colouration. Given that SY birds moult earlier in the
season than ASY birds, the stronger effects of July conditions make sense. SY birds typically grow

23

their first set of adult feathers in June or July, so wetter conditions in July may contribute towards
higher insect abundance and thus overall food intake for younger birds, resulting in more colourful
plumage. For SY birds that have already grown their adult feathers by August, temperature effects
on feathers may be unrelated to moult conditions and may instead be dependent on variation in
conditions throughout the season. Warnock (2017) found hotter temperatures in August were
associated with decreased rump brightness in SY females, although tail feathers were not
examined. The contrasting temperature effect could be due to differences in local climates: hotter
temperatures in August could have different effects on insect populations in more northern
climates. Our study was conducted at (50.70°N ), compared to Warnock’s (2017), which occurred
at 32.5889° N. Alternatively, factors other than insect abundance could be contributing towards
temperature effects on feather colour: Delhey et al. (2010) found an increase in the brightness of
structurally based crown plumage of Blue Tits after moult and continuing throughout the year, and
suggested that keratinolytic bacteria could alter the feathers’ nanostructures throughout the year,
thus altering brightness, while Gunderson et al. (2009) found that female Eastern Bluebirds were
particularly sensitive to changes in plumage colouration after exposure to feather-degrading
bacteria.
In conclusion, we found evidence of combined effects of individual- (age) and populationlevel (weather) processes influencing the expression of structural colouration in Mountain
Bluebirds. The effects of weather on feather colouration were more pronounced on female birds,
but in both sexes any effects of weather were dependent on age. Overall, rainfall appeared to have
more of an effect on feather colour than temperature, which we suggest is mostly due to an increase
in insect abundance during wetter summers influencing body condition during moult. Overall,
structural colours appear to be influenced by both age and weather conditions, and future studies

24

on the relationship between condition and structural colour as well as the effects of rainfall and
temperature on feather growth and microstructure could provide more insight into the factors that
can affect structural plumage ornamentation.

25

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