Inter-annual variation in American
redstart (Setophaga ruticilla) plumage
colour is associated with rainfall and
temperature during moult: an 11-year
study
Matthew W. Reudink, Ann E. McKellar,
Kristen L. D. Marini, Sarah L. McArthur,
Peter P. Marra & Laurene M. Ratcliffe
Oecologia
ISSN 0029-8549
Oecologia
DOI 10.1007/s00442-014-3167-4

1 23

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Author's personal copy
Oecologia
DOI 10.1007/s00442-014-3167-4

BEHAVIORAL ECOLOGY –ORIGINAL RESEARCH

Inter-annual variation in American redstart (Setophaga ruticilla)
plumage colour is associated with rainfall and temperature
during moult: an 11-year study
Matthew W. Reudink · Ann E. McKellar ·
Kristen L. D. Marini · Sarah L. McArthur ·
Peter P. Marra · Laurene M. Ratcliffe

Received: 6 May 2014 / Accepted: 14 November 2014
© Springer-Verlag Berlin Heidelberg 2014

Abstract Carotenoid-based colouration plays an important role in sexual signaling in animals as an honest indicator of individual quality during mate choice and competitive interactions. However, few studies have examined how
natural variation in weather conditions influences interannual variation in the expression of ornamentation, potentially through affecting the dietary availability of carotenoids. In this study, we examine variation in the expression
of carotenoid-based plumage colouration in relation to temperature and rainfall during the pre-moulting and moulting period over 11 years in a population of American redstarts, Setophaga ruticilla, breeding in eastern Canada.
We used reflectance spectrometry of tail feathers collected
from male and female redstarts to relate feather colour
with weather conditions the previous breeding season during the months over which redstarts are likely to moult

(June–September). At a population level, birds expressed
feathers with higher red chroma and lower brightness in
years following high July rainfall and low August temperature. The pattern was stronger in males, but was generally
consistent across ages and sexes. Analyses of feathers from
repeatedly captured birds indicated that the above patterns
could be explained by individual change in feather colour.
We suggest that higher rainfall during the moulting period
may increase insect abundance and the availability of dietary carotenoids. This is among the first studies to show
effects of weather conditions on a sexual signalling trait,
which may have important consequences for sexual selection, mate choice, and the reliability of putative signals.

Communicated by Indrikis Krams.

Introduction

M. W. Reudink and A. E. McKellar contributed equally to this
work.

The evolution of ornaments in animals has long fascinated
scientists interested in sexual selection and mate choice.
The colourful plumage of birds, in particular, has attracted
much attention, with hundreds of studies aimed at examining the function and evolution of ornamental traits in birds
(Hill and McGraw 2006). These traits have been shown to
function in inter- (reviewed in Hill 2006) and intra-sexual
(reviewed in Santos et al. 2011) interactions, often acting as honest indicators of individual condition or quality.
Carotenoid-based plumage in particular has garnered attention because carotenoid pigments, which are responsible
for producing yellow, orange, and red plumage, cannot be
synthesized de novo, but rather must be ingested as part of
the diet (Hill and Montgomerie 1994). Thus, individuals
must be capable of obtaining sufficient carotenoids during

M. W. Reudink (*) · K. L. D. Marini · S. L. McArthur
Department of Biological Sciences, Thompson Rivers University,
Kamloops, BC, Canada
e-mail: mreudink@tru.ca
A. E. McKellar
Environment Canada, 115 Perimeter Road, Saskatoon,
SK, Canada
P. P. Marra
Migratory Bird Center, Smithsonian Conservation Biology
Institute, National Zoological Park, Washington, DC, USA
L. M. Ratcliffe
Department of Biology, Queen’s University,
Kingston, ON, Canada

Keywords Plumage · Carotenoid · Climate · Long-term ·
Moult · Colour change

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moult through efficient foraging (Endler 1983; Hill 1992).
In addition, because of the antioxidant properties of carotenoids and their importance for immune function, a trade-off
exists whereby only individuals in good health and condition can afford to allocate carotenoids to both self-maintenance and the production of colourful plumage (Lozano
1994).
Because the display of colourful carotenoid-based
plumage is based in large part on the dietary availability of carotenoids, habitat selection during moult may be
an important factor determining plumage expression. For
example, great tits (Parus major) reared in deciduous forests moulted more yellow plumage compared to individuals
reared in coniferous forests (Slagsvold and Lifjeld 1985).
Follow-up studies demonstrated that this difference was
due primarily to comparatively low intake of carotenoidrich caterpillars in coniferous forests (Partali et al. 1987).
In a recent study, American redstart (Setophaga ruticilla)
males on their tropical wintering grounds regrew tail feathers with significantly lower red chroma after an original
tail feather (grown on the breeding grounds) was plucked
(Tonra et al. 2014). As chroma generally reflects carotenoid
content (Saks et al. 2003), the authors suggested the reduction in red chroma was likely due to a lack of dietary availability of carotenoids during the dry season in the Caribbean, which precluded the growth of high-quality plumage
(Tonra et al. 2014).
Despite much research into the function and evolution
of ornamental plumage, factors influencing annual variation in ornament expression have received relatively little
attention. Though a number of studies have examined the
role of reproductive trade-offs (Siefferman and Hill 2005;
Doutrelant et al. 2012) and contracted moult periods (Griggio et al. 2009; Newton and Dawson 2011; Stutchbury
et al. 2011) on the production of sub-optimal plumage
colouration, less is known about how natural variation in
climatic conditions influences population-level ornamentation, which may have important effects on inter- and intrasexual signaling during both the over-wintering and subsequent breeding periods. For example, years of low primary
productivity (as measured by Normalized Difference Vegetation Index, NDVI) on the African wintering grounds of
European barn swallows (Hirundo rustica) were associated
with shorter tails and subsequently later reproduction and
smaller clutches (Saino et al. 2004).
For species that undergo a complete post-breeding
moult, conditions and food availability at the period surrounding moult can be critically important for the acquisition of high-quality plumage (Hill and Montgomerie 1994;
Hill 2000). Scordato et al. (2012) recently observed complex cross-generational effects of climatic conditions on
the expression of a sexually selected trait (wing bar size)
in male Hume’s warblers (Phylloscopus humei). Wing bar

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size was influenced by temperature both during spring and
summer, with warm springs leading to larger wing bars and
warm summers leading to smaller wing bars in the following breeding season. Variation in the expression of sexually-selected traits at the population level through variation in climate can have significant impacts on the intensity
of sexual selection. For example, Cockburn et al. (2008)
demonstrated that sexual selection in superb fairy-wrens
(Malurus cyaneus) was much stronger when conditions
were good, i.e. in years of high rainfall. Thus, evidence is
mounting that understanding the impact of weather conditions on the expression of ornamental traits is important for
understanding how sexual selection acts to shape variation
in these traits.
In this study, we examined variation in plumage colouration in a population of American redstarts over an 11-year
period to determine whether temperature and rainfall during juvenile feather growth and adult post-breeding moult
influenced expression of a sexually-selected trait. In our
study population of American redstarts breeding in Ontario,
Canada, we have demonstrated that variation in plumage
colouration appears to have important inter- and intra-sexual signaling functions. Adult (referred to as after-secondyear, or ASY) males with brighter tails are more likely to be
polygynous (Reudink et al. 2009a), and both flank and tail
colouration are important predictors of extra-pair paternity
(Reudink et al. 2009a) and offspring provisioning (Germain et al. 2010). In addition, female tail brightness is positively associated with age and negatively associated with
parental care in ASY females and fledging success in firstyear (referred to as second-year, or SY) females (Osmond
et al. 2013). Finally, tail brightness in both SY and ASY
males is associated with winter territory quality, suggesting
a possible intra-sexual signalling function during the nonbreeding season (Reudink et al. 2009b). However, the role
of weather conditions in influencing population-level variation in plumage colouration remains unexplored.
We collected tail feathers from ASY and SY males
and females breeding at our study site and related them
to annual weather conditions over 11 years to determine
whether: (1) variation in weather (temperature, rainfall)
during nesting and/or moult predicted variation in plumage
colour the following season, (2) these patterns were consistent across ages and sexes, and (3) these patterns could
be explained by individual change in feather colour.

Materials and methods
Study species
American redstarts are small (7–8 g) migratory birds
that exhibit delayed plumage maturation. All females,

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and males throughout their first year (including their
first breeding season), exhibit grey plumage, with yellow
carotenoid-based patches on the tail, flanks, and wings.
Adult males exhibit similar patterning, but are black
with orange patches. Redstarts are insectivores that prey
on Homoptera, Diptera, Coleoptera, Lepidoptera, and
Hymenoptera species, from which they obtain carotenoids needed for plumage production (Robinson and Holmes 1982). Juveniles (SY) begin growing tail and wing
feathers while still in the nest, typically in June or July at
our study site, and retain those feathers through the next
breeding season (Pyle 1997). Adults (ASY) undergo a
single post-breeding moult that generally occurs between
July and September, usually just following successful or
unsuccessful breeding attempts (Pyle 1997; Sherry and
Holmes 1997).

Table 1 Annual sample sizes of American redstart (Setophaga ruticilla) feathers used in an analysis of the relationship between feather
colour and weather during moult
Year

ASY male

ASY female

SY male

SY female

2001
2002
2003
2004
2005
2006
2007
2008
2009
2010

2
16
18
27
43
38
44
22
25
19

0
0
3
21
13
10
15
1
12
0

10
24
13
23
37
14
10
16
15
13

0
0
1
15
12
17
11
0
9
0

2011

7

1

1

0

Field methods
Colour analysis
Fieldwork for this project was conducted at the Queen’s
University Biological Station (QUBS), Chaffey’s Lock,
ON, Canada (44°34′N, 76°19′W) from May to July 2001–
2011. Male American redstarts were captured shortly after
territory establishment using mist nets and simulated territorial intrusions (decoy with song playback). Females
were captured during feeding trips to the nest or by luring into nets using playback of fledgling distress calls. All
birds were banded with a unique combination of a single
Canadian Wildlife Service aluminum band and 2–3 colour
bands, then aged as either SY or ASY. Because redstart
males exhibit delayed plumage maturation, male age was
determined as SY if the males retained the juvenile grey/
yellow plumage patterning or ASY if they exhibited definitive black/orange colouration. Females were aged SY/ASY
based on the degree of wear and colouring on the rectrices
and moult limits (Pyle 1997). From each individual, a single tail feather (3rd rectrix) was then plucked and immediately stored in a manila coin envelope and placed in a
cardboard box to reduce light contamination prior to colour analysis in the laboratory. We observed no linear relationships between colour variables and year sampled, suggesting that time since plucking did not influence feather
colour; similarly, little effect of feather age had previously
been demonstrated with museum specimens, especially
those collected within the past 50 years (Armenta et al.
2008).
Our analysis involved a total of 578 tail feathers from
481 individuals, including 193 ASY males, 72 ASY
females, 176 SY males, and 65 SY females (Table 1). Fifty
ASY individuals (46 males and 4 females) were present
in >1 year (2.4 ± 0.7 years, range 2–4), and 25 SY individuals returned as ASYs in at least one subsequent year
(2.2 ± 0.4 years, range 2–3).

Plumage colouration was quantified by reflectance spectrometry using an Ocean Optics JAZ spectrometer with a
xenon light source and measuring reflectance across the
avian visual range (300–700 nm). Prior to colour analysis,
all feathers were mounted on low reflectivity (<5 %) black
paper (Colorline Ebony #142). Ten measurements from
each feather were taken haphazardly within the orange/
yellow region of the feather, not including the rachis, with
the probe held within a non-reflective sheath 5.9 mm from
the feather at a 90° angle. Measurements were standardized
between each successive feather using an Ocean Optics
WS-1 white standard and non-reflective dark standard.
Reflectance measurements were analysed in the R-based
colour analysis program RCLR v.28 (Montgomerie 2008).
Prior to calculating colour variables, we performed a smoothing function on all curves to eliminate noise and local peaks.
We then calculated brightness, red chroma, and hue. Brightness was calculated as the mean amount of light reflected
across the visual spectrum mean (R300–700), while red chroma
was calculated as (R605–700/R300–700). Hue was calculated
via segment classification (arctan{[(R415–510 − R320–415 )/
R320–700]/[(R575–700 − R415–575)/R320–700]}), and provides
information on the dominant wavelength of light reflected.
Both chroma and hue are generally considered good indicators of carotenoid content (Saks et al. 2003). Low values
indicate feathers shifted towards longer orange/red wavelengths, while higher values indicate feathers shifted towards
shorter, yellow, wavelengths. The 10 measurements for each
individual feather were then averaged to produce a single
brightness, red chroma, and hue value for each feather. Colour analysis methods and calculations follow previous work
by our group on American redstarts (e.g., Reudink et al.
2009a, b; Tonra et al. 2014).

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Table 2 Summary of the topranked models (<4 AICc units
from top model) explaining variation in feather brightness, red
chroma, and hue for ASY and
SY American redstarts (model
details at table bottom)

Model category

Top models

Sex + Jl.Rn + Jl.Tp + sex × Jl.Rn
Sex + Jl.Rn + Jl.Tp + sex × Jl.Tp

Sex + Jn.Rn + Jl.Tp + sex × Jl.Rn + sex × Jl.Tp
Sex + Jl.Rn + sex × Jl.Rn

Sex + Jl.Rn
Sex + Jl.Rn + Jl.Tp + sex × Jl.Tp

ASY red chroma
Sex + Jl.Rn + sex × Jl.Rn
Sex + A.Tpd + sex × A.Tp
Sex + Jl.Rn + Jl.Tp + sex × Jl.Rn

Sex + Jl.Rn + Jl.Tp + sex × Jl.Tp
Sex + A.Rn + A.Tp + sex × A.Tp
Sex + Jl.Rn
Sex + Jl.Rn + Jl.Tp + sex × Jl.Rn + sex × Jl.Tp
SY red chroma
Sex + Jl.Rn
Sex + Jl.Rn + Jl.Tp
Sex + Jl.Rn + sex × Jl.Rn
Sex + Jl.Rn + Jl.Tp + sex × Jl.Tp
Sex + Jl.Rn + Jl.Tp + sex × Jl.Rn + sex × Jl.Tp
ASY hue
Sex + A.Tp + sex × A.Tp
Sex + A.Rn + A.Tp + sex × A.Rn + sex × A.Tp
Sex + A.Rn + A.Tp + sex × A.Tp
Sex + Jl.Rn + Jl.Tp
Sex
SY hue
Sex + A.Rn + A.Tp + sex × A.Tp
Sex + S.Tp + sex × S.Tp
Sex + A.Rn + A.Tp
Sex + A.Rn

Sex + A.Tp + sex × A.Tp
Sex + S.Tp

Null model

Weather data
We obtained rainfall and temperature information for the
Ontario breeding grounds from a weather station at QUBS.
We used monthly total rainfall and mean air temperature from June–September as proxies for breeding season
weather in yearx−1, to be related to feather colour in yearx
(see “Statistical analyses”). Thus, we obtained weather data
for the years 2000–2010 in order to relate them to feather
colour in the years 2001–2011. However, QUBS weather

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wi

−1,089.9
−1,089.6

0.00
0.31

0.31
0.26

−1,088.9
−1,088.4

1.00
1.52

0.19
0.14

−645.8

0.00

0.33

1.54
1.90

0.25
0.13

−643.8
−643.2

2.03
2.64

0.12
0.09

−1,610.2
−1,608.7
−1,608.1

0.00
1.46
2.10

0.31
0.15
0.11

−1,608.0
−1,607.1
−1,607.0
−1,606.8

2.19
3.03
3.19
3.41

0.10
0.07
0.06
0.06

−1,298.6
−1,296.6
−1,296.5
−1,296.2
−1,295.3

0.00
1.98
2.10
2.35
3.24

0.40
0.15
0.14
0.12
0.08

−405.6
−404.1
−404.0
−401.9
−401.7

0.00
1.44
1.57
3.61
3.90

0.28
0.13
0.13
0.05
0.04

−115.2
−114.8
−114.7
−114.4

0.00
0.41
0.55
0.78

0.11
0.09
0.09
0.08

1.09
1.28

0.07
0.06

1.57

0.05

−645.3
−643.9

Sex + Jl.Rn + Jl.Tp + sex × Jl.Rn
Sex + Jl.Rn + sex × Jl.Rn

J June, Jl July, A August, S
September, Rn total rainfall,
Tp mean temperature (e.g.,
Jl.Tp = mean July temperature)

∆AICc

ASY brightness

SY brightness
Sex + Jl.Rn + Jl.Tp

The AICc value, difference in
AICc between the model and
the top model (∆AICc), and
model weights (wi) are shown.
Year and individual were
included as random effects in
all ASY models, and year was
included as a random effect in
all SY models. Note that only
the top 7 models are shown for
SY hue (18 models were within
4 AICc units)

AICc

−114.1
−114.0

−113.7

data were not available for 2010, and so instead we used
temperature and rainfall data from a nearby Environment
Canada weather station (Ottawa, Ontario, approximately
100 km to the northeast of our study site). Weather data
from the QUBS and Environment Canada stations were
generally highly correlated for 2000–2009 (see below),
and so we felt justified using Environment Canada data for
2010. Nonetheless, if weather data from 2010 (and feathers
from 2011) were excluded, our results remained unchanged
(note small sample size of feathers for 2011 compared to

−0.0063 (−0.014, 0.0017)

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Values in bold indicate that the 95 % CI did not overlap zero. See Table 2 for variable abbreviations

0.0085 (−0.047, 0.064)
0.045 (0.011, 0.079)

0.044 (−0.0088, 0.097)

0.013 (−0.09, 0.12)

0.034 (−0.058, 0.13)

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−0.0047 (−0.024, 0.015)

Oecologia

0.015 (0.0029, 0.034)
Sex × Jl.Tp

other years; Table 1). We also compared our weather data to
datasets from an additional two nearby weather stations—
Kingston, Ontario (approximately 40 km south and available for 2000–2006), and Toronto, Ontario (approximately
260 km southwest and available for 2000–2010). When
comparing all three Environment Canada stations to data
from QUBS, we found temperature data to be very highly
correlated (all r > 0.92). Correlations for rainfall were not
as high though were generally positive (mean r = 0.35),
likely reflecting more localized and variable levels of precipitation. In any case, we felt confident that our weather
dataset was a reasonable representation of local weather
from an area that would have encompassed the moulting
location of many American redstarts from our study site.
Indeed, the authors have often observed birds moulting in
the late season (July and August) on our study site, and stable isotope analysis suggests moult during migration may
be uncommon (Reudink et al. 2008).

0.0066 (0.00013, 0.013)

−0.016 (−0.032, −0.0002)

Sex × Jl.Rn

0.05 (−0.01, 0.12)

Sex × S.Tp

−0.073 (−0.15, 0.0043)

0.0036 (−0.0057, 0.013)

0.0022 (−0.018, 0.023)

A.Rn

−0.0078 (−0.02, 0.0048)

0.0015 (−0.0053, 0.0083)

−0.018 (−0.04, 0.0029)
Jl.Tp

−0.037 (−0.1, 0.025)
Sex × S.Rn

−0.038 (−0.12, 0.043)
−0.0015 (−0.0055, 0.0025)

A.Tp

−0.0026 (−0.0085, 0.0033)

-0.086 (−0.17, −0.0043)

0.0058 (0.0016, 0.01)

0.045 (−0.047, 0.14)
S.Tp

0.046 (−0.044, 0.13)

−0.036 (−0.12, 0.049)
0.0035 (−0.0048, 0.0012)
Jl.Rn

−0.0066 (−0.012, −0.00076)
S.Rn

0.0044 (−0.0017, 0.01)

−0.082 (−0.17, 0.0058)
−0.022 (−0.035, −0.0095)

−0.0085 (−0.03, 0.012)
Sex × Jn.Tp

0.022 (−0.0068, 0.051)
Sex × A.Tp

0.041 (−0.01, 0.092)

−0.033 (−0.09, 0.23)

−0.002 (−0.0088, 0.0047)

Sex × A.Rn

−0.018 (−0.12, 0.08)

−0.24 (−0.28, −0.21)
Jn.Tp

−0.0063 (−0.011, −0.0012)
0.011 (0.0054, 0.017)
0.044 (0.024, 0.063)
0.0019 (−0.012, 0.016)
Sex

SY hue
ASY hue
SY chroma
ASY chroma
SY brightness
ASY brightness

Statistical analyses
To examine population-level variation in feather colour
in association with weather, we constructed models that
related feather colour variables in yearx to total monthly
rainfall and mean monthly temperature for June–September
in yearx−1.
To avoid problems of multiple correlation and because
potential influences of weather variables on colour should
occur within the same month, only rainfall and temperature from a given month were allowed in the same model.
We included sex in all models and allowed for interactions
between sex and rainfall or sex and temperature. Thus, a
full model for the month of June would include sex, total
rainfall in June, mean temperature in June, sex × total
rainfall in June, and sex × mean temperature in June. The
total set of models we evaluated included all combinations
of the previous variables (with the requirement that sex be
included), for each of 4 months, for a total of 34 models,
including a null model and a sex-only model. We analysed
the three colour variables (brightness, red chroma, and
hue) separately and analysed ASY and SY birds separately,
since adults and juveniles are predicted to grow their feathers during different time periods. Year was included as a
random effect in all models, and individual was included as
a random effect in all ASY models. We standardized temperature and rainfall data prior to analysis by subtracting
values by the mean and dividing by the standard deviation
so that main effects would remain biologically interpretable when involved in interactions and coefficients would
be comparable in magnitude (Schielzeth 2010). We ranked
models with Akaike’s information criterion corrected for
small sample size (AICc) and considered models within
4 AICc units to be competitive (Burnham and Anderson

Table 3 Model-averaged parameter estimates and 95 % CI for variables included in the top-ranked models (<4 AICc units of best model) explaining variation in feather brightness, red chroma,
and hue for ASY and SY American redstarts

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2002). We tested models for multiple correlation and overdispersion. Standardized total rainfall and mean temperature
were not strongly correlated within models (all r < 0.02),
and we did not detect any overdispersion in our models (all
p > 0.5). To supplement the above analyses, we used linear regression analyses weighted by sample size of feathers to examine the relationship between mean annual colour variables and those mean weather variables identified
as important in model selection. Finally, to test whether
the above population-level changes could be explained by
change in individual feather colour, we used a subset of 42
ASY males that were present on our study site in at least 2
consecutive years. We modelled change in colour as a function of change in those weather variables deemed important in the above analyses. Individual was again included
as a random effect to account for those ASY males present across >2 years. We evaluated the significance of the
weather variables by removing each and comparing the
difference in deviance between the resultant model and the
full model. Differences in deviance approximately follow a
Chi square distribution with one degree of freedom (Zuur
et al. 2009). The same models could not be run for ASY
females due to very small numbers of consecutively returning birds (n = 3). All analyses were performed in R v.3.0.1
(R Development Core Team 2013).

Results
Brightness
The top model explaining variation in ASY feather
brightness in yearx included effects of sex, July rainfall
in yearx−1, July temperature in yearx−1, and the interaction between sex and July rainfall in yearx−1 (Table 2).
Other models within 4 AICc units included the above
effects in addition to June rainfall and the interaction
between sex and July temperature. These results indicate
that July weather appeared to have a stronger association
with feather brightness in comparison to weather from
other months and that ASY males and females may have
responded differently to July weather. Specifically, confidence intervals for model-averaged parameter estimates for
the interaction between sex and July rainfall did not overlap zero (Table 3) and indicated that lower July rainfall was
associated with brighter feathers more strongly in males
(i.e., steeper slopes) than in females (Fig. 1a). Across years,
lower July rainfall in yearx−1 was associated with brighter
feathers in ASY males in yearx, and no other relationships
were significant (rainfall: males R2 = 0.49, t9 = −2.96,
p = 0.016, females R2 = 0.09, t6 = 0.77, p = 0.470; temperature: males R2 = 0.00, t9 = −0.13, p = 0.900, females
R2 = 0.35, t6 = −1.81, p = 0.121; Fig. 2). Based on the

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above results we examined the relationship between individual change in ASY male feather brightness and annual
change in July rainfall, which revealed a significant effect
of July rainfall (χ12 = 8.79, p = 0.003).
The top model explaining variation in SY feather brightness in yearx included effects of sex, July rainfall, and July
temperature in yearx−1. Other competitive models included
interactions between sex and July rainfall or temperature
(Table 2). Model-averaged parameter estimates indicated
that SY males had brighter feathers than SY females and
that lower July rainfall was associated with brighter feathers (Table 3). Across years, lower July rainfall in yearx−1
was associated with brighter feathers in SY males in yearx,
and no other relationships were significant (Rainfall: males
R2 = 0.70, t9 = −4.54, p = 0.001, females R2 = 0.42,
t4 = −1.72, p = 0.161; temperature: males R2 = 0.00,
t9 = −0.07, p = 0.949, females R2 = 0.08, t4 = 0.59,
p = 0.585; Fig. 3).
Red chroma
The top model explaining variation in ASY feather red
chroma included effects of sex, July rainfall in yearx−1,
and their interaction. Other competitive models included
July temperature and its interaction with sex, as well as
August temperature, August rainfall, and the interaction
between sex and August temperature (Table 2). Modelaveraged parameter estimates indicated that male feathers
had higher red chroma than female feathers, and that male
feather chroma may have been more strongly associated
with weather than female feather chroma (Table 3). Specifically, ASY male chroma appeared to be more strongly
reduced following years of low July rainfall and years of
high August temperature in comparison to females (Fig. 1b,
c). Across years, higher July rainfall in yearx−1 was associated with higher red chroma in feathers of ASY males in
yearx, and no other relationships were significant (rainfall:
males R2 = 0.54, t9 = 3.24, p = 0.010, females R2 = 0.00,
t6 = 0.09, p = 0.934; temperature: males R2 = 0.16,
t9 = −1.30, p = 0.226, females R2 = 0.01, t6 = −0.24,
p = 0.820; Fig. 4a–d). Similarly, lower temperature in
August in yearx−1 was associated with higher red chroma in
feathers of ASY males in yearx, and no other relationships
were significant (rainfall: males R2 = 0.00, t9 = −0.18,
p = 0.864, females R2 = 0.28, t6 = −1.51, p = 0.182; temperature: males R2 = 0.50, t9 = −3.02, p = 0.015, females
R2 = 0.29, t6 = −1.56, p = 0.169; Fig. 4e–h). Based on
the above results, we examined relationships between both
July rainfall and August temperature and individual change
in ASY male feather chroma. We found that individual
change was significantly predicted by August temperature
(χ12 = 11.42, p < 0.001), but not by July rainfall (χ12 = 1.68,
p = 0.194).

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Oecologia
Fig. 1 Interaction effects from
models explaining variation
in ASY American redstart
(Setophaga ruticilla) feather
colour. a Feather brightness and
July rainfall, b red chroma and
July rainfall, c red chroma and
August temperature, and d hue
and August temperature. Shown
is the mean feather colour variable in yearx in years of high
(those above and including the
median) or low (those below the
median) rainfall or temperature
in yearx−1. Solid line males,
dashed line females

Fig. 2 Relationship between
mean annual feather brightness
(±SD) and July weather for
ASY male and female feathers. Data points are weighted
by sample size, and solid line
shows a significant relationship

Results for SY red chroma showed some similarities to
those for ASY birds, with sex and July rainfall included in
the top model and other competitive models including July
temperature and interactions between sex and July temperature or sex and July rainfall (Table 2). Model-averaged
parameter estimates indicated that higher July rainfall was
associated with higher red chroma, and male feathers had

higher red chroma than female feathers (Table 3). In contrast to ASY birds, no significant interactions were found
between sex and weather (Table 3). Across years, higher
July rainfall in yearx−1 was associated with higher values
of red chroma in SY male feathers in yearx, and all other
relationships between red chroma and July rainfall or
temperature across years were non-significant (rainfall:

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Oecologia
Fig. 3 Relationship between
mean annual feather brightness
(±SD) and July weather for SY
male and female feathers. Data
points are weighted by sample
size, and solid line shows a
significant relationship

males R2 = 0.75, t9 = 5.22, p < 0.001, females R2 = 0.32,
t4 = 1.36, p = 0.244; temperature: males R2 = 0.07,
t9 = −0.84, p = 0.422, females R2 = 0.43, t4 = −1.72,
p = 0.161; Fig. 5).
Hue
The top model explaining variation in ASY feather hue
included effects of sex, August temperature in yearx−1,
and their interaction. Other competitive models included
August rainfall and its interaction with sex, as well as July
rainfall and July temperature (Table 2). Model-averaged
parameter estimates indicated that ASY male feathers had
lower hue values compared to ASY female feathers and
appeared to respond more strongly to August temperature,
with higher hue following years of higher temperature
(Table 3; Fig. 1d). In contrast, higher July temperature was
associated with lower hue. No clear patterns were evident
across yearly means for July weather and ASY feather hue
(rainfall: males R2 = 0.08, t9 = −0.88, p = 0.401, females
R2 = 0.04, t6 = 0.47, p = 0.653; temperature: males
R2 = 0.08, t9 = −0.89, p = 0.397, females R2 = 0.30,
t6 = −1.62, p = 0.156; Fig. 6a–d) or August weather and
ASY feather hue (rainfall: males R2 = 0.00, t9 = −0.09,
p = 0.934, females R2 = 0.01, t6 = 0.25, p = 0.813; temperature: males R2 = 0.13, t9 = 1.16, p = 0.275, females
R2 = 0.05, t6 = −0.59, p = 0.579; Fig. 6e–h). Based on
the above results, we examined relationships between both
July temperature and August temperature and individual
change in ASY male feather hue. We found that individual

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change was significantly predicted by August temperature (χ12 = 4.40, p = 0.040), but not by July temperature
(χ12 = 2.54, p = 0.111).
Compared to feather brightness and red chroma, SY
feather hue appeared to be less influenced by weather.
Eighteen models were competitive with the top-ranked
model explaining variation in feather hue, indicating that
many different models were equally likely given the data,
and this set also included the null model (Table 2). We did
not make annual comparisons for SY birds due to no clear
weather variables appearing important in influencing individual variation in feather hue.

Discussion
Environmental factors, such as temperature and rainfall,
can have major impacts on migratory birds at both the
individual and population level. High primary productivity associated with increased rainfall on the wintering
grounds may result in high over-winter survival, which is
reflected in high breeding season abundance (Wilson et al.
2011). Similarly, productivity on the breeding grounds can
be strongly affected by climatic conditions (reviewed in
Newton 1998), with variation in temperature and rainfall
influencing the timing of breeding (Dunn 2004) and arrival
(McKellar et al. 2013), productivity (Wilson and Arcese 2004) and survival due to changes in food abundance
(Morrison and Bolger 2002; Collister and Wilson 2007).
These pathways can also be influenced by local climate

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Oecologia
Fig. 4 Relationship between
mean annual feather red chroma
(±SD) and July and August
weather for ASY male and
female feathers. Data points
are weighted by sample size,
and solid lines show significant
relationships

conditions en route (Tøttrup et al. 2008). In this study, we
demonstrate that weather conditions later in the breeding
season may also have important effects on plumage colouration. American redstarts showed associations between the
brightness, red chroma and, to a lesser extent, hue of their
tail feathers in relation to breeding season weather conditions the previous year. In general, individuals expressed
plumage with higher red chroma (Figs. 4, 5), lower brightness (Figs. 2, 3), and more orange-shifted hue (Fig. 6) in
years following high July rainfall or temperature and lower
August temperature.
In our population of American redstarts, individuals
begin breeding in mid- to late May, and by July have either
completed breeding attempts (due to not successfully pairing or raising offspring) or are in the latter stages of raising

nestlings or fledglings. By late July, many individuals have
begun the process of moult prior to fall migration, sometimes overlapping moult and nestling provisioning in late
nests (unpublished data). During this time, rainfall can be
especially variable, with July total rainfall ranging from 11
to 96 mm over 11 years. Across certain age and sex classes,
rainfall and temperature during moult appeared to predict
plumage colouration the subsequent season. We suggest
that the most likely mechanism for this pattern is differences in insect abundance, with higher insect abundance
resulting from high levels of rainfall (e.g., Moser 1967;
Boomsma and Leusink 1981). For example, work on great
tits demonstrated that the proportion of leaf-eating insects
(primarily carotenoid-rich Lepidoptera larvae) in the diet
of nestlings declined drastically over the breeding season;

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Fig. 5 Relationship between
mean annual feather red chroma
(±SD) and July weather SY
male and female feathers. Data
points are weighted by sample
size, and solid line shows a
significant relationship

correspondingly, carotenoid content was significantly
higher in first-brood nestlings than in later nestlings (Ilyina
et al. 2013). Similarly, Eeva et al. (2005, 2008) demonstrated that nestling great tits inhabiting regions with high
levels of industrial pollution and fewer Lepidopteran larvae
grew less intensely coloured yellow ventral feathers.
These findings parallel a substantial body of work in the
American redstart showing the influence of rainfall during
the wintering period on insect abundance and subsequent
body condition (Studds and Marra 2007; Angelier et al.
2011), migratory timing (Studds and Marra 2011; McKellar et al. 2013), and abundance (Wilson et al. 2011). However, we are not aware of other studies that have shown
weather-associated changes in feather colouration during
moult, although long-term changes in climatic conditions
have been linked to population-level changes in colouration
in some species. Melanin-based plumage redness in scops
owls (Otus scops) has increased significantly over the past
century—a period associated with increases in temperature
and rainfall (Galeotti et al. 2009). Globally, a similar pattern was observed across four owl genera [wood (Strix),
scops (Otus), screech (Megascops), and pygmy owls (Glaucidium)], with darker red phenotypes observed in lower
latitudes with warmer climates (Roulin et al. 2011).
American redstarts prey on a variety of insects including Lepidoptera larvae, which are highly concentrated in
carotenoid pigments (Eeva et al. 2010) and are one of the
major food sources during the breeding season (Sherry and
Holmes 1997). High July rainfall likely results in increased
insect availability, which may increase individual condition and also increase dietary carotenoid availability. We

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suggest that redstart feather colouration varies at an individual and a population level due to annual variation in the
availability of insects and dietary carotenoids, as increased
rainfall was positively associated with increased red
chroma and reduced brightness in ASY males, which may
positively reflect carotenoid content (Saks et al. 2003). Perhaps most importantly, the patterns that we observed were
consistent across age classes, though patterns were stronger
in males than in females (Table 2; Fig. 1), suggesting that
environmental effects, rather than solely individual-level
trade-offs (e.g., trade-offs with reproductive effort; Siefferman and Hill 2005; Doutrelant et al. 2012), can have
important influences on plumage colouration.
Plumage colouration of both ASY and SY individuals
was most often associated with conditions the previous July
and August, though weather from other months appeared
in some top-ranked models (Table 2). Hatch-year birds
(i.e., those who would be SY birds the following season)
may grow their feathers in the nest as early as June, though
many would certainly do so into July or even August. Adult
birds typically begin moult in July, though many likely
moult primarily in August and potentially into September, especially those individuals with late-season nesting
attempts (see “Introduction”). Further research would be
needed to identify the basis for the particular importance of
rainfall in July and temperature in August over conditions
in other months, perhaps as it relates to insect emergence
and abundance. In contrast, we found some evidence for
differential strength of effects of weather between males
and females (i.e., significant interactions in Table 2 and
differing slopes in Fig. 1). These relationships most often

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Oecologia
Fig. 6 Relationship between
mean annual feather hue (±SD)
and July and August weather for
ASY male and female feathers.
Data points are weighted by
sample size

indicated that female colouration may have responded
less strongly to weather conditions than male colouration,
though sample sizes were smaller for females (Table 1).
Possible reasons for differences in the strength of associations between feather colouration and weather include
sex-specific foraging preferences or differences in moult
patterns, in addition to obvious differences in feather carotenoid content between males and females. Additionally,
differences in selective pressures for signalling exerted on
males and females could result in a differential response to
weather conditions during moult.
In American redstarts, variation in plumage colouration
is strongly associated with variation in reproductive success
on the breeding grounds (Reudink et al. 2009a), as well as

winter territory quality on their tropical wintering grounds
(Reudink et al. 2009b). Our findings of individual change in
feather colouration in association with environmental conditions during moult raise interesting questions about the relative importance of environmental and genetic effects (e.g.,
genotype-by-environment interactions; Qvarnstrom 1999;
Roulin et al. 1998, 2008) and the condition-dependence
of signalling traits (Cotton et al. 2004). For example, male
American redstart (both SY and ASY) with brighter tail
colouration are more likely to occupy dominance-mediated
high-quality winter habitats in Jamaica, suggesting that tail
brightness may be acting as an intra-sexual signal (Reudink et al. 2009b). Because birds wintering in the Caribbean
breed across a broad range of latitudes in eastern North

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America (Norris et al. 2007), variation in environmental
conditions and their interaction with intrinsic factors could
influence the reliability of tail colouration as an honest signal of individual quality on the wintering grounds.
In conclusion, we demonstrate that, over an 11-year
period, American redstarts exhibited substantial inter-year
variation in plumage colouration that appeared to be associated with weather conditions during moult, perhaps due
to increased dietary availability of carotenoids following
higher insect abundance. While many studies have examined the influence of climate on population dynamics (e.g.,
survival, productivity), very few have examined the influence of environmental conditions on the expression of sexually selected traits. This work demonstrates a potentially
critical seasonal interaction, whereby weather conditions
during post-breeding moult may carry over to impact intersexual signalling during the non-breeding season and the
intensity of sexual selection during the subsequent breeding season. Future studies into environmentally-induced
variation in sexual signals and their effects on sexual selection, mate choice, and the reliability of putative signals
may reveal how these pressures could shift with a changing
global climate (e.g., Cockburn et al. 2008).
Acknowledgments We thank would like to thank the many field
assistants, technicians, and students that worked on this population
over the years. We also thank the editor and two anonymous reviewers
for helpful comments and suggestions. Funding for this project was
provided by the National Science Foundation (P.P.M) and Natural Sciences and Engineering Research Council of Canada Discovery Grants
to L.M.R. and M.W.R.

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