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# Evolution of female colors in birds: the role of female cost of reproduction and paternal care

Amélie Fargevieille, Arnaud Grégoire, Doris Gomez, Claire Doutrelant

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## To cite this version:

Amélie Fargevieille, Arnaud Grégoire, Doris Gomez, Claire Doutrelant. Evolution of female colors in birds: the role of female cost of reproduction and paternal care. Journal of Evolutionary Biology, 2023, 36 (3), pp.579-588. 10.1111/jeb.14150. hal-03960532

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https://hal.science/hal-03960532

Submitted on 27 Jan 2023

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# Evolution of female colors in birds: the role of female cost of reproduction and paternal care

Amélie Fargevieille, Arnaud Grégoire, Doris Gomez, Claire Doutrelant

► To cite this version:

Amélie Fargevieille, Arnaud Grégoire, Doris Gomez, Claire Doutrelant. Evolution of female colors in birds: the role of female cost of reproduction and paternal care. Journal of Evolutionary Biology, 2023, 36 (3), pp.579-588. 10.1111/jeb.14150. hal-03960532

HAL Id: hal-03960532

https://hal.science/hal-03960532v1

Submitted on 17 Feb 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# 1 Abstract

2 Female ornamentation is frequently observed in animal species and is sometimes found as more 3 evolutionary labile than male ornamentation. A complex array of factors may explain its presence 4 and variation. Here we assessed the role of female cost of reproduction and paternal care. Both 5 factors have been pinpointed as important by theoretical studies but have not been investigated yet 6 in details at the interspecific level. We worked on 133 species of North temperate Passeriformes 7 bird species for which both the clutch volume – here taken as the proxy of female cost of 8 reproduction - and amount of paternal care are relatively well known. Using spectrometry, we 9 measured the whole-body colored plumage patches and quantified three metrics corresponding to 10 brightness (i.e. achromatic component), color chromaticity (i.e. intensity), and color volume (i.e. 11 diversity). We found a strong association between male and female color metrics. Controlling for 12 this association, we found additional small but detectable effects of both cost of reproduction and 13 paternal care. First, females of species with more paternal care were slightly brighter. Second, the 14 interaction between the level of paternal care and egg volume was correlated with female color 15 intensity: females with more paternal care were more chromatic, with this association mostly 16 present when their investment in reproduction was low. Together these results suggest that female 17 cost of reproduction and paternal care are part of the multiple factors explaining variation of female 18 coloration, besides the strong covariation between male and female coloration.

19

20 *Keywords:* Sexual selection – female ornaments – paternal care – reproductive costs – avian visual
21 space – social selection – comparative analyses
1 Original article: Evolution of female colors in birds: the role of female cost of reproduction

2 and paternal care

3 *Authors:* Amélie Fargevieille (ORCID 0000-0003-0934-3563)§, Arnaud Grégoire (ORCID 0000-0001-6103-355X)§, Doris Gomez (ORCID 0000-0002-9144-3426)§, Claire Doutrelant (ORCID: 0000-0003-1893-3960)§

6 *Author institutional affiliations:* §CEFE-CNRS UMR 5175, Université de Montpellier, Campus du CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France

8 *Corresponding author contact details:* Amélie Fargevieille, CEFE-CNRS UMR5175, Campus du CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5; France. afargevieille@gmail.com

10 *Author contribution:* A.F., A.G. and C.D. designed the study; A.F. collected the data with help from D.G; A.F. analysed the data; A.F. wrote the first draft with several inputs from A.G., D.G. and C.D.

13 Acknowledgments.

14 We thank the National Museum of Natural History in Paris, especially Patrick Boussès and
15 Jérôme Fuchs who gave us access to the bird collection and facilitated the use of the specimens
16 selected. We thank Maria del Rey Granado for her help with recording spectrometric measures.

17 We last thank Marianne Elias and Benoît Nabholz for help with phylogenetic comparative methods,
18 Tim Janicke and anonymous reviewers for insightful comments on the manuscript, and Jessica
19 Pearce and Kaitlyn Murphy for English language editing and comments. Funding was provided by
20 the CNRS, the University of Montpellier, the French National Research Agency (CD: ANR 09-
21 JCJC-0050-0) and the regional government of Languedoc Roussillon (Chercheur-es d'Avenir grant
22 to CD).

23 *Conflict of interest statement:* We declare no conflict of interest.
## 22 Introduction

Males in many animal species display conspicuous ornaments and armaments and a large body of evolutionary research has aimed to understand the factors at the origin of this variation (Andersson, 1994). Females in many animal species also display conspicuous traits and in the past twenty years, research focusing on the evolution and variation in female ornaments has increased (Amundsen, 2000; Harrison & Poe, 2012; Nordeide et al., 2013; Odom et al., 2014; Dale et al., 2015; Riebel et al., 2019; Doutrelant et al., 2020).

Owing to the paradigm that inter- and intra-sexual selection is typically stronger on males (Bateman, 1948; Janicke et al., 2016; but see Hoquet, 2020), female ornamentation was historically considered as a genetic by-product of male ornamentation with natural selection (e.g. predation) leading to more cryptic phenotypes in females. Nowadays, there is a general agreement that natural, sexual and social selection also impact female ornamentation, and that studies need to understand how these complex arrays of forces interact and explain the observed variability (Kraaijeveld et al., 2007; Clutton-Brock, 2009; Tobias et al., 2012; Hare & Simmons, 2019; Cooney et al., 2022; Doutrelant et al., 2020).

Comparative studies have been a powerful tool to investigate the multiple drivers of female ornamentation. Overall most of the comparative studies focused on the selective forces driving the evolution of sexual dichromatism, which expresses the effect of sexual and natural selection acting on both sexes (Johnson et al., 2013; Dunn et al., 2015; Shultz & Burns, 2017). These studies clearly outlined three factors explaining the evolution of female ornamentation: predation, female competition, and selection acting on males. Increase of predation risk, migration and intense sexual selection on males are usually associated with lower coloration in females compared to males (Martin & Badyaev, 1996; Dale et al., 2015; Dunn et al., 2015) whereas higher female competition
24 *Data archival*: Data sets are deposited in the Dryad Digital Repository (URL:

25 https://doi.org/10.5061/dryad.9s4mw6mkh)
for access to reproduction in cooperative breeding species is associated with lower sexual dichromatism (Rubenstein & Lovette, 2009; Dale et al., 2015). To go further in understanding the variation in female ornamentation at the interspecific level, it is important to keep focusing on female ornamentation and to test the effect of female-related forces - i.e. intensity of selective pressures and constraints specifically acting on females (Rosvall et al., 2020; Cooney et al., 2022; Doutrelant et al., 2020).

Egg production, as a female-specific cost of reproduction is a factor expected by theoretical models to influence female ornamentation (Fitzpatrick et al., 1995). While predation risk has been well-studied as a cost of reproduction limiting the evolution of female ornaments (e.g.; Martin & Badyaev, 1996; Soler & Moreno, 2012; Dunn et al., 2015; Matysiokova et al., 2017; Cain et al., 2019), the consequences of investing more in the production of eggs remain overlooked by comparative studies. Theoretically, females paying higher reproductive costs are predicted to invest less in signaling (Chenoweth et al., 2006). This trade-off was shown experimentally in blue tits where females forced to lay more eggs during one breeding season produced less developed color ornaments in the consecutive molt (Doutrelant et al., 2012). At the interspecific level, this trade off was verified in the Hirundinidae family for a morphological trait, with female tail fork depth decreasing with increasing egg size (Hasegawa & Arai, 2017). In another study, females from tropical passerine species – typically associated with smaller clutch size than temperate species – were found to be more colorful (Dale et al., 2015). But yearlong territoriality and lifelong pairs are also more widespread in tropical species, and may also explain the evolution of female coloration through intrasexual selection and male mate choice. Thus, whether variation in female reproductive costs constrains the evolution of female coloration still needs attention.

Paternal care is by contrast a factor supposed to influence two female-related forces promoting
27 Female ornamentation is frequently observed in animal species and is sometimes found as more
28 evolutionary labile than male ornamentation. A complex array of factors may explain its presence
29 and variation. Here we assessed the role of female cost of reproduction and paternal care. Both
30 factors have been pinpointed as important by theoretical studies but have not been investigated yet
31 in details at the interspecific level. We worked on 133 species of North temperate Passeriformes
32 bird species for which both the clutch volume – here taken as the proxy of female cost of
33 reproduction - and amount of paternal care are relatively well known. Using spectrometry, we
34 measured the whole-body colored plumage patches and quantified three metrics corresponding to
35 brightness (i.e. achromatic component), color chromaticity (i.e. intensity), and color volume (i.e.
36 diversity). We found a strong association between male and female color metrics. Controlling for
37 this association, we found additional small but detectable effects of both cost of reproduction and
38 paternal care. First, females of species with more paternal care were slightly brighter. Second, the
39 interaction between the level of paternal care and egg volume was correlated with female color
40 intensity: females with more paternal care were more chromatic, with this association mostly
41 present when their investment in reproduction was low. Together these results suggest that female
42 cost of reproduction and paternal care are part of the multiple factors explaining variation of female
43 coloration, besides the strong covariation between male and female coloration.

44

45 **Keywords:** Sexual selection – female ornaments – paternal care – reproductive costs – avian visual
46 space – social selection – comparative analyses
the evolution of female ornaments. Theoretical models predict the more time a male invests in parental care, the choosier he is expected to be (Kokko & Johnstone, 2002), enabling the evolution of female ornaments through intersexual selection. In addition, paternal care can also be indirectly related to the evolution of female ornaments through intrasexual selection if females compete for access to paternal care (Lyu et al., 2017). At the interspecific level, the effect of paternal care has been rarely tested specifically, with mixed results across studies. Absence of paternal care was related to species with duller female upper part coloration in one study (Dale et al. 2015). In another study, paternal care was more widespread in more colorful monochromatic species, but with no change in female brightness and color (Dunn et al., 2015). Last, male participation to nest-building was associated with more intense female coloration (estimated by human eye) in a comparative analysis on 178 Western Palearctic passerines (Soler et al., 2019). In these three studies, paternal care was estimated as a binary index (presence/absence) while paternal investment can vary both within the stages of reproduction and across bird species. Thus, a necessary step to help clarifying the role of paternal investment in reproduction on the evolution of female ornament is to quantify variation in male investment across different stages of paternal care (from incubation to fledgling). Finally, it is important to estimate the effect of the interaction between costs of reproduction and paternal care, because the cost of female reproduction may reduce the expected expression of female coloration in species with high paternal care.

Here, we carried out phylogenetic comparative analyses to test whether interspecific variation in paternal care interacts with female reproductive costs to explain some of the variation in female coloration. We controlled for male coloration to account for genetic correlation between males and females. Excluding female coloration from the models is a common approach when working on the evolution of male ornamentation. Using this approach assumes that all coloration is
47 **Introduction**

Males in many animal species display conspicuous ornaments and armaments and a large body
of evolutionary research has aimed to understand the factors at the origin of this variation
(Andersson, 1994). Females in many animal species also display conspicuous traits and in the past
twenty years, research focusing on the evolution and variation in female ornaments has increased
(Amundsen, 2000; Harrison & Poe, 2012; Nordeide et al., 2013; Odom et al., 2014; Dale et al.,
2015; Riebel et al., 2019; Doutrelant et al., 2020).

Owing to the paradigm that inter- and intra-sexual selection is typically stronger on males
(Bateman, 1948; Janicke et al., 2016; but see Hoquet, 2020), female ornamentation was
historically considered as a genetic by-product of male ornamentation with natural selection (e.g.
predation) leading to more cryptic phenotypes in females. Nowadays, there is a general agreement
that natural, sexual and social selection also impact female ornamentation, and that studies need
to understand how these complex arrays of forces interact and explain the observed variability
(Kraaijeveld et al., 2007; Clutton-Brock, 2009; Tobias et al., 2012; Hare & Simmons, 2019;
Cooney et al., 2022; Doutrelant et al., 2020).

Comparative studies have been a powerful tool to investigate the multiple drivers of female
ornamentation. Overall most of the comparative studies focused on the selective forces driving the
evolution of sexual dichromatism, which expresses the effect of sexual and natural selection acting
on both sexes (Johnson et al., 2013; Dunn et al., 2015; Shultz & Burns, 2017). These studies clearly
outlined three factors explaining the evolution of female ornamentation: predation, female
competition, and selection acting on males. Increase of predation risk, migration and intense sexual
selection on males are usually associated with lower coloration in females compared to males
(Martin & Badyaev, 1996; Dale et al., 2015; Dunn et al., 2015) whereas higher female competition
potentially adaptive, which seems unrealistic for both sexes (Price, 1996). Here, we chose a
conservative perspective, and we studied female coloration while controlling for male coloration.
Yet, for the sake of comparison with the literature, we presented the results of analyses that
excluded male coloration in ESM and discussed both results.

We worked on passerine bird species from the temperate Northern hemisphere. The amount of
paternal care during different stages of reproduction (incubation, chick rearing and post fledgling)
is better described, thus more quantifiable for these species than for non-temperate ones. Likewise,
information on female reproductive costs - egg size and volume - is also better described in
temperate species, with temperate species also presenting more variability in reproductive costs
compared to tropical ones. We measured the whole-body plumage of females and males with a
spectrometer. Bird eye physiology allows many species of birds to see ultra-violet and to
discriminate colors differently from human (Vorobyev, 2003). We therefore used avian visual
perception models so that our color metrics reflect coloration as perceived by birds in their avian
color space (Stoddard & Prum, 2008; Renoult *et al.*, 2017). We predicted that paternal care should
enhance female conspicuous plumage coloration and female reproductive costs should constrain
it, with possible interactions between these two factors (Fitzpatrick *et al.* 1995; Kokko and
Johnstone 2002, Chenoweth *et al.* 2006).

108

109

## Material & Methods

We studied 133 Northern temperate passerine species (complete list in Table S1) for which we had
enough information on paternal care and cost of reproduction. Because evolutionary lability in
female color traits was historically detected in Icteridaes and Thraupidaes (Icteridae: Irwin, 1994;
for access to reproduction in cooperative breeding species is associated with lower sexual
dichromatism (Rubenstein & Lovette, 2009; Dale et al., 2015). To go further in understanding the
variation in female ornamentation at the interspecific level, it is important to keep focusing on
female ornamentation and to test the effect of female-related forces - i.e. intensity of selective
pressures and constraints specifically acting on females (Rosvall et al., 2020; Cooney et al., 2022;
Doutrelant et al., 2020).

Egg production, as a female-specific cost of reproduction is a factor expected by theoretical
models to influence female ornamentation (Fitzpatrick et al., 1995). While predation risk has been
well-studied as a cost of reproduction limiting the evolution of female ornaments (e.g.; Martin &
Badyaev, 1996; Soler & Moreno, 2012; Dunn et al., 2015; Matysiokova et al., 2017; Cain et al.,
2019), the consequences of investing more in the production of eggs remain overlooked by
comparative studies. Theoretically, females paying higher reproductive costs are predicted to
invest less in signaling (Chenoweth et al., 2006). This trade-off was shown experimentally in blue
tits where females forced to lay more eggs during one breeding season produced less developed
color ornaments in the consecutive molt (Doutrelant et al., 2012). At the interspecific level, this
trade off was verified in the Hirundinidae family for a morphological trait, with female tail fork
depth decreasing with increasing egg size (Hasegawa & Arai, 2017). In another study, females
from tropical passerine species – typically associated with smaller clutch size than temperate
species – were found to be more colorful (Dale et al., 2015). But yearlong territoriality and lifelong
pairs are also more widespread in tropical species, and may also explain the evolution of female
coloration through intrasexual selection and male mate choice. Thus, whether variation in female
reproductive costs constrains the evolution of female coloration still needs attention.

Paternal care is by contrast a factor supposed to influence two female-related forces promoting
Thraupidaes: Burns, 1998), we included species belonging to 17 families phylogenetically close to Icteridaes and Thraupidaes (Fig. S1). All selected species breed on the mainland to avoid some confounding effects of insularity syndrome (Doutrelant *et al.*, 2016).

## Response variables - Color analyses

We measured with a spectrophotometer (details in ESM) well preserved stuffed specimens of 118 adults with breeding plumage. These specimens belong to the collections at the National Museum 119 of Natural History in Paris and were selected by AF and DG. Spectrometry is a precise and 120 objective method that is strongly recommended when the human visual system differs from that 121 of the natural signal receivers (Odeen *et al.*, 2011). Previous analyses have shown that 122 spectrometric estimates of plumage coloration are similar for well-preserved specimens and 123 freshly collected specimens (Doucet & Hill, 2009). To estimate coloration across the whole body 124 in both males and females we measured the same twenty-one topographical zones on each 125 specimen (Fig. S2). 126

We selected one male (except for one species for which two males were selected) and one female 127 from each species/subspecies, focusing on the best-preserved birds (see details in ESM and Table 128 S1), a method which is similar to the common practice in recent comparative analyses of working 129 on 'typical' specimens from bookplates or photos (Rubenstein and Lovette 2009; Dale et al. 2015; 130 Negro et al. 2018; Soler et al. 2019, Merwin et al 2020, Delhey et al 2021), while gaining more 131 precision, specifically by assessing ultraviolet part of coloration and integrating avian vision 132 instead of human vision. 133

We analyzed the spectral data using the R (R Core Team, 2022) package *pavo* (Maia *et al.*, 2019). 134 We used both equal illuminance across wavelengths and the blue tit visual system (Stoddard & 135
the evolution of female ornaments. Theoretical models predict the more time a male invests in parental care, the choosier he is expected to be (Kokko & Johnstone, 2002), enabling the evolution of female ornaments through intersexual selection. In addition, paternal care can also be indirectly related to the evolution of female ornaments through intrasexual selection if females compete for access to paternal care (Lyu et al., 2017). At the interspecific level, the effect of paternal care has been rarely tested specifically, with mixed results across studies. Absence of paternal care was related to species with duller female upper part coloration in one study (Dale et al. 2015). In another study, paternal care was more widespread in more colorful monochromatic species, but with no change in female brightness and color (Dunn et al., 2015). Last, male participation to nest-building was associated with more intense female coloration (estimated by human eye) in a comparative analysis on 178 Western Palearctic passerines (Soler et al., 2019). In these three studies, paternal care was estimated as a binary index (presence/absence) while paternal investment can vary both within the stages of reproduction and across bird species. Thus, a necessary step to help clarifying the role of paternal investment in reproduction on the evolution of female ornament is to quantify variation in male investment across different stages of paternal care (from incubation to fledgling). Finally, it is important to estimate the effect of the interaction between costs of reproduction and paternal care, because the cost of female reproduction may reduce the expected expression of female coloration in species with high paternal care.

Here, we carried out phylogenetic comparative analyses to test whether interspecific variation in paternal care interacts with female reproductive costs to explain some of the variation in female coloration. We controlled for male coloration to account for genetic correlation between males and females. Excluding female coloration from the models is a common approach when working on the evolution of male ornamentation. Using this approach assumes that all coloration is
136 Prum, 2008) to reconstruct how colors would be perceived by birds and more specifically by
137 conspecifics.

138 For each male and female of each species, we quantified three coloration metrics to encompass
139 different aspects of coloration (Renoult et al., 2017): (i) mean brightness as an achromatic metric
140 of coloration, (ii) mean chromaticity as an index of the intensity of coloration, and (iii) color
141 volume as an index of the diversity of colors (see details in ESM and, Table S2 for correlation
142 between these metrics). Male and female color volumes were log transformed to fit a normal
143 distribution. The values of the coloration metrics were standardized to have a mean of zero and a
144 variance of one.

145 **<u>Explanatory variables.</u>**

146 Values for life-history traits were determined using the literature (Cramp & Simmons, 2009;
147 Rodewald, 2015).

148 *Egg production.* We used clutch volume (mm³; egg volume*clutch size) to estimate reproductive
costs associated with female initial investment in reproduction (referred as “egg production”)
149 (Watson *et al.*, 2015). This allowed us to account for both egg production strategies: large clutch
150 sizes or large eggs. We only used mean clutch size when median clutch size (a more common
151 statistic) was not available. Egg volume (mm³) was calculated using Hoyt’s formula (Hoyt, 1979):
152 egg volume=0.509*(egg length)*(egg width)². Since we used closely related songbird species, we
153 assumed there was no bias due to the potential asymmetry in egg shape (Hoyt, 1979).
154

155 *Paternal care.* In songbirds, male investment can vary continuously from a lack of paternal care
to equal share of duties with females. In some species, male's contribution relates only to post-
156 fledgling care whereas in other species, males also share incubation duty with females. To account
157
potentially adaptive, which seems unrealistic for both sexes (Price, 1996). Here, we chose a
116
conservative perspective, and we studied female coloration while controlling for male coloration.
117
Yet, for the sake of comparison with the literature, we presented the results of analyses that
118
excluded male coloration in ESM and discussed both results.
119

We worked on passerine bird species from the temperate Northern hemisphere. The amount of
120
paternal care during different stages of reproduction (incubation, chick rearing and post fledgling)
121
is better described, thus more quantifiable for these species than for non-temperate ones. Likewise,
122
information on female reproductive costs - egg size and volume - is also better described in
123
temperate species, with temperate species also presenting more variability in reproductive costs
124
compared to tropical ones. We measured the whole-body plumage of females and males with a
125
spectrometer. Bird eye physiology allows many species of birds to see ultra-violet and to
126
discriminate colors differently from human (Vorobyev, 2003). We therefore used avian visual
127
perception models so that our color metrics reflect coloration as perceived by birds in their avian
128
color space (Stoddard & Prum, 2008; Renoult *et al.*, 2017). We predicted that paternal care should
129
enhance female conspicuous plumage coloration and female reproductive costs should constrain
130
it, with possible interactions between these two factors (Fitzpatrick *et al.* 1995; Kokko and
131
Johnstone 2002, Chenoweth *et al.* 2006).
132
133
134

## Material & Methods

We studied 133 Northern temperate passerine species (complete list in Table S1) for which we had
136
enough information on paternal care and cost of reproduction. Because evolutionary lability in
137
female color traits was historically detected in Icteridaes and Thraupidaes (Icteridae: Irwin, 1994;
138
for this variation of investment in parental care, we characterized paternal care during three main reproductive stages: incubation, chick rearing (i.e., brooding and feeding), and post-fledgling.

We used the method developed by Webb et al. (2010) to quantify the relative male contribution during each stage ($P_{X male}$ in (1)) and then to define the relative male contribution to total parental care ($P_{T male}$ in (2)).

$$P_{X male} = \frac{\sum \text{male contribution at stage "X"}}{\sum \text{male contribution at stage "X" + female contribution at stage "X"}} \quad (1)$$

$$P_{T male} = \frac{P_{I male} * t_{I} + P_{R male} * t_{R} + P_{F male} * t_{F}}{t_{T}} \quad (2)$$

$P_{T male}$, $P_{I male}$, $P_{R male}$, and $P_{F male}$ in (2) refer to the relative contribution made by males to (T) total parental care, (I) incubation, (R) chick rearing, and (F) post-fledging, respectively. The variables $t_T$, $t_I$, $t_R$, and $t_F$ refer to the duration (in days) of these same periods, where $t_T=t_I+t_R+t_F$ (see ESM for details).

We based our scoring on quantitative estimates of contributions when available in (Webb et al., 2010). Otherwise, we estimated them as followed: when only descriptions were provided, we applied a standardized scoring procedure based on the use of adverbs and other qualifiers (rarely, often, female-only, etc.; details in Table S3). Median activity duration was used when mean activity duration was not available. When possible, we employed information corresponding to the subspecies selected for plumage measurement; otherwise, we used information corresponding to main subspecies. AF assigned scores. To control for bias in assigning scores, the list of species with adverbs and qualifiers used in the literature was provided to all co-authors who were asked
Thraupidaes: Burns, 1998), we included species belonging to 17 families phylogenetically close to Icteridaes and Thraupidaes (Fig. S1). All selected species breed on the mainland to avoid some confounding effects of insularity syndrome (Doutrelant *et al.*, 2016).

## Response variables - Color analyses

We measured with a spectrophotometer (details in ESM) well preserved stuffed specimens of 143 adults with breeding plumage. These specimens belong to the collections at the National Museum 144 of Natural History in Paris and were selected by AF and DG. Spectrometry is a precise and 145 objective method that is strongly recommended when the human visual system differs from that 146 of the natural signal receivers (Odeen *et al.*, 2011). Previous analyses have shown that 147 spectrometric estimates of plumage coloration are similar for well-preserved specimens and 148 freshly collected specimens (Doucet & Hill, 2009). To estimate coloration across the whole body 149 in both males and females we measured the same twenty-one topographical zones on each 150 specimen (Fig. S2). 151

We selected one male (except for one species for which two males were selected) and one female 152 from each species/subspecies, focusing on the best-preserved birds (see details in ESM and Table 153 S1), a method which is similar to the common practice in recent comparative analyses of working 154 on 'typical' specimens from bookplates or photos (Rubenstein and Lovette 2009; Dale et al. 2015; 155 Negro et al. 2018; Soler et al. 2019, Merwin et al 2020, Delhey et al 2021), while gaining more 156 precision, specifically by assessing ultraviolet part of coloration and integrating avian vision 157 instead of human vision. 158

We analyzed the spectral data using the R (R Core Team, 2022) package *pavo* (Maia *et al.*, 2019). 159 We used both equal illuminance across wavelengths and the blue tit visual system (Stoddard & 160
180 to assign a score to each species based on the qualifiers. There was a high correlation among co-  
181 authors in the way of assigning scores (>0.82). The scoring performed by AF was kept for analyses.

182 For all species in the dataset, the total female contribution to parental care was equal to or higher
183 than the total male contribution. There were no species with male-only parental care; we thus
184 avoided rare cases of reverse sexual selection, which have already received significant attention
185 (Edward & Chapman, 2011).

186 **Controlled covariates.** Our analyses also included additional factors likely to affect the evolution
187 of female coloration. Controlled covariates corresponding to life-history traits were determined
188 using the literature (Cramp & Simmons, 2009; Rodewald, 2015).

189 *Male coloration:* genetic correlation and constraints on male coloration is predicted to affect
190 female coloration. We used a conservative approach and included male coloration (corresponding
191 color metric) as a covariable in our analysis to quantify this correlation.

192 *Nest characteristics:* nest height and openness were related to color evolution (Soler & Moreno,
193 2012) by increasing vulnerability to predation during nest attendance. We used an index of nest
194 characteristics, combining nest openness and nest height, to control its effect on female coloration
195 in our analysis. We assigned a score from 0 to 2 to both nest position and openness, for each species
196 (details in Table S4). We then summed the two scores to an overall score ranging from 0 to 4, with
197 0 characterizing the lowest level of vulnerability and 4 the highest level of vulnerability of nest
198 characteristics.

199 *Female body size:* Since female body size can influence egg production (Bennett & Owens, 2002)
200 as well as plumage coloration (Dale *et al.*, 2015; Cooney *et al.*, 2022), we also included in our
161 Prum, 2008) to reconstruct how colors would be perceived by birds and more specifically by
162 conspecifics.

163 For each male and female of each species, we quantified three coloration metrics to encompass
164 different aspects of coloration (Renoult et al., 2017): (i) mean brightness as an achromatic metric
165 of coloration, (ii) mean chromaticity as an index of the intensity of coloration, and (iii) color
166 volume as an index of the diversity of colors (see details in ESM and, Table S2 for correlation
167 between these metrics). Male and female color volumes were log transformed to fit a normal
168 distribution. The values of the coloration metrics were standardized to have a mean of zero and a
169 variance of one.

170 **<u>Explanatory variables.</u>**

171 Values for life-history traits were determined using the literature (Cramp & Simmons, 2009;
172 Rodewald, 2015).

173 ***Egg production.*** We used clutch volume (mm³; egg volume*clutch size) to estimate reproductive
costs associated with female initial investment in reproduction (referred as “egg production”)
(Watson *et al.*, 2015). This allowed us to account for both egg production strategies: large clutch
sizes or large eggs. We only used mean clutch size when median clutch size (a more common
statistic) was not available. Egg volume (mm³) was calculated using Hoyt’s formula (Hoyt, 1979):
egg volume=0.509*(egg length)*(egg width)². Since we used closely related songbird species, we
assumed there was no bias due to the potential asymmetry in egg shape (Hoyt, 1979).

180 ***Paternal care.*** In songbirds, male investment can vary continuously from a lack of paternal care
to equal share of duties with females. In some species, male's contribution relates only to post-
fledgling care whereas in other species, males also share incubation duty with females. To account
analyses mean female body mass for each species and log transformed the values to fit a normal
distribution.

*Migration*: migratory behavior has also been shown to affect color evolution (Friedman *et al.*, 2009; Dale *et al.*, 2015; Dunn *et al.*, 2015; Delhey *et al.*, 2021; Cooney *et al.*, 2022). Three levels were defined for this variable: strictly resident, partially migrant and strictly migrant.

*Breeding period duration (in days)*: Breeding period duration was also added as a covariable because it may influence reproductive investment and thus plumage coloration (Bokony & Liker, 2005). It was calculated by multiplying the amount of time needed to produce a single brood by the mean number of broods per breeding season. This value was log transformed to fit a normal distribution.

211 **Phylogenetic comparative methods**

We carried out phylogenetic generalized least squares (PGLS) (Grafen, 1989) using the R
packages *phytools* (Revell, 2018) and *phylolm* (Ho & Ane, 2014). PGLS analyses make it possible
to run a model and estimate the phylogenetic signal in the model residuals at the same time (Revell,
2010). To account for potential phylogenetic relationships among the species in our dataset
(Felsenstein, 1985), we extracted 500 random phylogenetic trees (available file “phylogenies.tre”
in dryad repository) with Hackett backbones from the birdtree.org website (Jetz *et al.*, 2012) using
the R package *ape* (Paradis *et al.*, 2018).

For each of the three color metrics, we ran a linear model including our two predictors of interest
(paternal care and egg production), allowing their interaction, and the five covariates defined
above. All the predictors and covariates tested in our analysis are presented in Figure 1 and their
values were similarly standardized. We tested for multicollinearity among them using the R
for this variation of investment in parental care, we characterized paternal care during three main reproductive stages: incubation, chick rearing (i.e., brooding and feeding), and post-fledgling.

We used the method developed by Webb et al. (2010) to quantify the relative male contribution during each stage ($P_{X male}$ in (1)) and then to define the relative male contribution to total parental care ($P_{T male}$ in (2)).

$$P_{X male} = \frac{\sum \text{male contribution at stage "X"}}{\sum \text{male contribution at stage "X" + female contribution at stage "X"}} \quad (1)$$

$$P_{T male} = \frac{P_{I male} * t_{I} + P_{R male} * t_{R} + P_{F male} * t_{F}}{t_{T}} \quad (2)$$

$P_{T male}$, $P_{I male}$, $P_{R male}$, and $P_{F male}$ in (2) refer to the relative contribution made by males to (T) total parental care, (I) incubation, (R) chick rearing, and (F) post-fledging, respectively. The variables $t_T$, $t_I$, $t_R$, and $t_F$ refer to the duration (in days) of these same periods, where $t_T=t_I+t_R+t_F$ (see ESM for details).

We based our scoring on quantitative estimates of contributions when available in (Webb et al., 2010). Otherwise, we estimated them as followed: when only descriptions were provided, we applied a standardized scoring procedure based on the use of adverbs and other qualifiers (rarely, often, female-only, etc.; details in Table S3). Median activity duration was used when mean activity duration was not available. When possible, we employed information corresponding to the subspecies selected for plumage measurement; otherwise, we used information corresponding to main subspecies. AF assigned scores. To control for bias in assigning scores, the list of species with adverbs and qualifiers used in the literature was provided to all co-authors who were asked
package yhat (Nimon et al., 2013). Apart from a strong correlation between female body mass and egg production (r=0.683), multicollinearity was low (r<0.07).

For each color metric set as the variable to explain, we ran PGLS analyses on the models using each of the 500 phylogenetic trees and simultaneously estimated Pagel's lambda for tree (Table 1). To include uncertainty (Paradis, 2012; Garamszegi & Mundry, 2014) within the songbird phylogeny, we used a model-averaging procedure (Garamszegi & Mundry, 2014) that was based on the AIC weights associated with each tree and calculated the estimate values and 95% confidence intervals for all the predictors in the models accounting for phylogenetic uncertainty (Garamszegi & Mundry, 2014) based on 500 phylogenetic trees.

## Results

The three female color metrics were strongly associated (all estimates ~0.7) with their corresponding male color metrics (Figure 1, Table 1). More colored females were found in species with more colored males.

Female brightness and female chromaticity were slightly correlated to paternal care (brightness: 0.14 [0.03; 0.26] 95% CI, chromaticity: 0.13 [0.002; 0.26] 95% CI) independently of egg production (Figure 1a, Figure 2, Table 1). Species with more paternal care had brighter females and higher female chromaticity. The interaction between paternal care and egg production was in addition marginally associated with female chromaticity (-0.14 [-0.26; -0.02] 95% CI), Figure 1b, Table 1). At low egg production, female chromaticity increased when paternal care increased. When paternal care was relatively high, female chromaticity decreased when egg production increased (Figure 3). We found no effect of paternal care or egg production on female color volume (Figure 1c, Table 1).
to assign a score to each species based on the qualifiers. There was a high correlation among co-authors in the way of assigning scores (>0.82). The scoring performed by AF was kept for analyses.

For all species in the dataset, the total female contribution to parental care was equal to or higher than the total male contribution. There were no species with male-only parental care; we thus avoided rare cases of reverse sexual selection, which have already received significant attention (Edward & Chapman, 2011).

**Controlled covariates.** Our analyses also included additional factors likely to affect the evolution of female coloration. Controlled covariates corresponding to life-history traits were determined using the literature (Cramp & Simmons, 2009; Rodewald, 2015).

*Male coloration:* genetic correlation and constraints on male coloration is predicted to affect female coloration. We used a conservative approach and included male coloration (corresponding color metric) as a covariable in our analysis to quantify this correlation.

*Nest characteristics:* nest height and openness were related to color evolution (Soler & Moreno, 2012) by increasing vulnerability to predation during nest attendance. We used an index of nest characteristics, combining nest openness and nest height, to control its effect on female coloration in our analysis. We assigned a score from 0 to 2 to both nest position and openness, for each species (details in Table S4). We then summed the two scores to an overall score ranging from 0 to 4, with 0 characterizing the lowest level of vulnerability and 4 the highest level of vulnerability of nest characteristics.

*Female body size:* Since female body size can influence egg production (Bennett & Owens, 2002) as well as plumage coloration (Dale et al., 2015; Cooney et al., 2022), we also included in our
Among the covariates, only migratory behavior was correlated with chromaticity. Females from strictly resident species had less intense coloration than partially migrant and migrant species (-0.43 [-0.83; -0.03] 95% CI) (Figure 1 and Table 1).

## Discussion

This study aimed at determining whether female cost of reproduction and paternal care explained part of the evolution of female ornaments. The results suggest modest but detectable male-independent associations between these two factors and two proxies of female coloration. Female brightness slightly covaries with paternal care, and female intensity of coloration (chromaticity) is associated with paternal care and the interaction between the two factors. In addition, our results confirm strong coevolution between male and female coloration, which represents a major source of variation for the coloration of both sexes.

In two former very large-scale comparative analyses, presence/absence of paternal care during nestling stage tested in association with other proxies of male sexual or natural selection, was associated with female coloration, once positively and once negatively (Dale et al., 2015; Dunn et al., 2015). In one smaller scale comparative analysis, the presence/absence of paternal care during incubation was in addition positively associated with female coloration (Soler et al. 2019). Our analysis, also conducted at a small scale but using a continuous index of paternal care, encompassing incubation to post-fledging care, suggests that interspecific variation in time invested by males in parental care is associated to both female brightness and female chromaticity.

In the Northern temperate passerine birds included in our study, paternal care is slightly positively related to female brightness (Figure 2). A positive association may result from the action of two non-exclusive forces: through male mate choice as males are predicted to be choosier when they invest more in paternal behaviors (Kokko & Johnstone, 2002), and through intra-sexual
analyses mean female body mass for each species and log transformed the values to fit a normal distribution.

*Migration*: migratory behavior has also been shown to affect color evolution (Friedman *et al.*, 2009; Dale *et al.*, 2015; Dunn *et al.*, 2015; Delhey *et al.*, 2021; Cooney *et al.*, 2022). Three levels were defined for this variable: strictly resident, partially migrant and strictly migrant.

*Breeding period duration (in days)*: Breeding period duration was also added as a covariable because it may influence reproductive investment and thus plumage coloration (Bokony & Liker, 2005). It was calculated by multiplying the amount of time needed to produce a single brood by the mean number of broods per breeding season. This value was log transformed to fit a normal distribution.

## Phylogenetic comparative methods

We carried out phylogenetic generalized least squares (PGLS) (Grafen, 1989) using the R packages *phytools* (Revell, 2018) and *phylolm* (Ho & Ane, 2014). PGLS analyses make it possible to run a model and estimate the phylogenetic signal in the model residuals at the same time (Revell, 2010). To account for potential phylogenetic relationships among the species in our dataset (Felsenstein, 1985), we extracted 500 random phylogenetic trees (available file "phylogenies.tre" in dryad repository) with Hackett backbones from the birdtree.org website (Jetz *et al.*, 2012) using the R package *ape* (Paradis *et al.*, 2018).

For each of the three color metrics, we ran a linear model including our two predictors of interest (paternal care and egg production), allowing their interaction, and the five covariates defined above. All the predictors and covariates tested in our analysis are presented in Figure 1 and their values were similarly standardized. We tested for multicollinearity among them using the R
269 competition where females compete to access paternal care (Tobias et al., 2012; Cain, 2014). For 270 female chromaticity, our study suggests in addition a small but detectable role of female 271 investment in egg production. In species with low egg production, the chromaticity of female 272 coloration increases with paternal care, but in species with the highest male investment, the 273 chromaticity of female coloration decreases when investment in egg production increases (Figure 274 3). This interaction suggests a threshold beyond which the cost of reproduction in females could 275 no longer favor the positive coevolution of female coloration and male investment in parental care. 276 The colors of ornaments, i.e. their chromaticity, are often costly to produce (McGraw, 2006), and 277 a trade-off between female investment in offspring and female investment in costly ornaments is 278 predicted to limit the evolution of female coloration at some point (Fitzpatrick et al., 1995; 279 Chenoweth et al., 2006). This trade-off can even lead female traits to become non-functional 280 (Nordeide et al., 2013). Nonetheless, most of the species included in our study have some form of 281 paternal care, and only few species have large egg production, (Figure 3). Therefore, to confirm 282 the detected trade-off between investing in eggs or color intensity, future comparative analyses 283 may include more species with low or no paternal care and high level of female investment in 284 eggs. 285

We found no association between female brightness and egg production. This suggests that the 285 production of achromatic ornamentation may be less costly and thus less constrained by the cost 286 of egg production than color intensity. In an experiment in blue tits, an increase in reproductive 287 costs in females changed the chromatic component (hue) of the blue crown but did not affect 288 brightness of the same patch (Doutrelant et al., 2012). Experiments aimed at relating the effect of 289 reproductive costs on both achromatic and chromatic components of coloration are needed to 290 confirm this suggestion. 291
package yhat (Nimon et al., 2013). Apart from a strong correlation between female body mass and egg production (r=0.683), multicollinearity was low (r<0.07).

For each color metric set as the variable to explain, we ran PGLS analyses on the models using each of the 500 phylogenetic trees and simultaneously estimated Pagel's lambda for tree (Table 1). To include uncertainty (Paradis, 2012; Garamszegi & Mundry, 2014) within the songbird phylogeny, we used a model-averaging procedure (Garamszegi & Mundry, 2014) that was based on the AIC weights associated with each tree and calculated the estimate values and 95% confidence intervals for all the predictors in the models accounting for phylogenetic uncertainty (Garamszegi & Mundry, 2014) based on 500 phylogenetic trees.

The three female color metrics were strongly associated (all estimates ~0.7) with their corresponding male color metrics (Figure 1, Table 1). More colored females were found in species with more colored males.

Female brightness and female chromaticity were slightly correlated to paternal care (brightness: 0.14 [0.03; 0.26] 95% CI, chromaticity: 0.13 [0.002; 0.26] 95% CI) independently of egg production (Figure 1a, Figure 2, Table 1). Species with more paternal care had brighter females and higher female chromaticity. The interaction between paternal care and egg production was in addition marginally associated with female chromaticity (-0.14 [-0.26; -0.02] 95% CI), Figure 1b, Table 1). At low egg production, female chromaticity increased when paternal care increased. When paternal care was relatively high, female chromaticity decreased when egg production increased (Figure 3). We found no effect of paternal care or egg production on female color volume (Figure 1c, Table 1).
Color volume was not associated with any of our tested variables. Color volume relates to the diversity of coloration (Stoddard & Prum, 2008), and it is sometimes thought that greater volume indicates more diverse mechanisms of color production. The fact that paternal care and female investment in reproduction does not affect this diversity of coloration in females when controlling for male coloration may suggest diversity of coloration in females is not under strong intra- and intersexual selection in females and its evolution may be related to other selective forces. To date, comparative analyses found an association between color volume and sex, documenting higher diversity of colors in males than females (Delhey, 2015; Shultz & Burns, 2017), and males from sexually dichromatic species to be more colorful than males from less dichromatic species (Cooney et al., 2022). Additionally, they found for each sex an association between color volume or colorfulness and type of habitats (Shultz & Burns, 2017; Cooney et al., 2022), or between color volume and the number of sympatric species from the same family (Doutrelant et al., 2016). Thus, habitat selection and interspecific competition may be the main factors associated with the evolution of color volume, with stronger selection in males than females.

For our three proxies of coloration, male coloration was the factor explaining most of the variation in female coloration. This result was also found in several former comparative studies (e.g. Dale et al. 2015; e.g. Delhey 2015; Shultz and Burns 2017; Cooney et al. 2019). This strong phenotypic correlation is potentially in favor of the prevailing role of genetic correlation (Lande, 1980) i.e. shared genetic architecture, in the level of expression of color traits in females. For instance, the coloration of the red bill of zebra finches is strongly correlated between fathers and daughters (Price, 1996). However, phenotypic correlation may also be the result of other color-correlated factors such as shared diets (Amundsen et al., 2006) as well as shared natural, social, and sexual selective pressures acting on both sexes (Shultz & Burns, 2017). It could also be the
Among the covariates, only migratory behavior was correlated with chromaticity. Females from strictly resident species had less intense coloration than partially migrant and migrant species (-0.43 [-0.83; -0.03] 95% CI) (Figure 1 and Table 1).

## Discussion

This study aimed at determining whether female cost of reproduction and paternal care explained part of the evolution of female ornaments. The results suggest modest but detectable male-independent associations between these two factors and two proxies of female coloration. Female brightness slightly covaries with paternal care, and female intensity of coloration (chromaticity) is associated with paternal care and the interaction between the two factors. In addition, our results confirm strong coevolution between male and female coloration, which represents a major source of variation for the coloration of both sexes.

In two former very large-scale comparative analyses, presence/absence of paternal care during nestling stage tested in association with other proxies of male sexual or natural selection, was associated with female coloration, once positively and once negatively (Dale et al., 2015; Dunn et al., 2015). In one smaller scale comparative analysis, the presence/absence of paternal care during incubation was in addition positively associated with female coloration (Soler et al. 2019). Our analysis, also conducted at a small scale but using a continuous index of paternal care, encompassing incubation to post-fledging care, suggests that interspecific variation in time invested by males in parental care is associated to both female brightness and female chromaticity.

In the Northern temperate passerine birds included in our study, paternal care is slightly positively related to female brightness (Figure 2). A positive association may result from the action of two non-exclusive forces: through male mate choice as males are predicted to be choosier when they invest more in paternal behaviors (Kokko & Johnstone, 2002), and through intra-sexual
result of divergent types of selection in males and females that select for similar ornamentation in both sexes, for instance social selection for females and sexual selection for males (Tobias *et al.*, 2012). So the part of variance explained by the phenotypic correlation in our models can also overestimate the importance of genetic correlation. Female coloration may be driven by female-specific selective pressure even in species sharing similar colors, with different forces leading to similar colorations (Amundsen *et al.*, 2006; Enbody *et al.*, 2017).

By including male coloration in our analyses, we have statistically “corrected” for correlated plumage similarities between sexes and the effect of genetic correlation, but we have also drastically reduced our ability to detect the real importance of selective pressures. When removing male corresponding color metric from our models (See Table S5), we found that estimates for egg production were much bigger in relation to female brightness (0.435 [0.063;0.806] 95% CI), suggesting we potentially underestimated the effect of female investment on female coloration. Estimate values remained similar for paternal care and the interaction between paternal care and egg production (See Table S5). So female reproductive costs may drive the evolution of female coloration (constraining chromaticity and favoring brightness) as a higher level than what we find with our conservative approach including male coloration in the statistical model. Yet, we think it is better to retain the conclusion of the model controlling for male coloration, to include the acknowledged effect of genetic correlation.

We assumed that species showing higher values of brightness and chromaticity could be seen as more ornamented/conspicuous This is reasonable for conspicuous carotenoid and structural colors (red, blue, yellow, blue, violet), but works poorly for dark to black ornaments (Marcondes & Brumfield, 2019). If males produce black ornaments and females not, female brightness and chromaticity will be higher than male brightness and chromaticity but in fact female would appear
294 competition where females compete to access paternal care (Tobias et al., 2012; Cain, 2014). For 295 female chromaticity, our study suggests in addition a small but detectable role of female 296 investment in egg production. In species with low egg production, the chromaticity of female 297 coloration increases with paternal care, but in species with the highest male investment, the 298 chromaticity of female coloration decreases when investment in egg production increases (Figure 299 3). This interaction suggests a threshold beyond which the cost of reproduction in females could 300 no longer favor the positive coevolution of female coloration and male investment in parental care. 301 The colors of ornaments, i.e. their chromaticity, are often costly to produce (McGraw, 2006), and 302 a trade-off between female investment in offspring and female investment in costly ornaments is 303 predicted to limit the evolution of female coloration at some point (Fitzpatrick et al., 1995; 304 Chenoweth et al., 2006). This trade-off can even lead female traits to become non-functional 305 (Nordeide et al., 2013). Nonetheless, most of the species included in our study have some form of 306 paternal care, and only few species have large egg production, (Figure 3). Therefore, to confirm 307 the detected trade-off between investing in eggs or color intensity, future comparative analyses 308 may include more species with low or no paternal care and high level of female investment in 309 eggs. 310

310 We found no association between female brightness and egg production. This suggests that the 311 production of achromatic ornamentation may be less costly and thus less constrained by the cost 312 of egg production than color intensity. In an experiment in blue tits, an increase in reproductive 313 costs in females changed the chromatic component (hue) of the blue crown but did not affect 314 brightness of the same patch (Doutrelant et al., 2012). Experiments aimed at relating the effect of 315 reproductive costs on both achromatic and chromatic components of coloration are needed to 316 confirm this suggestion.
338 duller. But only three species of our data set correspond to males with large amount of black
339 coloration (using a method adapted from (Osorio et al., 2004) that defines black as a coloration
340 that produces less than 0.05 percentage reflectance of the white standard at each photoreceptors),
341 so it unlikely affected our results.

342 Regarding ecological factors included as covariate in our study, we found a correlation between
343 migratory behavior and intensity of coloration. In accordance with previous work (Dale et al.,
344 2015; Dunn et al., 2015), females from strictly resident species were less likely to have intense
345 colors (chromaticity metric). Unlike Cooney et al (2022), we did not find a correlation between
346 migratory behavior and female color volume, which may be a consequence of our dataset,
347 restricted to species breeding in northern temperate areas. We did not find any correlation between
348 female color metrics and nest characteristics. Different studies had found that birds breeding in
349 higher and more open nests have less conspicuous females (Martin & Badyaev, 1996; Soler &
350 Moreno, 2012), but other recent studies failed to find a correlation between predation risk or nest
351 openness and plumage conspicuousness (Matysiokova et al., 2017; Cain et al., 2019; Simpson et
352 al., 2020). This lack of consistency among studies can be explained by the difficulty to build
353 adequate metrics to estimate predation risk. It can also be related to variation in the quantification
354 of coloration (human vision, color from bookplates, spectrometry) or to the decision of treating
355 males and females together or separately.

356 To conclude, our work confirms the strong covariation between male and female coloration. In
357 addition, it reveals a moderate association between paternal care and female coloration when
358 female costs are reduced, either with lower cost of egg production, or lower cost of producing
359 female ornament. These costs are likely important to consider and should be incorporated when
360 testing other potential factors that drives the evolution of female ornamentation (such as female
Color volume was not associated with any of our tested variables. Color volume relates to the diversity of coloration (Stoddard & Prum, 2008), and it is sometimes thought that greater volume indicates more diverse mechanisms of color production. The fact that paternal care and female investment in reproduction does not affect this diversity of coloration in females when controlling for male coloration may suggest diversity of coloration in females is not under strong intra- and intersexual selection in females and its evolution may be related to other selective forces. To date, comparative analyses found an association between color volume and sex, documenting higher diversity of colors in males than females (Delhey, 2015; Shultz & Burns, 2017), and males from sexually dichromatic species to be more colorful than males from less dichromatic species (Cooney et al., 2022). Additionally, they found for each sex an association between color volume or colorfulness and type of habitats (Shultz & Burns, 2017; Cooney et al., 2022), or between color volume and the number of sympatric species from the same family (Doutrelant et al., 2016). Thus, habitat selection and interspecific competition may be the main factors associated with the evolution of color volume, with stronger selection in males than females.

For our three proxies of coloration, male coloration was the factor explaining most of the variation in female coloration. This result was also found in several former comparative studies (e.g. Dale et al. 2015; e.g. Delhey 2015; Shultz and Burns 2017; Cooney et al. 2019). This strong phenotypic correlation is potentially in favor of the prevailing role of genetic correlation (Lande, 1980) i.e. shared genetic architecture, in the level of expression of color traits in females. For instance, the coloration of the red bill of zebra finches is strongly correlated between fathers and daughters (Price, 1996). However, phenotypic correlation may also be the result of other color-correlated factors such as shared diets (Amundsen et al., 2006) as well as shared natural, social, and sexual selective pressures acting on both sexes (Shultz & Burns, 2017). It could also be the
361 territoriality). Paternal care is associated with both female brightness and chromaticity showing the importance of considering this factor in further analyses. More generally, this study shows the importance of testing factors related to female-specific forces of selection and costs, to understand and determine the array of factors leading to the evolution of female ornaments.

365
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By including male coloration in our analyses, we have statistically “corrected” for correlated plumage similarities between sexes and the effect of genetic correlation, but we have also drastically reduced our ability to detect the real importance of selective pressures. When removing male corresponding color metric from our models (See Table S5), we found that estimates for egg production were much bigger in relation to female brightness (0.435 [0.063;0.806] 95% CI), suggesting we potentially underestimated the effect of female investment on female coloration. Estimate values remained similar for paternal care and the interaction between paternal care and egg production (See Table S5). So female reproductive costs may drive the evolution of female coloration (constraining chromaticity and favoring brightness) as a higher level than what we find with our conservative approach including male coloration in the statistical model. Yet, we think it is better to retain the conclusion of the model controlling for male coloration, to include the acknowledged effect of genetic correlation.

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418 Fitzpatrick, S., Berglund, A. & Rosenqvist, G. 1995. Ornaments or Offspring - Costs to Reproductive Success. *Restrict Sexual Selection Processes*. *Biol. J. Linnean Soc.* **55**: 251-260.

420 Garamszegi, L.Z. & Mundry, R. 2014. Multimodel-Inference in Comparative Analyses. In: *Modern phylogenetic comparative methods and their application in evolutionary biology.*, pp. 305-331. Springer, New York.

423 Grafen, A. 1989. The Phylogenetic Regression. *Philos. Trans. R. Soc. Lond. Ser. B-Biol. Sci.* **326**: 119-157.
363 duller. But only three species of our data set correspond to males with large amount of black
364 coloration (using a method adapted from (Osorio et al., 2004) that defines black as a coloration
365 that produces less than 0.05 percentage reflectance of the white standard at each photoreceptors),
366 so it unlikely affected our results.

367 Regarding ecological factors included as covariate in our study, we found a correlation between
368 migratory behavior and intensity of coloration. In accordance with previous work (Dale et al.,
369 2015; Dunn et al., 2015), females from strictly resident species were less likely to have intense
370 colors (chromaticity metric). Unlike Cooney et al (2022), we did not find a correlation between
371 migratory behavior and female color volume, which may be a consequence of our dataset,
372 restricted to species breeding in northern temperate areas. We did not find any correlation between
373 female color metrics and nest characteristics. Different studies had found that birds breeding in
374 higher and more open nests have less conspicuous females (Martin & Badyaev, 1996; Soler &
375 Moreno, 2012), but other recent studies failed to find a correlation between predation risk or nest
376 openness and plumage conspicuousness (Matysiokova et al., 2017; Cain et al., 2019; Simpson et
377 al., 2020). This lack of consistency among studies can be explained by the difficulty to build
378 adequate metrics to estimate predation risk. It can also be related to variation in the quantification
379 of coloration (human vision, color from bookplates, spectrometry) or to the decision of treating
380 males and females together or separately.

381 To conclude, our work confirms the strong covariation between male and female coloration. In
382 addition, it reveals a moderate association between paternal care and female coloration when
383 female costs are reduced, either with lower cost of egg production, or lower cost of producing
384 female ornament. These costs are likely important to consider and should be incorporated when
385 testing other potential factors that drives the evolution of female ornamentation (such as female
424 Hare, R.M. & Simmons, L.W. 2019. Sexual selection and its evolutionary consequences in female
425 animals. *Biological Reviews* **94**: 929–956.

426 Harrison, A. & Poe, S. 2012. Evolution of an ornament, the dewlap, in females of the lizard genus Anolis.
427 *Biological Journal of the Linnean Society* **106**: 191–201.

428 Hasegawa, M. & Arai, E. 2017. Egg size decreases with increasing female tail fork depth in family
429 Hirundinidae. *Evol. Ecol.* **31**: 559–569.

430 Ho, L.S.T. & Ane, C. 2014. A linear-time algorithm for Gaussian and non-Gaussian trait evolution models.
431 *Systematic Biology* **63**: 397–408.

432 Hoquet, T. 2020. Bateman (1948): rise and fall of a paradigm? *Animal Behaviour* **164**: 223–231.

433 Hoyt, D. 1979. Practical Methods of Estimating Volume and Fresh Weight of Bird Eggs. *AUK* **96**: 73–77.

434 Irwin, R. 1994. The Evolution of Plumage Dichromatism in the New-World Blackbirds - Social Selection
435 on Female Brightness. *Am. Nat.* **144**: 890–907.

436 Janicke, T., Haederer, I.K., Lajeunesse, M.J. & Anthes, N. 2016. Darwinian sex roles confirmed across the
437 animal kingdom. *Sci. Adv.* **2**: UNSP e1500983.

438 Jetz, W., Thomas, G.H., Joy, J.B., Hartmann, K. & Mooers, A.O. 2012. The global diversity of birds in space
439 and time. *Nature* **491**: 444–448.

440 Johnson, A.E., Price, J.J. & Pruett-Jones, S. 2013. Different modes of evolution in males and females
441 generate dichromatism in fairy-wrens (Maluridae). *Ecol. Evol.* **3**: 3030–3046.

442 Kokko, H. & Johnstone, R.A. 2002. Why is mutual mate choice not the norm? Operational sex ratios, sex
443 roles and the evolution of sexually dimorphic and monomorphic signalling. *Philos. Trans. R. Soc.
444 Lond. Ser. B-Biol. Sci.* **357**: 319–330.

445 Kraaijeveld, K., Kraaijeveld-Smit, F.J.L. & Komdeur, J. 2007. The evolution of mutual ornamentation.
446 *Anim. Behav.* **74**: 657–677.

447 Lande, R. 1980. Sexual Dimorphism, Sexual Selection, and Adaptation in Polygenic Characters. *Evolution*
448 **34**: 292–305. [Society for the Study of Evolution, Wiley].

449 Lyu, N., Servedio, M.R., Lloyd, H. & Sun, Y.-H. 2017. The evolution of postpairing male mate choice.
450 *Evolution* **71**: 1465–1477.

451 Maia, R., Gruson, H., Endler, J.A. & White, T.E. 2019. pavo 2: New tools for the spectral and spatial
452 analysis of colour in r. *Methods in Ecology and Evolution* **10**: 1097–1107.

453 Marcondes, R.S. & Brumfield, R.T. 2019. Fifty shades of brown: Macroevolution of plumage brightness in
454 the Furnariida, a large clade of drab Neotropical passerines. *Evolution* **73**: 704–719.

455 Martin, T.E. & Badyaev, A.V. 1996. Sexual Dichromatism in Birds: Importance of Nest Predation and Nest
456 Location for Females Versus Males. *Evolution* **50**: 2454–2460.
386 territoriality). Paternal care is associated with both female brightness and chromaticity showing
387 the importance of considering this factor in further analyses. More generally, this study shows the
388 importance of testing factors related to female-specific forces of selection and costs, to understand
389 and determine the array of factors leading to the evolution of female ornaments.

390
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443 Fitzpatrick, S., Berglund, A. & Rosenqvist, G. 1995. Ornaments or Offspring - Costs to Reproductive Success. *Restrict Sexual Selection Processes*. *Biol. J. Linnean Soc.* **55**: 251-260.

445 Garamszegi, L.Z. & Mundry, R. 2014. Multimodel-Inference in Comparative Analyses. In: *Modern phylogenetic comparative methods and their application in evolutionary biology.*, pp. 305-331. Springer, New York.

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495 Rubenstein, D.R. & Lovette, I.J. 2009. Reproductive skew and selection on female ornamentation in social species. *Nature* **462**: 786–U106.

497 Shultz, A.J. & Burns, K.J. 2017. The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae). *Evolution* **71**: 1061–1074.

499 Simpson, R.K., Mistakidis, A.F. & Doucet, S.M. 2020. Natural and sexual selection shape the evolution of colour and conspicuousness in North American wood-warblers (Parulidae). *Biol J Linn Soc* **130**: 500–501 89–100. Oxford Academic.

502 Soler, J.J., Morales, J., Javier Cuervo, J. & Moreno, J. 2019. Conspicuousness of passerine females is associated with the nest-building behaviour of males. *Biol. J. Linnean Soc.* **126**: 824–835.

504 Soler, J.J. & Moreno, J. 2012. Evolution of sexual dichromatism in relation to nesting habits in European passerines: a test of Wallace's hypothesis. *J. Evol. Biol.* **25**: 1614–1622.

506 Stoddard, M.C. & Prum, R.O. 2008. Evolution of avian plumage color in a tetrahedral color space: A phylogenetic analysis of new world buntings. *Am. Nat.* **171**: 755–776.

508 Tobias, J.A., Montgomerie, R. & Lyon, B.E. 2012. The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition. *Philos. Trans. R. Soc. B-Biol. Sci.* **367**: 2274–2293.

511 Vorobyev, M. 2003. Coloured oil droplets enhance colour discrimination. *Proceedings of the Royal Society of London. Series B: Biological Sciences* **270**: 1255–1261. Royal Society.

513 Watson, D.M., Anderson, S.E. & Olson, V. 2015. Reassessing Breeding Investment in Birds: Class-Wide Analysis of Clutch Volume Reveals a Single Outlying Family. *PLoS One* **10**: e0117678.

515 Webb, T.J., Olson, V.A., Szekely, T. & Freckleton, R.P. 2010. Who cares? Quantifying the evolution of division of parental effort. *Methods Ecol. Evol.* **1**: 221–230.

517
449 Hare, R.M. & Simmons, L.W. 2019. Sexual selection and its evolutionary consequences in female animals. *Biological Reviews* **94**: 929–956.

451 Harrison, A. & Poe, S. 2012. Evolution of an ornament, the dewlap, in females of the lizard genus Anolis.
*Biological Journal of the Linnean Society* **106**: 191–201.

453 Hasegawa, M. & Arai, E. 2017. Egg size decreases with increasing female tail fork depth in family Hirundinidae. *Evol. Ecol.* **31**: 559–569.

455 Ho, L.S.T. & Ane, C. 2014. A linear-time algorithm for Gaussian and non-Gaussian trait evolution models.
*Systematic Biology* **63**: 397–408.

457 Hoquet, T. 2020. Bateman (1948): rise and fall of a paradigm? *Animal Behaviour* **164**: 223–231.

458 Hoyt, D. 1979. Practical Methods of Estimating Volume and Fresh Weight of Bird Eggs. *AUK* **96**: 73–77.

459 Irwin, R. 1994. The Evolution of Plumage Dichromatism in the New-World Blackbirds - Social Selection on Female Brightness. *Am. Nat.* **144**: 890–907.

461 Janicke, T., Haederer, I.K., Lajeunesse, M.J. & Anthes, N. 2016. Darwinian sex roles confirmed across the animal kingdom. *Sci. Adv.* **2**: UNSP e1500983.

463 Jetz, W., Thomas, G.H., Joy, J.B., Hartmann, K. & Mooers, A.O. 2012. The global diversity of birds in space and time. *Nature* **491**: 444–448.

465 Johnson, A.E., Price, J.J. & Pruett-Jones, S. 2013. Different modes of evolution in males and females generate dichromatism in fairy-wrens (Maluridae). *Ecol. Evol.* **3**: 3030–3046.

467 Kokko, H. & Johnstone, R.A. 2002. Why is mutual mate choice not the norm? Operational sex ratios, sex roles and the evolution of sexually dimorphic and monomorphic signalling. *Philos. Trans. R. Soc. Lond. Ser. B-Biol. Sci.* **357**: 319–330.

470 Kraaijeveld, K., Kraaijeveld-Smit, F.J.L. & Komdeur, J. 2007. The evolution of mutual ornamentation.
*Anim. Behav.* **74**: 657–677.

472 Lande, R. 1980. Sexual Dimorphism, Sexual Selection, and Adaptation in Polygenic Characters. *Evolution* **34**: 292–305. [Society for the Study of Evolution, Wiley].

474 Lyu, N., Servedio, M.R., Lloyd, H. & Sun, Y.-H. 2017. The evolution of postpairing male mate choice.
*Evolution* **71**: 1465–1477.

476 Maia, R., Gruson, H., Endler, J.A. & White, T.E. 2019. pavo 2: New tools for the spectral and spatial analysis of colour in r. *Methods in Ecology and Evolution* **10**: 1097–1107.

478 Marcondes, R.S. & Brumfield, R.T. 2019. Fifty shades of brown: Macroevolution of plumage brightness in the Furnariida, a large clade of drab Neotropical passerines. *Evolution* **73**: 704–719.

480 Martin, T.E. & Badyaev, A.V. 1996. Sexual Dichromatism in Birds: Importance of Nest Predation and Nest Location for Females Versus Males. *Evolution* **50**: 2454–2460.
**Table and Figure legends**

**Table 1:** Estimated values from the PGLS analyses. Variables are sorted in the same order as in Figure 1. 95% confidence intervals (displayed in brackets) include phylogenetic uncertainty. Estimates with confidence intervals not overlapping zero are represented in bold (except for Pagel's Lambda values).

**Figure 1:** Forest plots for a) brightness, b) chromaticity, and c) color volume in females, representing the mean estimates and their confidence intervals obtained by model averaging (see ESM). Mean estimates integrated control for phylogenies (PGLS analyses) for b) and c). Confidence intervals included phylogenetic uncertainty with 95% confidence intervals. "Male coloration" refers to the corresponding coloration metric. Larger circles were used to emphasize the main proxies of interest in our studies (smaller circles for other tested covariates). Estimates associated with confidence intervals not overlapping zero are displayed in black along with corresponding covariate (on the left). Estimates associated with confidence intervals overlapping zero are in grey along with the corresponding covariate.

**Figure 2:** Relationship between paternal care and mean female plumage brightness. The black crosses are the raw values for each species, the best-fit line is dashed with grey area representing the 95% confidence intervals. We also tested the represented relationship excluding the four outliers (low paternal care values), and we obtained equivalent trend (0.10 [95% CI: -0.02;0.23]).
482 Matysiokova, B., Remes, V. & Cockburn, A. 2017. Broad-scale variation in sexual dichromatism in
483 songbirds is not explained by sex differences in exposure to predators during incubation. *J.
484 Avian Biol.* **48**: 1322–1330.

485 McGraw, K.J. 2006. Mechanics of Carotenoid-Based Coloration. In: *Bird coloration - Mechanisms and
measurements* (G. E. Hill & K. J. McGraw, eds), pp. 177–242. Harvard University Press,
486 Cambridge, Massachusetts.

488 Negro, J.J., Figueroa-Luque, E. & Galvan, I. 2018. Melanin-based sexual dichromatism in the Western
489 Palearctic avifauna implies darker males and lighter females. *J. Avian Biol.* **49**: UNSP e01657.

490 Nimon, K., Oswald, F. & Roberts, and J.K. 2013. yhat: Interpreting Regression Effects.

491 Nordeide, J.T., Kekalainen, J., Janhunen, M. & Kortet, R. 2013. Female ornaments revisited - are they
492 correlated with offspring quality? *J. Anim. Ecol.* **82**: 26–38.

493 Odeen, A., Hastad, O. & Alstrom, P. 2011. Evolution of ultraviolet vision in the largest avian radiation -
494 the passerines. *BMC Evol. Biol.* **11**: 313.

495 Odom, K.J., Hall, M.L., Riebel, K., Omland, K.E. & Langmore, N.E. 2014. Female song is widespread and
496 ancestral in songbirds. *Nat Commun* **5**: 3379. Nature Publishing Group.

497 Osorio, D., Smith, A.C., Vorobyev, M. & Buchanan-Smith, H.M. 2004. Detection of Fruit and the Selection
498 of Primate Visual Pigments for Color Vision. *The American Naturalist* **164**: 696–708.
499 The University of Chicago Press.

500 Paradis, E. 2012. Analysis of Macroevolution with Phylogenies - 6.1 Phylogenetic Comparative Methods.
501 In: *Analysis of phylogenetics and evolution with R*, pp. 203–247. New York.

502 Paradis, E., Blomberg, S., Bolker, B., Brown, J., Claude, J., Cuong, H.S., et al. 2018. ape: Analyses of
503 Phylogenetics and Evolution.

504 Price, D.K. 1996. Sexual selection, selection load and quantitative genetics of zebra finch bill colour.
505 *Proceedings of the Royal Society of London. Series B: Biological Sciences* **263**: 217–221. Royal
506 Society.

507 R Core Team. 2022. *R: A Language and Environment for Statistical Computing*. R Foundation for
508 Statistical Computing, Vienna, Austria.

509 Renoult, J.P., Kelber, A. & Schaefer, H.M. 2017. Colour spaces in ecology and evolutionary biology. *Biol.
510 Rev.* **92**: 292–315.

511 Revell, L.J. 2010. Phylogenetic signal and linear regression on species data. *Methods Ecol. Evol.* **1**: 319–
512 329.

513 Revell, L.J. 2018. phytools: Phylogenetic Tools for Comparative Biology (and Other Things).
**Figure 3:** Depiction of the two-way interaction between paternal care and egg production and its effect on interspecific variation in mean female plumage chromaticity. The equation includes estimates for paternal care, egg production and their interaction. The black stars are the raw values for each species.
514 Riebel, K., Odom, K.J., Langmore, N.E. & Hall, M.L. 2019. New insights from female bird song: towards an integrated approach to studying male and female communication roles. *Biology Letters* **15**: 20190059. Royal Society.

517 Rodewald, P. (ed). 2015. The Birds of North America. Cornell Laboratory of Ornithology, Ithaca NY.

518 Rosvall, K.A., Bentz, A.B. & George, E.M. 2020. How research on female vertebrates contributes to an expanded challenge hypothesis. *Hormones and Behavior* **123**: 104565.

520 Rubenstein, D.R. & Lovette, I.J. 2009. Reproductive skew and selection on female ornamentation in social species. *Nature* **462**: 786-U106.

522 Shultz, A.J. & Burns, K.J. 2017. The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae). *Evolution* **71**: 1061-1074.

524 Simpson, R.K., Mistakidis, A.F. & Doucet, S.M. 2020. Natural and sexual selection shape the evolution of colour and conspicuousness in North American wood-warblers (Parulidae). *Biol J Linn Soc* **130**: 89-100. Oxford Academic.

527 Soler, J.J., Morales, J., Javier Cuervo, J. & Moreno, J. 2019. Conspicuousness of passerine females is associated with the nest-building behaviour of males. *Biol. J. Linnean Soc.* **126**: 824-835.

529 Soler, J.J. & Moreno, J. 2012. Evolution of sexual dichromatism in relation to nesting habits in European passerines: a test of Wallace's hypothesis. *J. Evol. Biol.* **25**: 1614-1622.

531 Stoddard, M.C. & Prum, R.O. 2008. Evolution of avian plumage color in a tetrahedral color space: A phylogenetic analysis of new world buntings. *Am. Nat.* **171**: 755-776.

533 Tobias, J.A., Montgomerie, R. & Lyon, B.E. 2012. The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition. *Philos. Trans. R. Soc. B-Biol. Sci.* **367**: 2274-2293.

536 Vorobyev, M. 2003. Coloured oil droplets enhance colour discrimination. *Proceedings of the Royal Society of London. Series B: Biological Sciences* **270**: 1255-1261. Royal Society.

538 Watson, D.M., Anderson, S.E. & Olson, V. 2015. Reassessing Breeding Investment in Birds: Class-Wide Analysis of Clutch Volume Reveals a Single Outlying Family. *PLoS One* **10**: e0117678.

540 Webb, T.J., Olson, V.A., Szekely, T. & Freckleton, R.P. 2010. Who cares? Quantifying the evolution of division of parental effort. *Methods Ecol. Evol.* **1**: 221-230.

542
<table><caption>**Table 1:** Estimated values from the PGLS analyses. Variables are sorted in the same order as in Figure 1. 95% confidence intervals (displayed in brackets) include phylogenetic uncertainty. Estimates with confidence intervals not overlapping zero are represented in bold (except for Pagel's Lambda values).</caption><caption>Estimates with confidence intervals</caption><thead><tr><th>Variable</th><th>Brightness</th><th>Chromaticity</th><th>Color volume</th></tr></thead><tbody><tr><td>Paternal care</td><td><strong>0.143 [0.028;0.258]</strong></td><td><strong>0.132 [0.002;0.262]</strong></td><td>-0.004 [-0.132;0.123]</td></tr><tr><td>Egg production</td><td>0.066 [-0.178;0.309]</td><td>-0.004 [-0.347;0.259]</td><td>0.159 [-0.133;0.452]</td></tr><tr><td>Egg production x Paternal care</td><td>0.059 [-0.045;0.164]</td><td><strong>-0.135 [-0.255;-0.015]</strong></td><td>0.019 [-0.096;0.133]</td></tr><tr><td>Male coloration</td><td><strong>0.733 [0.618;0.848]</strong></td><td><strong>0.672 [0.541;0.803]</strong></td><td><strong>0.670 [0.530;0.810]</strong></td></tr><tr><td>Nest characteristics</td><td>0.038 [-0.079;0.155]</td><td>0.040 [-0.098;0.177]</td><td>-0.055 [-0.186;0.076]</td></tr><tr><td>Female weight (log)</td><td>-0.065 [-0.310;0.180]</td><td>-0.009 [-0.312;0.294]</td><td>-0.108 [-0.406;0.190]</td></tr><tr><td>Breeding period (log)</td><td>-0.049 [-0.160;0.063]</td><td>0.068 [-0.055;0.191]</td><td>-0.069 [-0.188;0.050]</td></tr><tr><td>Migration</td><td></td><td></td><td></td></tr><tr><td>Strictly migrant</td><td>0.021 [-0.220;0.262]</td><td>-0.217 [-0.490;0.056]</td><td>-0.176 [-0.562;0.207]</td></tr><tr><td>Partially migrant</td><td>-0.101 [-0.342;0.140]</td><td>-0.047 [-0.320;0.226]</td><td>0.104 [-0.160;0.369]</td></tr><tr><td>Strictly resident</td><td>0.334 [-0.019;0.687]</td><td><strong>-0.431 [-0.831;-0.031]</strong></td><td>-0.177 [-0.562;0.207]</td></tr><tr><td>Pagel's Lambda</td><td>1.10<sup>-6</sup> [1.10<sup>-6</sup>; 1.10<sup>-6</sup>]</td><td>0.355 [0.351;0.358]</td><td>0.444 [0.440;0.448]</td></tr></tbody></table>
**Table and Figure legends**

**Table 1:** Estimated values from the PGLS analyses. Variables are sorted in the same order as in Figure 1. 95% confidence intervals (displayed in brackets) include phylogenetic uncertainty. Estimates with confidence intervals not overlapping zero are represented in bold (except for Pagel's Lambda values).

**Figure 1:** Forest plots for a) brightness, b) chromaticity, and c) color volume in females, representing the mean estimates and their confidence intervals obtained by model averaging (see ESM). Mean estimates integrated control for phylogenies (PGLS analyses) for b) and c). Confidence intervals included phylogenetic uncertainty with 95% confidence intervals. "Male coloration" refers to the corresponding coloration metric. Larger circles were used to emphasize the main proxies of interest in our studies (smaller circles for other tested covariates). Estimates associated with confidence intervals not overlapping zero are displayed in black along with corresponding covariate (on the left). Estimates associated with confidence intervals overlapping zero are in grey along with the corresponding covariate.

**Figure 2:** Relationship between paternal care and mean female plumage brightness. The black crosses are the raw values for each species, the best-fit line is dashed with grey area representing the 95% confidence intervals. We also tested the represented relationship excluding the four outliers (low paternal care values), and we obtained equivalent trend (0.10 [95% CI: -0.02;0.23]).
<figure><img src="image_4.png" /></figure>

**Figure 1:** Forest plots for a) brightness, b) chromaticity, and c) color volume in females, representing the mean estimates and their confidence intervals obtained by model averaging (see ESM). Mean estimates integrated control for phylogenies (PGLS analyses) for b) and c). Confidence intervals included phylogenetic uncertainty with 95% confidence intervals. "Male coloration" refers to the corresponding coloration metric. Larger circles were used to emphasize the main proxies of interest in our studies (smaller circles for
**Figure 3:** Depiction of the two-way interaction between paternal care and egg production and its effect on interspecific variation in mean female plumage chromaticity. The equation includes estimates for paternal care, egg production and their interaction. The black stars are the raw values for each species.
other tested covariates). Estimates associated with confidence intervals not overlapping zero are displayed in black along with
corresponding covariate (on the left). Estimates associated with confidence intervals overlapping zero are in grey along with the
corresponding covariate.
<table><caption>**Table 1:** Estimated values from the PGLS analyses. Variables are sorted in the same order as in Figure 1. 95% confidence intervals (displayed in brackets) include phylogenetic uncertainty. Estimates with confidence intervals not overlapping zero are represented in bold (except for Pagel's Lambda values).</caption><thead><tr><th rowspan="2">Variable</th><th colspan="3">Estimates with confidence intervals</th></tr><tr><th>Brightness</th><th>Chromaticity</th><th>Color volume</th></tr></thead><tbody><tr><td>Paternal care</td><td><strong>0.143 [0.028;0.258]</strong></td><td><strong>0.132 [0.002;0.262]</strong></td><td>-0.004 [-0.132;0.123]</td></tr><tr><td>Egg production</td><td>0.066 [-0.178;0.309]</td><td>-0.004 [-0.347;0.259]</td><td>0.159 [-0.133;0.452]</td></tr><tr><td>Egg production x Paternal care</td><td>0.059 [-0.045;0.164]</td><td><strong>-0.135 [-0.255;-0.015]</strong></td><td>0.019 [-0.096;0.133]</td></tr><tr><td>Male coloration</td><td><strong>0.733 [0.618;0.848]</strong></td><td><strong>0.672 [0.541;0.803]</strong></td><td><strong>0.670 [0.530;0.810]</strong></td></tr><tr><td>Nest characteristics</td><td>0.038 [-0.079;0.155]</td><td>0.040 [-0.098;0.177]</td><td>-0.055 [-0.186;0.076]</td></tr><tr><td>Female weight (log)</td><td>-0.065 [-0.310;0.180]</td><td>-0.009 [-0.312;0.294]</td><td>-0.108 [-0.406;0.190]</td></tr><tr><td>Breeding period (log)</td><td>-0.049 [-0.160;0.063]</td><td>0.068 [-0.055;0.191]</td><td>-0.069 [-0.188;0.050]</td></tr><tr><td>Migration</td><td></td><td></td><td></td></tr><tr><td>Strictly migrant</td><td>0.021 [-0.220;0.262]</td><td>-0.217 [-0.490;0.056]</td><td>-0.176 [-0.562;0.207]</td></tr><tr><td>Partially migrant</td><td>-0.101 [-0.342;0.140]</td><td>-0.047 [-0.320;0.226]</td><td>0.104 [-0.160;0.369]</td></tr><tr><td>Strictly resident</td><td>0.334 [-0.019;0.687]</td><td><strong>-0.431 [-0.831;-0.031]</strong></td><td>-0.177 [-0.562;0.207]</td></tr><tr><td>Pagel's Lambda</td><td>1.10<sup>-6</sup> [1.10<sup>-6</sup> ; 1.10<sup>-6</sup>]</td><td>0.355 [0.351 ;0.358]</td><td>0.444 [0.440 ;0.448]</td></tr></tbody></table>
<figure><img src="image_5.png" /></figure>

**Figure 2:** Relationship between paternal care and mean female plumage brightness. The black crosses are the raw values for each species, the best-fit line is dashed with grey area representing the 95% confidence intervals. We also tested the represented relationship excluding the four outliers (low paternal care values), and we obtained equivalent trend (0.10 [95% CI: -0.02;0.23]).
<figure><img src="image_2.png" /><figcaption>**Figure 1:** Forest plots for a) brightness, b) chromaticity, and c) color volume in females, representing the mean estimates and their confidence intervals obtained by model averaging (see ESM). Mean estimates integrated control for phylogenies (PGLS analyses) for b) and c). Confidence intervals included phylogenetic uncertainty with 95% confidence intervals. "Male coloration" refers to the corresponding coloration metric. Larger circles were used to emphasize the main proxies of interest in our studies (smaller circles for</figcaption></figure>
<figure><img src="image_6.png" /></figure>
other tested covariates). Estimates associated with confidence intervals not overlapping zero are displayed in black along with
corresponding covariate (on the left). Estimates associated with confidence intervals overlapping zero are in grey along with the
corresponding covariate.
**Figure 3:** Depiction of the two-way interaction between paternal care and egg production and its effect on interspecific variation in mean female plumage chromaticity. The equation includes estimates for paternal care, egg production and their interaction. The black stars are the raw values for each species
<figure><img src="image_3.png" /><figcaption>**Figure 2:** Relationship between paternal care and mean female plumage brightness. The black crosses are the raw values for each species, the best-fit line is dashed with grey area representing the 95% confidence intervals. We also tested the represented relationship excluding the four outliers (low paternal care values), and we obtained equivalent trend (0.10 [95% CI: -0.02;0.23]).</figcaption></figure>
<figure><img src="image_4.png" /></figure>
**Supplementary information**
**Figure 3:** Depiction of the two-way interaction between paternal care and egg production and its effect on interspecific variation in mean female plumage chromaticity. The equation includes estimates for paternal care, egg production and their interaction. The black stars are the raw values for each species
<table><caption>**Table S1:** Bird species/subspecies/families included in the analyses. The scientific names are those used by birdtree.org. The collection point is where the bird was collected (nearest town/city (state), country; the list of country abbreviations is at the bottom of the table). The number of measurements is the total number of color measurements carried out by spectrometry on the bird, where *3 indicates that the measurements were replicated three times to test repeatability. (1) Two male specimens of *Myadestes townsendi* were used; measurements were carried out on the second specimen in zones that were stained on the first specimen (except for the stain on its abdomen, the first specimen was in better condition than the second specimen).</caption><thead><tr><th rowspan="2">Scientific name</th><th rowspan="2">Family</th><th rowspan="2">Collection point</th><th colspan="2">Male</th><th rowspan="2">Collection point</th><th colspan="2">Female</th></tr><tr><th>Voucher number</th><th>Number of measurements</th><th>Voucher number</th><th>Number of measurements</th></tr></thead><tbody><tr><td>Agelaius phoeniceus</td><td>Icteridae</td><td>Auburn (NY), USA</td><td>1921-140</td><td>38</td><td>Rochester (NY), USA</td><td>1938-360</td><td>38</td></tr><tr><td>Aimophila ruficeps</td><td>Passerellidae</td><td>Dulzura (CA), USA</td><td>NA</td><td>33</td><td>Dulzura (CA), USA</td><td>NA</td><td>33</td></tr><tr><td>Amphispiza bilineata deserticola</td><td>Passerellidae</td><td>Tucson (AZ), USA</td><td>1949-417</td><td>28</td><td>El Rosario, MX</td><td>1956-1109</td><td>28*3</td></tr><tr><td>Quiscalus mexicanus</td><td>Icteridae</td><td>Chimalapa, MX</td><td>1953-656</td><td>23</td><td>Tlatizapan, MX</td><td>1937-443</td><td>23</td></tr><tr><td>Anthus pratensis pratensis</td><td>Motacillidae</td><td>Plougasnou, FR</td><td>1960-2798</td><td>36</td><td>Plougasnou, FR</td><td>1960-2818</td><td>36</td></tr><tr><td>Anthus spinoletta spinoletta</td><td>Motacillidae</td><td>Urdos, FR</td><td>1979-25</td><td>27</td><td>Saint-Jean-de-Luz, FR</td><td>1979-19</td><td>27</td></tr><tr><td>Xanthocephalus xanthocephalus</td><td>Icteridae</td><td>Silver Lake (MI), USA</td><td>1962-3448</td><td>25</td><td>Bixby (MO), USA</td><td>1955-376</td><td>25*3</td></tr><tr><td>Bombycilla garrulous garrulous</td><td>Bombycillidae</td><td>Uppsala, SE</td><td>1928-1890</td><td>28*3</td><td>Uppsala, SE</td><td>1928-1891</td><td>28</td></tr><tr><td>Miliaria calandra</td><td>Emberezidae</td><td>Pons, FR</td><td>1933-223</td><td>37</td><td>Jonzac, FR</td><td>1933-224</td><td>37</td></tr><tr><td>Cardinalis cardinalis coccineus</td><td>Cardinalidae</td><td>MX</td><td>1999-2482</td><td>27</td><td>Chimalapa, MX</td><td></td><td>27</td></tr><tr><td>Carduelis cannabina cannabina</td><td>Fringillidae</td><td>Lion-sur-Mer, FR</td><td>1965-2876</td><td>38</td><td>Le Puy-Notre Dame, FR</td><td>1978-270</td><td>38</td></tr><tr><td>Carduelis carduelis carduelis</td><td>Fringillidae</td><td>Eure-et-Loir, FR</td><td>1933-658</td><td>35</td><td>Andeville, FR</td><td>1960-1580</td><td>35*3</td></tr><tr><td>Carduelis chloris chloris</td><td>Fringillidae</td><td>Plougasnou, FR</td><td>1960-1583</td><td>27*3</td><td>Remomeix, FR</td><td>1960-76</td><td>27</td></tr><tr><td>Carduelis flammea flammea</td><td>Fringillidae</td><td>Armentières-en-Brie, FR</td><td>1977-56</td><td>43</td><td>Le Mage, FR</td><td>1977-51</td><td>43</td></tr></tbody></table>
<table><tr><td>Carduelis pinus</td><td>Fringillidae</td><td>Auburn (NY), 1921-189</td><td>39</td><td>Auburn (NY), 1922-18</td><td>39</td></tr><tr><td>Carduelis psaltria</td><td>Fringillidae</td><td>Dulzura (CA), 2001-1148</td><td>25</td><td>Dulzura (CA), 2001-1147</td><td>25</td></tr><tr><td>Carduelis spinus</td><td>Fringillidae</td><td>Vannes, FR 1973-506</td><td>39</td><td>Remomeix, FR 1960-75</td><td>39</td></tr><tr><td>Carduelis tristis</td><td>Fringillidae</td><td>Auburn (NY), 1962-3445</td><td>24*3</td><td>Kamouraska (QC), 1921-133</td><td>24</td></tr><tr><td>Carpodacus mexicanus</td><td>Fringillidae</td><td>Dulzura (CA), 1879-2771</td><td>37</td><td>La Paz, MX 2003-1580</td><td>37</td></tr><tr><td>Carpodacus purpureus purpureus</td><td>Fringillidae</td><td>Ithaca (NY), USA 1953-338</td><td>37</td><td>Ithaca (NY), USA 1953-339</td><td>37</td></tr><tr><td>Catharus guttatus</td><td>Turdidae</td><td>Reading (NY), 2002-42</td><td>27</td><td>Jackson County 1939-7</td><td>27</td></tr><tr><td>Catharus ustulatus ustulatus</td><td>Turdidae</td><td>Los Angeles 1956-1047</td><td>26</td><td>Long Beach (CA), 1955-268</td><td>26</td></tr><tr><td>Emberiza citrinella citrinella</td><td>Emberezidae</td><td>Plougasnou, FR 1960-2470</td><td>42</td><td>Noisiel, FR 1962-4110</td><td>42</td></tr><tr><td>Certhia familiaris macrodactyla</td><td>Certhiidae</td><td>Saint-Cassin, FR 1993-215</td><td>39</td><td>Lucéram, FR 2008-678</td><td>39</td></tr><tr><td>Certhia brachydactyla megarhynchos</td><td>Certhiidae</td><td>Le Puy-Notre Dame, FR 1979-228</td><td>38</td><td>Le Puy-Notre Dame, FR 1979-229</td><td>38</td></tr><tr><td>Cinclus cinclus cinclus</td><td>Cinclidae</td><td>Condat-sur-Vienne, FR 1956-754</td><td>28</td><td>Solignac, FR 1948-519</td><td>28</td></tr><tr><td>Cistothorus palustris palustris</td><td>Troglodytidae</td><td>Auburn (NY), 1921-164</td><td>34</td><td>Cayuga (ON), CA 1921-165</td><td>34</td></tr><tr><td>Coccothraustes coccothraustes coccothraustes</td><td>Fringillidae</td><td>Southwestern FR 1990-575</td><td>32</td><td>Tencin, FR 2008-723</td><td>32</td></tr><tr><td>Dendroica coronate auduboni</td><td>Parulidae</td><td>Pasadena (CA), 1955-302</td><td>33</td><td>El Monte (CA), 1955-304</td><td>33</td></tr><tr><td>Dendroica petechia castaneiceps</td><td>Parulidae</td><td>La Paz, MX 1932-110</td><td>28*3</td><td>La Paz, MX 1932-111</td><td>28</td></tr><tr><td>Dumetella carolinensis</td><td>Mimidae</td><td>Ixtacomitan, MX 1951-800</td><td>23</td><td>Ixtacomitan, MX 1951-803</td><td>23*3</td></tr><tr><td>Luscinia luscinia</td><td>Muscicapidae</td><td>Amani Forest, TZ 1967-675</td><td>23</td><td>Joinville-le-Pont, FR 1989-246</td><td>23</td></tr><tr><td>Emberiza cia cia</td><td>Emberezidae</td><td>Aussois, FR 1969-1103</td><td>37</td><td>Urdos, FR 1978-170</td><td>37</td></tr><tr><td>Emberiza cirlus</td><td>Emberezidae</td><td>Trois-Monts, FR 1965-2767</td><td>42</td><td>Le Puy-Notre Dame, FR 1978-180</td><td>42</td></tr></table>
<table><tr><td>Sitta canadensis</td><td>Sittidae</td><td>Cayuga (OR), USA</td><td>1921-171</td><td>29</td><td>New Haven (CT), USA</td><td>1972-1646</td><td>29</td></tr><tr><td>Emberiza hortulana</td><td>Emberezidae</td><td>Cellule, FR</td><td>1947-67</td><td>34</td><td>Fronsac, FR</td><td>1997-518</td><td>34</td></tr><tr><td>Emberiza leucocephalos</td><td>Emberezidae</td><td>Baikal Lake, RU</td><td>1960-1440</td><td>42</td><td>Zaisan-Nor, KZ</td><td>2009-982</td><td>42</td></tr><tr><td>Emberiza schoeniclus schoeniclus</td><td>Emberezidae</td><td>Saint-Jean-du-Doigt, FR</td><td>1960-2450</td><td>42</td><td>Tonneins, FR</td><td>1987-496</td><td>42</td></tr><tr><td>Erithacus rubecula</td><td>Muscicapidae</td><td>Saix, FR</td><td>1978-878</td><td>28</td><td>Morsang-sur-Orge, FR</td><td>2013-69</td><td>28</td></tr><tr><td>Icterus bullockii</td><td>Icteridae</td><td>(CA), USA</td><td>2002-1430</td><td>30*3</td><td>Dulzura (CA), USA</td><td>2002-1429</td><td>30</td></tr><tr><td>Spizella arborea</td><td>Passerellidae</td><td>Auburn (NY), USA</td><td>1921-137</td><td>35</td><td>Rochester (NY), USA</td><td>1999-1956</td><td>35</td></tr><tr><td>Ficedula albicollis albicollis</td><td>Muscicapidae</td><td>Laconie, GR</td><td>1979-985</td><td>29</td><td>Laconie, GR</td><td>1979-991</td><td>29</td></tr><tr><td>Ficedula hypoleuca hypoleuca</td><td>Muscicapidae</td><td>Bouillé-Loretz, FR</td><td>1978-785</td><td>27</td><td>Chaponost, FR</td><td>1978-768</td><td>27</td></tr><tr><td>Ficedula parva parva</td><td>Muscicapidae</td><td>Teheran, IR</td><td>1969-2330</td><td>24</td><td>Teheran, IR</td><td>1957-875</td><td>24</td></tr><tr><td>Fringilla coelebs coelebs</td><td>Fringillidae</td><td>Brunoy, FR</td><td>1995-34</td><td>31</td><td>Plougasnou, FR</td><td>1960-1501</td><td>31</td></tr><tr><td>Fringilla montifringilla</td><td>Fringillidae</td><td>Le Croyot, FR</td><td>1997-1452</td><td>36</td><td>Plougasnou, FR</td><td>1960-2570</td><td>36</td></tr><tr><td>Icterus spurius</td><td>Icteridae</td><td>Ixtacomitan, MX</td><td>1951-963</td><td>29</td><td>Ixtacomitan, MX</td><td>1951-969</td><td>29*3</td></tr><tr><td>Coccothraustes vespertinus vespertinus</td><td>Fringillidae</td><td>Watkins Glen (NY), USA</td><td>1953-342</td><td>27</td><td>Lewis County (NY), USA</td><td>1953-343</td><td>27</td></tr><tr><td>Hylocichla mustelina</td><td>Turdidae</td><td>Yaxchilan, MX</td><td>1975-910</td><td>28</td><td>Yaxchilan, MX</td><td>1975-915</td><td>28</td></tr><tr><td>Sitta carolinensis</td><td>Sittidae</td><td>Auburn (NY), USA</td><td>1921-100</td><td>34</td><td>Auburn (NY), USA</td><td>1921-169</td><td>34</td></tr><tr><td>Euphagus cyanocephalus</td><td>Icteridae</td><td>El Monte (CA), USA</td><td>1955-379</td><td>22</td><td>Whittier (CA), USA</td><td>1956-1121</td><td>22</td></tr><tr><td>Icterus galbula</td><td>Icteridae</td><td>Motzorongo, MX</td><td>1940-648</td><td>29</td><td>Comitan, MX</td><td>1939-1177</td><td>29</td></tr><tr><td>Icterus parisorum</td><td>Icetrida</td><td>El Triumfo, MX</td><td>1930-19</td><td>33*3</td><td>El Triumfo, MX</td><td>1930-20</td><td>33</td></tr><tr><td>Spizella pusilla</td><td>Passerellidae</td><td>Reading (NY), USA</td><td>1999-137</td><td>29</td><td>Wheatland (WY), USA</td><td>1886-87</td><td>29</td></tr><tr><td>Zoothera naevia meruloides</td><td>Turdidae</td><td>Nicasio (CA), USA</td><td>1921-161</td><td>30*3</td><td>Pasadena (CA), USA</td><td>1955-263</td><td>30</td></tr><tr><td>Junco hyemalis aikeni</td><td>Passerellidae</td><td>Newcastle (WY), USA</td><td>1939-50</td><td>25</td><td>Newcastle (WY), USA</td><td>1939-51</td><td>25*3</td></tr></table>
<table><tr><td>Loxia curvirostra curvirostra</td><td>Fringillidae</td><td>Jurques, FR</td><td>1965-2824</td><td>37</td><td>Pierre-de-Bresse, FR</td><td>1899-670</td><td>37</td></tr><tr><td>Anthus trivialis trivialis</td><td>Motacillidae</td><td>Remomeix, FR</td><td>1947-81</td><td>36</td><td>Trois-Monts, FR</td><td>1965-2519</td><td>36</td></tr><tr><td>Luscinia megarhynchos megarhynchos</td><td>Muscicapida e</td><td>Saint-Jean-de-Luz, FR</td><td>1978-932</td><td>22</td><td>Wissous, FR</td><td>1967-1735</td><td>22</td></tr><tr><td>Luscinia svecica cyanecula</td><td>Muscicapida e</td><td>Nancy, FR</td><td>1972-1279</td><td>29*3</td><td>Ansauville, FR</td><td>1929-1752</td><td>29</td></tr><tr><td>Melospiza georgiana</td><td>Passerellidae</td><td>Reading (NY), USA</td><td>1999-1860</td><td>40</td><td>Reading (NY), USA</td><td>1999-1861</td><td>40</td></tr><tr><td>Melospiza lincolnii lincolnii</td><td>Passerellidae</td><td>Chippewa (OH), USA</td><td>1939-56</td><td>45</td><td>Fleming (OH), USA</td><td>1939-59</td><td>45</td></tr><tr><td>Melospiza melodia</td><td>Passerellidae</td><td>Dulzura (CA), USA</td><td>1999-1873</td><td>40</td><td>Dulzura (CA), USA</td><td>1999-1871</td><td>40</td></tr><tr><td>Mimus polyglottos leucopterus</td><td>Mimidae</td><td>MX</td><td>1933-306</td><td>25</td><td>Los Angeles (CA), USA</td><td>1956-1050</td><td>25</td></tr><tr><td>Monticola saxatilis</td><td>Muscicapida e</td><td>Tizi n'Taka, MA</td><td>1963-495</td><td>39</td><td>Mwaktau, KE</td><td>1969-1320</td><td>39</td></tr><tr><td>Monticola solitarius solitarius</td><td>Muscicapida e</td><td>Nice, FR</td><td>1960-2231</td><td>34*3</td><td>Marseille, FR</td><td>1960-1234</td><td>34</td></tr><tr><td>Motacilla alba alba</td><td>Motacillidae</td><td>Hendaye, FR</td><td>1978-123</td><td>29</td><td>Ranville, FR</td><td>1965-2434</td><td>29</td></tr><tr><td>Motacilla cinerea</td><td>Motacillidae</td><td>Rennes-les-Bains, FR</td><td>1978-110</td><td>29</td><td>Saint-Jean-du-Doigt, FR</td><td>1978-108</td><td>29</td></tr><tr><td>Icterus cucullatus igneus</td><td>Icteridae</td><td>Yucatan, MX</td><td>2002-1378</td><td>29</td><td>Mérida, MX</td><td>2002-1387</td><td>29*3</td></tr><tr><td>Oenanthe hispanica hispanica</td><td>Muscicapida e</td><td>Douar Zaara, TN</td><td>1978-488</td><td>26*3</td><td>Redeyef, TN</td><td>1986-253</td><td>26</td></tr><tr><td>Muscicapa striata striata</td><td>Muscicapida e</td><td>Ahetze, FR</td><td>1987-8</td><td>27</td><td>Trois-Monts, FR</td><td>1965-1959</td><td>27</td></tr><tr><td>Myadestes townsendi (1)</td><td>Turdidae</td><td>Paradise (AZ), USA<br>Silver City (NM), USA</td><td>1939-8<br>2000-2088</td><td>25</td><td>Cochise County (NV), USA</td><td>1939-9</td><td>25</td></tr><tr><td>Chondestes grammacus</td><td>Passerellidae</td><td>Dulzura (CA), USA</td><td>1955-329</td><td>36</td><td>Pasadena (CA), USA</td><td>1999-1974</td><td>36</td></tr><tr><td>Oenanthe isabellina</td><td>Muscicapida e</td><td>Ito, TD</td><td>1966-2280</td><td>26</td><td>100 km north of Rosso, MR</td><td>1954-106</td><td>26</td></tr><tr><td>Oenanthe oenanthe oenanthe</td><td>Muscicapida e</td><td>Remomeix, FR</td><td>1960-60</td><td>34</td><td>Biarritz, FR</td><td>1989-218</td><td>34*3</td></tr></table>
<table><tr><td>Geothlypis trichas</td><td>Parulidae</td><td>Auburn (NY), 1921-123</td><td>30</td><td>Auburn (NY), 1921-124</td><td>30</td></tr><tr><td>Sitta europaea caesia</td><td>Sittidae</td><td>Saumur, FR</td><td>1978-1525</td><td>Mouliherne, FR</td><td>1978-1547</td><td>27*3</td></tr><tr><td>Passer montanus montanus</td><td>Passeridae</td><td>Chamigny, FR</td><td>1988-583</td><td>Saint-Jean-de-Luz, FR</td><td>1978-156</td><td>32</td></tr><tr><td>Passerculus sandwichensis princeps</td><td>Passerellidae</td><td>South Duxbury (MA), USA</td><td>1886-77</td><td>South Duxbury (MA), USA</td><td>1886-76</td><td>38</td></tr><tr><td>Motacilla flava flava</td><td>Motacillidae</td><td>Saint-Jean-de-Luz, FR</td><td>1979-76</td><td>Remomeix, FR</td><td>1960-450</td><td>26</td></tr><tr><td>Passerella iliaca</td><td>Passerellidae</td><td>Auburn (NY), 1921-192</td><td>36</td><td>Reading (NY), USA</td><td>1999-1901</td><td>36</td></tr><tr><td>Passerina amoena</td><td>Cardinalidae</td><td>Santa Barbara (CA), USA</td><td>1955-356</td><td>(CA), USA</td><td>1999-2622</td><td>28</td></tr><tr><td>Passerina caerulea</td><td>Cardinalidae</td><td>MX</td><td>1999-2591</td><td>MX</td><td>1999-2592</td><td>41</td></tr><tr><td>Passerina ciris</td><td>Cardinalidae</td><td>Naha, MX</td><td>1975-801</td><td>Naha, MX</td><td>1975-803</td><td>27</td></tr><tr><td>Passerina cyanea</td><td>Cardinalidae</td><td>Cayuga (TX), USA</td><td>1921-134</td><td>Auburn (NY), USA</td><td>1921-135</td><td>24</td></tr><tr><td>Passerina versicolor</td><td>Cardinalidae</td><td>Ixmilquilpan, MX</td><td>1940-604</td><td>Ixmilquilpan, MX</td><td>1940-601</td><td>26</td></tr><tr><td>Petronia petronia</td><td>Passeridae</td><td>Tataouine, TN</td><td>1997-479</td><td>Tunis, TN</td><td>2008-711</td><td>37</td></tr><tr><td>Pheucticus ludovicianus</td><td>Cardinalidae</td><td>Cayuga (TX), USA</td><td>1921-788</td><td>Naha, MX</td><td>1975-787</td><td>37</td></tr><tr><td>Pheucticus melanocephalus</td><td>Cardinalidae</td><td>(CO), USA</td><td>1999-2477</td><td>(CO), USA</td><td>1999-2479</td><td>37</td></tr><tr><td>Phoenicurus ochruros gibraltariensis</td><td>Muscicapidae</td><td>Lorraine, FR</td><td>1976-1301</td><td>Le Puy-Notre Dame, FR</td><td>1978-911</td><td>26</td></tr><tr><td>Phoenicurus phoenicurus phoenicurus</td><td>Muscicapidae</td><td>Trois-Monts, FR</td><td>1965-2208</td><td>Lagny-sur-Marne, FR</td><td>1951-367</td><td>28</td></tr><tr><td>Pipilo maculatus megaloyx</td><td>Passerellidae</td><td>Dulzura (CA), USA</td><td>1999-2273</td><td>Dulzura (CA), USA</td><td>1999-2275</td><td>27*3</td></tr><tr><td>Piranga flava hepatica</td><td>Cardinalidae</td><td>La Parada, MX</td><td>1999-2894</td><td>Oaxaca, MX</td><td>1999-2896</td><td>25</td></tr><tr><td>Piranga olivacea</td><td>Cardinalidae</td><td>Ann Arbor (MI), USA</td><td>1939-42</td><td>Lorne Park (ON), CA</td><td>1891-738</td><td>23</td></tr><tr><td>Piranga rubra</td><td>Cardinalidae</td><td>Tepic, MX</td><td>1901-458</td><td>Tepic, MX</td><td>1901-457</td><td>25*3</td></tr><tr><td>Anthus campestris campestris</td><td>Motacillidae</td><td>Manonville, FR</td><td>1965-2527</td><td>FR</td><td>1933-2196</td><td>30</td></tr><tr><td>Prunella collaris collaris</td><td>Prunellidae</td><td>CH</td><td>1972-1350</td><td>Isgoun Ouagouns, MA</td><td>1963-62</td><td>32</td></tr></table>
<table><tr><td>Prunella modularis modularis</td><td>Prunellidae</td><td>Trois-Monts, FR</td><td>1965-2412</td><td>32</td><td>Trois-Monts, FR</td><td>1965-2422</td><td>32</td></tr><tr><td>Pipilo crissalis</td><td>Passerellidae</td><td>Pasadena (CA), USA</td><td>1955-360</td><td>24</td><td>Pasadena (CA), USA</td><td>1955-362</td><td>24</td></tr><tr><td>Pyrrhula pyrrhula pyrrhula</td><td>Fringillidae</td><td>Ahetze, FR</td><td>1988-585</td><td>26*3</td><td>Ahetze, FR</td><td>1981-881</td><td>26</td></tr><tr><td>Turdus iliacus iliacus</td><td>Turdidae</td><td>Ahetze, FR</td><td>1982-956</td><td>30</td><td>Argenton l'Eglise, FR</td><td>1970-1052</td><td>30</td></tr><tr><td>Quiscalus quiscula aeneus</td><td>Icteridae</td><td>Webster (TX), USA</td><td>1939-66</td><td>23</td><td>Auburn (NY), USA</td><td>1921-141</td><td>23</td></tr><tr><td>Pooecetes gramineus</td><td>Passerellidae</td><td>Rochester (NY), USA</td><td>1999-1980</td><td>38</td><td>Rochester (NY), USA</td><td>1999-1978</td><td>38</td></tr><tr><td>Regulus ignicapilla ignicapilla</td><td>Regulidae</td><td>Saint-Pern, FR</td><td>1975-1703</td><td>33</td><td>Saumur, FR</td><td>1978-1283</td><td>33</td></tr><tr><td>Regulus regulus regulus</td><td>Regulidae</td><td>Remomeix, FR</td><td>1949-34</td><td>32</td><td>Magny-les-Hameaux, FR</td><td>1985-231</td><td>32</td></tr><tr><td>Salpinctes obsoletus</td><td>Troglodytidae</td><td>Tehuantepec, MX</td><td>1996-1122</td><td>32</td><td>Dulzura (CA), USA</td><td>1996-1123</td><td>32</td></tr><tr><td>Saxicola rubetra</td><td>Muscicapidae</td><td>Remomeix, FR</td><td>1949-499</td><td>40</td><td>Solignac, FR</td><td>1960-2101</td><td>40</td></tr><tr><td>Saxicola torquatus rubicola</td><td>Muscicapidae</td><td>Meuvaines, FR</td><td>1965-2227</td><td>39</td><td>Meuvaines, FR</td><td>1965-2233</td><td>39</td></tr><tr><td>Seiurus noveboracensis</td><td>Parulidae</td><td>Auburn (NY), USA</td><td>1921-177</td><td>28</td><td>Rochester (NY), USA</td><td>2001-1869</td><td>28*3</td></tr><tr><td>Serinus serinus</td><td>Fringillidae</td><td>Saumur, FR</td><td>1978-135</td><td>38</td><td>Plougasnou, FR</td><td>1960-2590</td><td>38</td></tr><tr><td>Setophaga ruticilla</td><td>Parulidae</td><td>Ixtacomitan, MX</td><td>1951-834</td><td>26</td><td>Ixtacomitan, MX</td><td>1951-836</td><td>26*3</td></tr><tr><td>Sialia currucoides</td><td>Turdidae</td><td>Basset (VA), USA</td><td>1955-269</td><td>25*3</td><td>Coachella (CA), USA</td><td>1955-270</td><td>25</td></tr><tr><td>Sialia mexicana</td><td>Turdidae</td><td>Dulzura (CA), USA</td><td>2000-2068</td><td>27</td><td>Dulzura (CA), USA</td><td>2000-2067</td><td>27*3</td></tr><tr><td>Sialia sialis fulva</td><td>Turdidae</td><td>Comitan, MX</td><td>1951-92</td><td>28</td><td>Comitan, MX</td><td>1951-97</td><td>28</td></tr><tr><td>Regulus calendula calendula</td><td>Regulidae</td><td>Kern River (CA), USA</td><td>1956-1045</td><td>28</td><td>Pasadena (CA), USA</td><td>1955-295</td><td>28*3</td></tr><tr><td>Erythropygia galactotes galactotes</td><td>Muscicapidae</td><td>Rabat, MA</td><td>1970-574</td><td>29</td><td>Berkane, MA</td><td>1963-53</td><td>29</td></tr><tr><td>Calamospiza melanocorys</td><td>Passerellidae</td><td>Colorado Springs (CO), USA</td><td>1930-23</td><td>39</td><td>Chihuahua, MX</td><td>1930-24</td><td>39</td></tr><tr><td>Motacilla citreola</td><td>Motacillidae</td><td>Irkoutsk, RU</td><td>1898-790</td><td>27*3</td><td>Petchora Valley, RU</td><td>1905-36</td><td>27</td></tr></table>
<table><tr><td>Sturnella neglecta neglecta</td><td>Icteridae</td><td>Summer Lake (OR), USA</td><td>1962-3450</td><td>43</td><td>Garnsey (OR), USA</td><td>1955-374</td><td>43</td></tr><tr><td>Thryomanes bewickii</td><td>Troglodytidae</td><td>Santa Rita Mountains (AZ), USA</td><td>1886-36</td><td>26</td><td>Santa Rita Mountains (AZ), USA</td><td>1886-35</td><td>26</td></tr><tr><td>Toxostoma rufum</td><td>Mimidae</td><td>Ithaca (NY), USA</td><td>1953-332</td><td>28</td><td>Fort Benning (GA), USA</td><td>1975-1679</td><td>28*3</td></tr><tr><td>Troglodytes troglodytes troglodytes</td><td>Troglodytidae</td><td>Plougasnou, FR</td><td>1970-2300</td><td>34</td><td>Morlaix, FR</td><td>1970-2302</td><td>34</td></tr><tr><td>Oreoscoptes montanus</td><td>Mimidae</td><td>Menifee (CA), USA</td><td>1927-1872</td><td>31</td><td>Cajon Wash (CA), USA</td><td>1927-1893</td><td>31</td></tr><tr><td>Turdus merula merula</td><td>Turdidae</td><td>Île-de-France, FR</td><td>1990-569</td><td>27</td><td>Paris, FR</td><td>2000-978</td><td>27</td></tr><tr><td>Turdus migratorius</td><td>Turdidae</td><td>Cayuga (TX), USA</td><td>2001-2398</td><td>29</td><td>Rochester (NY), USA</td><td>1921-103</td><td>29</td></tr><tr><td>Turdus philomelos philomelos</td><td>Turdidae</td><td>Bourron-Marlotte, FR</td><td>1985-80</td><td>30</td><td>Biarritz, FR</td><td>1985-242</td><td>30</td></tr><tr><td>Turdus pilaris</td><td>Turdidae</td><td>Le Puy-Notre Dame, FR</td><td>1978-1489</td><td>32</td><td>Orly, FR</td><td>1984-489</td><td>32*3</td></tr><tr><td>Turdus torquatus torquatus</td><td>Turdidae</td><td>Pyrénées-Atlantiques, FR</td><td>1988-580</td><td>31</td><td>Saint-Jean-du-Doigt, FR</td><td>1960-2230</td><td>31</td></tr><tr><td>Turdus viscivorus viscivorus</td><td>Turdidae</td><td>Aussois, FR</td><td>1969-1034</td><td>32</td><td>Aussois, FR</td><td>1969-1035</td><td>32</td></tr><tr><td>Vermivora celata sordida</td><td>Parulidae</td><td>Pasadena (CA), USA</td><td>1955-300</td><td>26</td><td>CA</td><td>1955-301</td><td>26</td></tr><tr><td>Passer domesticus domesticus</td><td>Passeridae</td><td>Signy-Signets, FR</td><td>1976-1507</td><td>37</td><td>Trois-Monts, FR</td><td>1976-2967</td><td>37</td></tr><tr><td>Zonotrichia albicollis</td><td>Passerellidae</td><td>Reading (NY), USA</td><td>1999-1907</td><td>35</td><td>CA</td><td>1999-1906</td><td>35</td></tr><tr><td>Zonotrichia leucophrys gambeli</td><td>Passerellidae</td><td>Pasadena (CA), USA</td><td>1955-348</td><td>30*3</td><td>Santa Barbara (CA), USA</td><td>1955-349</td><td>30</td></tr></table>

CA=Canada; CH=Switzerland; FR=France; GR=Greece; IR=Iran; KE=Kenya; KZ=Kazakhstan; MA=Morocco; MR=Mauritania; MX=Mexico; RU=Russia; SE=Sweden; TD=Chad; TN=Tunisia; TZ=Tanzania; USA=United States of America.
<figure><img src="image_7.png" /></figure>

**Figure S1.** Phylogenetic tree representing the major families of Passerines from the Northern Hemisphere. Highlighted families correspond to the 17 families included in the analyses and show they are more closely related to each other than to the other families represented on the tree. The species of tanagers included in our analyses were formerly identified as “Thraupidae” but are now included in the sister-family of Cardinalidae. Therefore, we did not include “Thraupidae” in the phylogenetic tree.
# 1/ Specimen collection

We only selected specimens for which sex, adult status, subspecies, and collection area were specified. If the collection area but not the subspecies was specified, we looked in the Handbook of the Birds of the World (Del Hoyo *et al.*, 2004) to see if we could identify the subspecies; we did not use specimens for which there was any taxonomic doubt because of subspecies sympatry. When possible, we selected males and females from the same locality (details in Table S6). When this was impossible, we selected males and females of the same subspecies coming from locations as close together as possible.
## 2/ Coloration measurement

**Spectrometry:** A single person (AF) performed all the spectrometric measurements, using a Jaz spectrometer (Ocean Optics, Dunedin FL, USA) with a Jaz light source covering a spectral range of 300 nm to 700 nm and a 200 µm optical probe (FCR-7UV200-2-45-ME, Avantes, Apledoorn, NL) whose tip contained a 45° quartz window, which guaranteed a constant distance between the light and the sample and highly repeatable measurements. Reflectance was calculated in relative terms, via comparison with a black and a white reference (WS1, Ocean Optics, Dunedin FL, USA).

Before measuring the whole plumage, we first characterized zone coloration for each bird (see Figure S2). When a bird had distinct color patches or a patch was composed of different colors (up to four) in any of the 21 zones, we measured each color. Although the preliminary assessment of distinct patches or colors was carried out with the naked eye (and thus affected by human visual bias), we double-checked the assessment by examining the spectrometric spectra (Gomez & Thery, 2004; Stoddard & Prum, 2008; Doutrelant *et al.*, 2016). Ultimately, number of measurements ranged from 22 to 45 depending on the species (details in Table S1); the mean number of measurements per bird was 33. Within species, measurements number was the same for both sexes, even if one sex displayed fewer colors (Shultz & Burns, 2017). For some specimens we repeated all measurements three times to test measurement repeatability (details in Table S1).

For specimens in which we tested repeatability, we calculated the mean spectra per patch after controlling for concordance in spectral shape among the replicates.
<figure><img src="image_8.png" /><figcaption>**Figure S2:** Topography of a house sparrow (*Passer domesticus*). In this study, we started characterizing coloration in 22 zones (abbreviations for zone names in brackets). The lore zone (LO) was initially included but then later removed because it was poorly preserved in a large number of specimens. Consequently, only 21 zones were used in the final analyses.</figcaption></figure>

20

21

22

23

24

25
26

**Visual model:** colors are points in the avian tetrahedron color space, and plumage is a cloud of points.

27

The center of the color space is where all the cones receive equal stimulation, and a color is thus

28

perceived as achromatic (Maia *et al.*, 2019). A color is perceived as more intense or vivid as its distance

29

from the center of the color space increases. Mean plumage brightness was the mean relative reflectance

30

over the entire spectral range (300 nm to 700 nm) and corresponded to the number of photons caught by

31

double cones (Stoddard & Prum, 2008). We calculated mean plumage chromaticity based on the mean

32

distance from the center of the color space. We calculated color volume as the convex hull that included

33

all the points in a plumage cloud. Different colors differentially stimulate the four cones responsible for

34

color detection, which leads to point dispersal and increased color volume (Renoult *et al.*, 2017). Hence,

35

color volume expresses color diversity in bird plumage. We also calculated the mean distance between

36

all color pairs (Stoddard & Prum, 2008) as a metric of mean within-plumage contrast. This mean distance

37

was highly correlated with color volume (see Table S2), so we omitted it from later analyses.
<table><caption>**Table S2:** Pearson correlation coefficients for coloration metrics. Coefficients for female coloration metrics are in bold, and coefficients for male coloration metrics are in italics. * indicates a p-value of less than 0.05, and *** a p-value of less than 0.001.</caption><thead><tr><th></th><th>Brightness</th><th>Chromaticity</th><th>Color volume</th><th>Contrast</th></tr></thead><tbody><tr><th>Brightness</th><td></td><td>-0.17*</td><td>-0.06</td><td>-0.08</td></tr><tr><th>Chromaticity</th><td>0.02</td><td></td><td>0.40***</td><td>0.41***</td></tr><tr><th>Color volume</th><td>-0.15</td><td>0.33***</td><td></td><td>0.77**</td></tr><tr><th>Contrast</th><td>-0.04</td><td>0.39***</td><td>0.80***</td><td></td></tr></tbody></table>

38

39

40
## 3/ Life history traits

<table><caption>**Table S3:** Scoring of descriptive contributions of males and females to parental care</caption><thead><tr><th>Activity description</th><th>Male score</th><th>Female score</th><th>Comments</th></tr></thead><tbody><tr><td>Female only</td><td>0</td><td>1</td><td></td></tr><tr><td>Both</td><td>1</td><td>1</td><td></td></tr><tr><td>Female mostly/mainly/primarily/Greater by female</td><td>0.5</td><td>1</td><td></td></tr><tr><td>Male occasionally/rarely helps/may help</td><td>0.2</td><td>1</td><td></td></tr><tr><td>Male mostly/mainly/primarily/ Greater by males</td><td>1</td><td>0.5</td><td></td></tr><tr><td>Only male if female starts 2nd brood</td><td>1</td><td>1</td><td>Female is still invested in reproduction even if it is a 2nd brood</td></tr><tr><td>Both, male does more when female broods</td><td>1</td><td>1</td><td>Female is still highly invested</td></tr><tr><td>Male involvement varies</td><td>0.5</td><td>1</td><td></td></tr><tr><td>Mainly male at first</td><td>1</td><td>1</td><td>"at first" means female is invested in brooding hatchlings</td></tr><tr><td>Some males observed covering eggs for considerable periods</td><td>0.2</td><td>1</td><td>Few males, little information on male incubation duration</td></tr><tr><td>Female more than male</td><td>0.7</td><td>1</td><td></td></tr><tr><td>Female leaves first</td><td>1</td><td>0.5</td><td>Post-fledging period</td></tr><tr><td>Male may cover eggs briefly</td><td>0.2</td><td>1</td><td></td></tr><tr><td>Male observed incubating</td><td>0.5</td><td>1</td><td></td></tr><tr><td>Male provides more supplies</td><td>1</td><td>0.7</td><td></td></tr><tr><td>One observation of male feeding</td><td>0.1</td><td>1</td><td></td></tr><tr><td>Male with small harem; male if fledgling in territory</td><td>0.1</td><td>1</td><td>Very rare situation</td></tr></tbody></table>

### Miscellaneous

"Several weeks" was estimated to be 21 days.

When information on activity or duration was missing for a species, we used the median value for the whole data set.

<table><caption>**Table S4:** Scoring nest characteristic (from lowest to highest vulnerability)</caption><thead><tr><th>Ecological factor</th><th>Description</th><th>Score</th><th>Comment</th></tr></thead><tbody><tr><td rowspan="3">Nest position</td><td>Tree</td><td>0</td><td>&gt; 2 m</td></tr><tr><td>Shrub</td><td>1</td><td>50 cm–2 m</td></tr><tr><td>Ground</td><td>2</td><td>&lt; 50 cm</td></tr><tr><td rowspan="3">Nest openness</td><td>Closed, located in hole</td><td>0</td><td></td></tr><tr><td>Partially open, well concealed</td><td>1</td><td></td></tr><tr><td>Open</td><td>2</td><td></td></tr></tbody></table>
## 4/ Phylogenetic analyses.

**Table S5:** Estimated values from the PGLS analyses when excluding corresponding male coloration from the models. Variables are sorted in the same order as in Figure 1 (excluding male coloration). 95% confidence intervals (displayed in brackets) include phylogenetic uncertainty. Estimates with confidence intervals not overlapping zero are represented in bold (except for Pagel's Lambda values).

**Estimates with confidence intervals**

<table><thead><tr><th>Variable</th><th>Brightness</th><th>Chromaticity</th><th>Color volume</th></tr></thead><tbody><tr><td>Paternal care</td><td><strong>0.165 [0.008;0.323]</strong></td><td>0.112 [-0.051;0.275]</td><td>-0.133 [-0.294;0.027]</td></tr><tr><td>Egg production</td><td><strong>0.435 [0.063;0.806]</strong></td><td>-0.225 [-0.621;0.170]</td><td>-0.001 [-0.381;0.379]</td></tr><tr><td>Egg production x Paternal care</td><td>0.079 [-0.064;0.222]</td><td>-0.165 [-0.311;0.019]</td><td>0.039 [-0.106;0.185]</td></tr><tr><td>Nest characteristics</td><td>0.111 [-0.054;0.277]</td><td><strong>0.227 [0.051;0.403]</strong></td><td>0.020 [-0.149;0.189]</td></tr><tr><td>Female weight (log)</td><td><strong>-0.575 [-0.960;-0.189]</strong></td><td>-0.011[-0.432;0.409]</td><td>-0.027 [-0.423;0.370]</td></tr><tr><td>Breeding period (log)</td><td>-0.099 [-0.247;0.049]</td><td>0.040 [-0.114;0.194]</td><td>-0.134 [-0.286;0.018]</td></tr><tr><td>Migration</td><td></td><td></td><td></td></tr><tr><td>Strictly migrant</td><td>0.014 [-0.310;0.339]</td><td><strong>-0.365 [-0.694;-0.036]</strong></td><td>-0.166 [-0.497;0.165]</td></tr><tr><td>Partially migrant</td><td>-0.245 [-0.570;0.080]</td><td>-0.116 [-0.446;0.213]</td><td>0.323 [-0.008;0.653]</td></tr><tr><td>Strictly resident</td><td><strong>0.492 [0.012;0.972]</strong></td><td>-0.155 [-0.661;0.351]</td><td>-0.180 [-0.672;0.312]</td></tr><tr><td>Pagel's lambda</td><td>0.573 [0.570;0.575]</td><td>0.747 [0.744;0.749]</td><td>0.587 [0.584;0.589]</td></tr></tbody></table>

## References

* Del Hoyo, J., Arlott, N., Burn, H. & Cox, J. (eds). 2004. *Handbook of the birds of the world*. Lynx, Barcelona.

* Doutrelant, C., Paquet, M., Renoult, J.P., Grégoire, A., Crochet, P.-A. & Covas, R. 2016. Worldwide patterns of bird colouration on islands. *Ecology Letters* **19**: 537–545.

* Gomez, D. & Thery, M. 2004. Influence of ambient light on the evolution of colour signals: comparative analysis of a Neotropical rainforest bird community. *Ecol. Lett.* **7**: 279–284.

* Maia, R., Gruson, H., Endler, J.A. & White, T.E. 2019. pavo 2: New tools for the spectral and spatial analysis of colour in r. *Methods in Ecology and Evolution* **10**: 1097–1107.

* Renoult, J.P., Kelber, A. & Schaefer, H.M. 2017. Colour spaces in ecology and evolutionary biology. *Biol. Rev.* **92**: 292–315.

* Shultz, A.J. & Burns, K.J. 2017. The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae). *Evolution* **71**: 1061–1074.

* Stoddard, M.C. & Prum, R.O. 2008. Evolution of avian plumage color in a tetrahedral color space: A phylogenetic analysis of new world buntings. *Am. Nat.* **171**: 755–776.
**Original article: Evolution of female colors in birds: the role of female cost of reproduction and paternal care**

*Authors:* Amélie Fargevieille (ORCID 0000-0003-0934-3563)§, Arnaud Grégoire (ORCID 0000-0001-6103-355X)§, Doris Gomez (ORCID 0000-0002-9144-3426)§, Claire Doutrelant (ORCID: 0000-0003-1893-3960)§

*Author institutional affiliations:* §CEFE-CNRS UMR 5175, Université de Montpellier, Campus du CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France

*Corresponding author contact details:* Amélie Fargevieille, CEFE-CNRS UMR5175, Campus du CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5; France. afargevieille@gmail.com

*Author contribution:* A.F., A.G. and C.D. designed the study; A.F. collected the data with help from D.G; A.F. analysed the data; A.F. wrote the first draft with several inputs from A.G., D.G. and C.D.

**Acknowledgments.**

We thank the National Museum of Natural History in Paris, especially Patrick Boussès and Jérôme Fuchs who gave us access to the bird collection and facilitated the use of the specimens selected. We thank Maria del Rey Granado for her help with recording spectrometric measures. We last thank Marianne Elias and Benoît Nabholz for help with phylogenetic comparative methods, Tim Janicke and anonymous reviewers for insightful comments on the manuscript, and Jessica Pearce and Kaitlyn Murphy for English language editing and comments. Funding was provided by the CNRS, the University of Montpellier, the French National Research Agency (CD: ANR 09-JCJC-0050-0) and the regional government of Languedoc Roussillon (Chercheur-es d'Avenir grant to CD).

*Conflict of interest statement:* We declare no conflict of interest.
*Data archival:* Data sets are deposited in the Dryad Digital Repository (URL: https://doi.org/10.5061/dryad.9s4mw6mkh)