Monitoring steroid hormone patterns via excreted metabolites is an effective, non-invasive method for assessing physiological states in wild animals. This method allows for the monitoring of individuals or social groups with minimal disturbance. Here we aimed at elucidating the measurable glucocorticoids and androgens in excreta, for monitoring stress and reproductive hormones in the common waxbill (Estrilda astrild), a small estrildid finch. We measured the excreted metabolites in response to physiologically induced stress and reproductive challenges, both in males and females. For this, we challenged 12 captive common waxbills with adrenocorticotropic hormone (ACTH) and another 12 with luteinizing hormone-releasing hormone (LHRH). Steroid concentrations in droppings were analyzed using liquid chromatography mass spectrometry (LC-MS) before and up to six hours after each challenge. We found that enzymatic hydrolysis prior to LC-MS increased the recovery of steroid metabolites from common waxbills excreta. The ACTH challenge resulted in a significant increase of excreted glucocorticoids with a cortisone standard within 2-3 hours, suggesting that metabolites with a cortisone-similar structure are a reliable measure for monitoring physiological stress in this species. Although the LHRH challenge was inconclusive for androgens, our results provide useful technical details for measuring glucocorticoid metabolites in common waxbill excreta. This enables future studies of behavior in this bird species with minimum disturbance or handling bias and ensuring animal welfare requirements.

Sexual dimorphism in bird ornamentation is well-documented, with male traits often emphasized in sexual selection studies, while female ornamentation remains understudied. Investigating European serins (Serinus serinus) and common waxbills (Estrilda astrild), I summarize experimental evidence revealing that sex differences in ornamentation arise from sex-specific ecological and physiological constraints, particularly affecting females. In European serins, female ornamentation does not function as a sexual or dominance signal, contrasting with males. In common waxbills, female bill colouration is highly plastic and sensitive to ecological conditions and oestradiol levels, leading to context-dependent sexual dichromatism that can be reduced or eliminated under favourable conditions. Together, these results challenge the view that sexual dichromatism is predominantly genetically fixed and male driven, highlighting female colouration plasticity as a key source of variation in ornamentation and social signalling. These findings underscore the need to integrate female colour variation for understanding the evolution of sexual and social signals.

Glucocorticoids mediate behavioral and physiological responses to environmental stressors, supporting individuals' survival and fitness. Corticosterone, as the primary avian glucocorticoid, modulates these physiological responses. We focus on the common waxbill Estrilda astrild, a highly social and gregarious passerine that feeds and flocks together year-round, providing an excellent model to explore the impact of glucocorticoids on social behavior, especially in aggression. To investigate this, we conducted a correlational study in captive birds, examining baseline corticosterone plasma levels and three behavioral assays: 1) the tonic immobility test, a standard indicator of fear; 2) the mirror test, an assay to assess the reactive-proactive personality axis; and 3) food competition tests, which evaluate social behavior and aggression. Subsequently, we experimentally manipulated the common waxbill corticosterone levels by using exogenous high and low dosages of corticosterone to induce a short-term glucocorticoid challenge and evaluate its effects on behavior. Our results revealed that birds with higher baseline corticosterone plasma levels were less aggressive, but an acute increase in corticosterone levels increased aggression. Furthermore, corticosterone receptor antagonist reduced activity but did not affect feeding. These results suggest that corticosterone can influence aggression and activity in common waxbills, leading to a role in regulating social behavior in both sexes of this gregarious passerine species.

Animals can use information on social dominance to manage aggressive interactions, but information on strangers is lacking or uncertain. Strategies for dealing with this uncertainty are still being discovered, and experimental research is needed to understand their dynamics. Common waxbills, Estrilda astrild, were recently found to increase aggressiveness when less familiar bystanders are present, mostly by bullying subordinate flock members. We tested whether aggressive show-off also happens when strangers are at close range. In assays of competition for food in a confined space, waxbills decreased aggressiveness among familiar individuals when unfamiliar birds were present, and aggression was more likely among strangers. We found evidence for redirected aggression only from unfamiliar to familiar individuals: aggression towards familiar birds was more likely after receiving aggression from strangers. Thus, while waxbills did not show-off aggressiveness preventively near strangers, possibly because of the risk of being challenged, they still showed off reactively after receiving aggression from strangers. This study indicates that waxbills use aggressive show-offs towards third parties to signal dominance to unfamiliar individuals and do so in a context-dependent manner that appears to weigh the risk of being challenged at proximity. (c) 2025 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

In interspecific signalling, vivid colours have been interpreted to enable species recognition and maximise signal detection. Recently, it has been shown that vivid colours can also convey information on individual fitness, which could be advantageous for receivers. In the marine cleaning mutualism between the Indo-Pacific bluestreak cleaner wrasse (Labroides dimidiatus) and their reef fish visitors (known as clients), the colour of cleaners may provide behavioural cues, which clients could use to make decisions, as higher blue saturations are associated with better cleaning services. Here, we asked whether clients perceived differences in blue saturation when evaluating individual cleaners, and whether they used this information to choose them. To do this, we introduced the client threadfin butterflyfish (Chaetodon auriga) to three differently saturated video sequences of cleaners (minimum saturation, control and maximum saturation). We found that clients actively preferred video sequences of more saturated cleaners when played against lower saturations (i.e. spending more time in the preference area, facing more frequently and for longer periods, and entering the preference areas near the more saturated monitors more frequently), which provided more evidence that the blue colouration of cleaners may be a true signal of individual quality. By being able to distinguish these saturations, it is likely that in the wild, clients use this signal to make decisions and select the cleaners that provide a better cleaning service.

Color ornamentation is often sexually selected as a signal of condition, which may be affected by the environment and senescence, but long-term studies in mutually ornamented species are rare, limiting our understanding of sex-specific senescence. We measured color ornamentation of 60 common waxbills (Estrilda astrild) after capture from the wild and in 4 autumns in a large open-air mesocosm. We found longitudinal decreases in the size of the red mask and redness of plumage and bill color, consistent with senescence. Bill color decreased more in females. In males, there was a longitudinal increase in the size of the red breast patch. We also found increased color ornamentation, especially in females, when moving from the wild to the mesocosm, likely due to environmental effects. Monitoring waxbills in bird cages suggested that bill and plumage redness peak in the breeding season. Stronger senescence and environmental effects in females indicate cautious investment in ornamentation, while weaker senescence in males could be due to stronger sexual selection. Overall, although similarly ornamented, the sexes differ in their temporal investment in ornamentation, in line with their life histories. We found stronger senescence and environmental effects on female than male color in a mutually ornamented bird, the common waxbill. Results indicate plastic and cautious investment in costly coloration by females, likely to preserve body condition. In males, absence of senescence in color and even investing more later in life in one color trait indicate an attempt to always express high ornamentation, possibly due to stronger sexual selection on males.

Microbiota can both influence or be influenced by social structure and interactions between individual hosts. The use of social network analyses to test for associations between social structure and microbial diversity has only been applied to a few host species. Such an approach could help to disentangle conflicting findings on the effect of social transmission of microbial communities. Here, we explored the following hypotheses: i) that the social network centrality of hosts increases bacterial diversity (alpha diversity), ii) that the strength of social associations between hosts or iii) roosting in the same locations increases microbiota similarity between individuals (beta diversity). For this, we investigated cloaca- and feather-associated microbiotas of wild-caught common waxbills living in a semi-natural environment. We found no evidence for influences of sociality or roosting location on bacterial alpha or beta diversities. We nonetheless detected differences between birds roosting primarily indoors or outdoors at the amplicon sequence variant (for 16S rRNA) and genus levels of feather bacteria, with the birds roosting primarily outdoors having a higher prevalence and abundance of taxa associated with vegetation and the environment, while birds roosting indoors had a higher prevalence and abundance of some bacteria reported as animal-associated. The absence of major effects of host social network on microbiota suggests that environmental effects and individual host idiosyncrasy, perhaps mediated by genetic factors, are more important than within-group sociality driving microbial variation in common waxbills. These results thus call for research on hosts’ genetic diversity and how it may influence their microbiota. The social structure of gregarious animals may influence the similarity and diversity of their microbiota. Social proximity between hosts may lead to more similar microbiota, and centrality in the host social network may contribute to more diverse microbiota. Despite these expectations, we found no effects of social structure on microbial diversity in a flock of wild common waxbills sharing an open-air mesocosm environment. We found indications that roosting outdoors made waxbills more prone to host environment-associated bacteria and roosting indoors to animal-associated bacteria. The absence of major effects of social structure on the microbiome suggests that, even in gregarious species, environmental factors and individual host idiosyncrasies, perhaps genetic-based, can be the most important for understanding microbial variation.

Sexual selection theory has provided explanation for the evolution and function of elaborate male traits, yet less effective in explaining female trait expression. Consequently, there remains a significant gap in our understanding of how female ornamentation is maintained and what factors drive its evolution. The European serin (Serinus serinus) displays a dichromatic carotenoid-based plumage, with a yellow patch on the breast in both males and females, although with females displaying less conspicuous colouration. While yellow colouration in male serins serves a dual function in social contexts, both sexual and nonsexual, the potential signal role of this colouration in females remains unknown. This study investigates the relationship between female traits, measures of fitness and the potential sexual and social function of female plumage colouration. To achieve this, we conducted a male mate choice experiment and a social competition experiment for access to limited food, similar to those previously employed with male serins. We found a relation between yellow plumage colouration and age classes, with older females displaying more colourful plumage. The mate choice experiment did not reveal a male sexual preference based on carotenoid-based plumage colouration in females. Furthermore, while females established steep hierarchies within groups, dominance over food was not associated with plumage colour. These findings suggest that in the European serin, female plumage may be subject to direct natural selection for less conspicuous colours or may result from genetic correlation of a male-selected trait. Understanding of factors influencing the expression of female traits remains limited. This study addresses this knowledge gap by investigating the putative sexual and social functions of female plumage coloration in the European serin, a species displaying dichromatic carotenoid-based plumage. Our findings reveal a correlation between carotenoid-based yellow plumage coloration and age classes in females, indicating potential age-related changes in ornamental traits. We found no evidence of male preference for female plumage coloration, nor association with resource dominance. We discuss potential mechanisms driving the maintenance of these traits, such as natural selection favouring less conspicuous colours, or genetic correlations with male-selected traits. These findings contribute to our understanding of the complex mechanisms shaping sexual dimorphism.

The amino acid tryptophan (Trp) is a precursor of the neurotransmitter serotonin. Trp supplementation or other forms of serotonergic enhancement generally promote pro-social behavior, decreasing aggression, and also feeding in different animals. However, past research has been conducted in confined spaces, and there is little work in naturalistic conditions where animals move and associate more freely. We gave a Trp-enriched diet to a free-flying flock of common waxbills (Estrilda astrild) in semi-natural conditions and monitored group foraging, aggressions during feeding, and the social network. Contrary to expectations, aggressiveness and feeding increased during Trp supplementation. Consistent with the prediction of increased social associations, foraging groups became larger, and individuals joined more foraging groups, but these changes appear driven by increased appetite during Trp treatment. Also, the mean strength of associations in the social network did not change. Overall, Trp supplementation affected group behavior in this free-flying flock, but mostly in directions unanticipated based on research conducted in small spaces. To harmonize our results with those found in small confined spaces, we hypothesize that free-flying birds have energetic requirements not experienced in lab-housed individuals, which may impact social behavior and responses to Trp. The neurotransmitter serotonin and its precursor tryptophan (Trp) are known to modulate social behavior in animals. Trp supplementation typically has calming effects, diminishing aggression, movement, and appetite, but past research was conducted in artificial or confined spaces. Here, Trp supplementation of gregarious birds in more natural conditions showed different effects on group foraging, including more feeding and aggressiveness foraging, allowing better inferences about the regulation of ecologically relevant group behavior.

The influence of the neuropeptide arginine vasotocin (AVT) has been demonstrated across various species, on an ample range of behaviors, yet the results appear to be highly species-specific. In this study, we aimed to test how AVT influences both social and non-social behaviors in the common waxbill Estrilda astrild, a highly social estrildid finch. Through a within-subject design study, we experimentally manipulated AVT pathways through muscular injections of both an agonist and an antagonist of AVT at different dosages, and performed competition over food tests to assess behavioral changes. Our observations reveal a decrease in birds' movements with both low and high dosages of AVT. Additionally, the higher AVT dosage led to a significant decrease in birds' feeding, aggressive behavior, and allopreening. Conversely, the lower AVT dosage increased the duration of allopreening, which is a proxy for affiliation. The use of Manning Compound, a V1a antagonist, did not produce any changes in behavior, however, the absence of affinity studies for this compound in birds makes it difficult to interpret these results. It is plausible that in common waxbills, AVT V1b or V1a receptors may be involved in regulating movement, feeding, aggressive behavior, and allopreening, rather than V2 AVT receptors.

Microbial diversity may affect host phenotypes, including morphology and behaviour, but it remains unclear to which extent those could be mediated by host differences in diet or habitat use. We used a population of wild common waxbills living in a large open-air mesocosm to control for such differences and test whether phenotypic variation is associated with cloaca bacteria diversity. We also tested correlations between feather-associated bacteria diversity and plumage colour ornamentation. Albeit weakly correlated, individuals sharing more similar cloaca microbiotas were more similar in their carotenoid-based plumage and bill red colour, suggesting that the gut microbiota can influence ornamentation even when individuals are in the same habitat and have access to the same food resources, perhaps mediating individual differences in feeding or digestion. There were no associations between feather bacteria and plumage colour, nor between cloaca bacteria and body size or several behavioural phenotypes. These results further highlight that investigating associations between microbiota and behaviour in nature should account for variation in extrinsic factors.

In marine interspecific cleaning mutualisms, small fish known as "cleaners" inspect the surface, gills and sometimes the mouth of "client" reef fish, eating ectoparasites, mucus, scales and dead or infected tissue. These cleaner fish species share similar vivid coloration that makes them recognizable by clients. To gain insight on additional communication roles of cleaner fish color, we tested if differences in color among cleaner wrasse Labroides dimidiatus individuals captured from the wild are explained by sex, body size or parasite loads. We found that males were larger, heavier and tended to have more saturated blue tails than females. We also found blue color saturation to be positively correlated with size, and that this size-dependence explains the trend for the sexes to differ in color. Parasite loads did not predict individual differences in color, but fish in our sample were not heavily parasitized. Other color traits (including yellow color saturation and black color brightness) did not differ between the sexes and were associated with morphological differences. Size-dependence of blue color may thus suggest that it is a condition-dependent signal that could be used in various types of inter- and intraspecific social interactions.

The oxytocin family of neuropeptides is implicated in the regulation of sociality across vertebrates. Non-mammalian homologs of oxytocin, such as isotocin in fish and mesotocin in amphibians, reptiles and birds, all play crucial roles modulating social and reproductive behavior. In this study, we exogenously manipulated the mesotocinergic system in a highly social bird, the common waxbill Estrild astrild, and tested the effects on affiliative and aggressive behavior by performing tests of competition over food. Birds treated with mesotocin decreased almost all the behaviors we studied (movement, feeding, allopreening), while birds treated with an oxytocin antagonist showed a reduction only in social behaviors (aggressions and allopreening). We also found two sex-specific effects: mesotocin reduced allopreening more in males than females, and the oxytocin antagonist reduced aggressiveness only in females. Our results suggest sex-specific effects in the modulation of affiliative and aggressive behaviors via mesotocinergic pathways.

Serotonin or 5-hydroxytryptamine (5-HT) is a monoaminergic neurotransmitter that is known to influence behaviour in various animal species. Its actions, however, are complex and not well-understood yet. Here, we tested whether and how two 5-HT receptor agonists and a 5-HT receptor antagonist influence behaviour in common waxbills (Estrilda astrild), focusing on aggression, movement and feeding. We applied acute administration of either 8-OH-DPAT (a 5-HT1A receptor agonist), fluoxetine (a selective serotonin reuptake inhibitor; SSRI) or WAY 100,635 (a 5-HT1A receptor antagonist), and then quantified behaviour in the context of competition for food. Waxbills treated with the SSRI fluoxetine showed an overall decrease of aggressive behaviour, activity and feeding, while we found no significant effects of treatment with the other serotonergic enhancer (8-OH-DPAT) or with the antagonist WAY 100,635. Since both 8-OH-DPAT and WAY 100,635 act mainly on 5-HT1A receptor pathways, while fluoxetine more generally affects 5-HT pathways, our results suggest that receptors other than 5-HT1A are important for serotonergic modulation of waxbill behaviour.Significance statementThe serotonergic system is of interest for current behavioural research due to its influence on a range of behaviours, including aggression, affiliative behaviour, feeding and locomotion in various species. There are, however, numerous discrepancies regarding the behavioural effects of serotonin across studies. We used acute pharmacological manipulations of the serotonergic system in common waxbills, using two serotonin enhancers (8-OH-DPAT and fluoxetine) and a serotonin blocker (WAY 100,635). Behavioural effects of these pharmacological manipulations on aggressiveness, movement and feeding, during tests of competition over food, indicated an anxiogenic-like effect of fluoxetine, but not of 8-OH-DPAT and WAY 100,635. This suggests a distinct role for different serotonergic pathways on waxbill behaviour.

Host‐associated microbial communities are essential for host homeostasis and health. Most research on microbiotas is restricted to model host species and, since microbe–host dynamics can be species‐specific, we still lack a good taxonomic, ecological and evolutionary understanding of microbiotas in wild hosts, including birds. Given the diversity of avian host species, their habitat specializations, ecological requirements and life‐history traits, we need to characterize the dynamics of microbiotas from taxonomically diverse bird species. Here we describe cloaca and feather microbiotas of a population of common waxbills Estrilda astrild living in a large open‐air mesocosm, and compare it to microbiotas sampled one year later for a subset of this population, and to the microbiotas of free‐ranging waxbills from the same region. We found clear differences between microbiota from the cloaca, which contained some taxa that may be found in seeds, and from feathers, which contained some taxa that have been previously found in plants and soil. We found limited evidence for sexual differences in both cloaca and feather microbiotas, likely due to the gregariousness, similar behaviour and habitat use of both male and female waxbills. Microbial diversities of waxbills from the mesocosm were representative of those found in free‐ranging individuals. Also, structural differences between microbiota from mesocosm and free‐ranging waxbills seemed to be small and comparable to differences found in microbiota from the mesocosm in consecutive years. These results indicate that this mesocosm population could be used as a model in future studies investigating microbiota–host dynamics.

Carotenoid-based colour signals can be costly to produce and maintain, and trade-offs between signalling and other fitness traits are expected. In mutually ornamented species, trade-offs with reproduction may be stronger for females than males, because females often dedicate more resources to offspring production, which may lead to plastic investment in colour signals and plastic sexual dichromatism. Oestradiol is a candidate mediator of this trade-off because it regulates reproductive physiology and may also influence the expression of coloration. We tested this hypothesis by giving female common waxbills (Estrilda astrild) either oestradiol (17β-oestradiol) or empty implants during the early breeding season and measured spectral reflectance of carotenoid-based bill coloration weekly for two months. Using a model of avian vision, we found that bill colour in oestradiol-implanted females became less saturated, less red in hue and brighter, compared with control females and with unimplanted males. This resulted in a change in bill sexual dichromatism from imperceptible to perceptible. Results support the hypothesis that female reproductive physiology influences investment in coloration through changes in oestradiol and show a form of female-driven plastic sexual dichromatism. Greater sensitivity of female colour to physiological and/or environmental conditions helps explain why differences in sexual dichromatism among species differing in ecology often evolve owing to changes in female rather than male phenotype.

The dopaminergic (DAergic) system has well-known influences on behavioral and cognitive functions. Previous work with common waxbills (Estrilda astrild) reported context-specific DAergic effects that could have been due to social environment. Manipulating the dopamine D2-like receptor family (D2R) pathways had opposing effects on behavior depending on whether waxbills were tested alone or in a small cage with a mirror as a social stimulus. As waxbills are highly gregarious, it was hypothesized that being alone or perceiving that they have a companion might explain this context dependence. To test context-dependent DAergic effects, we compared behavioral effects of D2R manipulation in waxbills in the same familiar environment, but either alone or with a familiar, samesex companion. We found that D2R agonism decreased movement and feeding, similar to previous results when testing waxbills alone. However, contrary to the hypothesis of dependence on social context, we found that the behavioral effects of the D2R agonist were unchanged when waxbills were tested with a companion. The context dependence reported earlier might thus be due to other factors, such as the stress of being in a novel environment (small cage) or with an unfamiliar social stimulus (mirror image). In tests with a companion, we also found a sex-specific social effect of D2R manipulation: D2R blocking tended to decrease aggression in males but to increase it in females. Together with past work, our results suggest that DAergic effects on behavior involve different types of context or sex dependence.

Animal coloration is a key biological trait in social communication and animal behavior research. Quantifying color differences among individuals using reflectance spectrophotometry has become standard practice but can be challenging due to measurement inaccuracies and color plasticity, especially when color differences are small. Choosing robust color metrics, among the many receiver-independent and visual-system-dependent metrics available, is thus an important consideration. Here, we measured reflectance spectra from carotenoid-based red bill and breast plumage of 18 females and 6 males common waxbills repeatedly throughout 1 year, computed alternative color metrics for each of three color properties (brightness, hue, and saturation), including 9 receiver-independent metrics and 4 metrics based on a tetrahedral model of avian vision, and also computed Principal Components (PCs) of spectral reflectance. Then, we estimated the adjusted repeatability of individual color metrics and correlations among metrics of the same color property for bill and plumage separately. Except for metrics based on PCs, all receiver-independent color metrics were strongly correlated to visual model metrics of the same color property. Individual differences in color were significantly repeatable across days, except when using PCs, but the repeatability estimates differed for alternative metrics of the same color property. Overall, visual model metrics were the most consistently repeatable in different tissues, while the repeatability of receiver-independent metrics differed depending on tissue and color property. Our results suggest that visual model metrics are appropriate to study subtle differences in animal coloration, perhaps even in the absence of detailed information on the performance of alternative color metrics. Significance statement Reflectance spectrophotometry is a standard method for objectively measuring animal coloration, and several alternative metrics are available to quantify color properties from reflectance spectra. However, we lack systematic testing of the reliability of different metrics to capture small color differences among individuals, despite measurement inaccuracies and a degree of color plasticity. We repeatedly measured the carotenoid-based red color in the bill and plumage of common waxbills throughout 1 year, to test which metrics captured individual differences in color more consistently. Metrics based on modeling the receiver visual system retrieved consistently repeatable individual differences in hue, saturation and brightness, in either tissue, while the repeatability of receiver-independent metrics often differed depending on the tissue and color property. Thus, compared to receiver-independent metrics, metrics based on visual models may generally be appropriate to detect and study subtle differences in carotenoid-based coloration.

Dominance hierarchies reduce the costs of competitive aggression in social animals, and testosterone is a proposed mediator of aggressive behaviour and social dominance. While the effects of testosterone on male behaviour are well studied, effects on female aggression and dominance within mixed-sex groups are less clear. We tested how short-term testosterone treatment influences aggressive behaviour and dominance hierarchies in the common waxbill, Estrilda astrild, a bird with mild social hierarchies. In captive mixed-sex groups, we administered intramuscular testosterone to either males or females and, using a test of competition for food, compared aggressive behaviour and social dominance to those of a control treatment (saline) with no testosterone administered. Testosterone treatment to males increased their social dominance, without increasing aggression. We observed a decrease in female aggression towards treated males, suggesting that females were reacting to subtle cues of male dominance (e.g. vocal cues). In contrast, female treatment with testosterone did not increase their dominance, which could either be due to a lack of effect on females, or because males reacted to and opposed cues of female dominance. Together, our results indicate a sex-dependent role of testosterone on social hierarchies.

Sex differences in ornamentation are common and, in species with conventional sex roles, are generally thought of as stable, due to stronger sexual selection on males. Yet, especially in gregarious species, ornaments can also have non-sexual social functions, raising the possibility that observed sex differences in ornamentation are plastic. For example, females may invest in costly ornamentation more plastically, to protect body and reproductive ability in more adverse ecological conditions. We tested this hypothesis with experimental work on the mutually-ornamented common waxbill (Estrilda astrild), supplementing their diets either with pigmentary (lutein, a carotenoid) or non-pigmentary (vitamin E) antioxidants, or alleviating winter cold temperature. We found that both lutein and vitamin E supplementation increased red bill colour saturation in females, reaching the same mean saturation as males, which supports the hypothesis that female bill colour is more sensitive to environmental or physiological conditions. The effect of vitamin E, a non-pigment antioxidant, suggests that carotenoids were released from their antioxidant functions. Alleviating winter cold did not increase bill colour saturation in either sex, but increased the stability of female bill colour over time, suggesting that female investment in bill colour is sensitive to cold-mediated stress. Together, results show that waxbill bill sexual dichromatism is not stable. Instead, sexual dichromatism can be modulated, and even disappear completely, due to ecology-mediated plastic adjustments in female bill colour.