Psittaciformes (parrots, cockatoos and lorikeets) comprise one of the most colourful clades of birds. Their unique pigments and safe cavity nesting habits are two potential explanations for their colourful character. However, plumage colour varies substantially between parrot species and sometimes also between males and females of the same species. Here, we use comparative analyses to evaluate what factors correlate with colour elaboration, colour diversity and sexual dichromatism. Specifically, we test the association between different aspects of parrot colouration and (a) the intensity of sexual selection and social interactions, (b) variation along the slow‐fast life‐history continuum and (c) climatic variation. We show that larger species and species that live in warm environments display more elaborated colours, yet smaller species have higher levels of sexual dichromatism. Larger parrots tend to have darker and more blue and red colours. Parrots that live in wetter environments are darker and redder, whereas species inhabiting warm regions have more blue plumage colours. In general, each of the variables we considered explain small to moderate amounts of variation in parrot colouration (up to 15%). Our data suggest that sexual selection may be acting more strongly on males in small, short‐lived parrots leading to sexual dichromatism. More elaborate colouration in both males and females of the larger, long‐lived species with slow tropical life histories suggests that mutual mate choice, social selection and reduced selection for crypsis may be important in these species, as has been shown for passerines.

Sperm competition is an important component of post‐copulatory sexual selection that has shaped the evolution of sperm morphology. Previous studies have reported that sperm competition has a concurrently directional and stabilizing effect on sperm size. For example, bird species that show higher levels of extrapair paternity and larger testes (proxies for the intensity of sperm competition) have longer sperm and lower coefficients of variation in sperm length, both within and between males. For this reason, these sperm traits have been proposed as indexes to estimate the level of sperm competition in species for which other measures are not available. The relationship between sperm competition and sperm morphology has been explored mostly for bird species that breed in temperate zones, with the main focus on passerine birds. We measured sperm morphology in 62 parrot species that breed mainly in the tropics and related variation in sperm length to life‐history traits potentially indicative of the level of sperm competition. We showed that sperm length negatively correlated with the within‐male coefficient of variation in sperm length and positively with testes mass. We also showed that sperm is longer in sexually dichromatic and in gregarious species. Our results support the general validity of the hypothesis that sperm competition drives variation in sperm morphology. Our analyses suggest that post‐copulatory sexual selection is also important in tropical species, with more intense sperm competition among sexually dichromatic species and among species that breed at higher densities.

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17Psittaciformes (parrots, cockatoos and lorikeets) comprise one of the most colourful clades of birds. Their 18 unique pigments and cavity nesting habits are two potential explanations for their colourful character. 19However, plumage colour varies substantially between parrot species and sometimes also between males 20 and females of the same species. Here, we use comparative analyses to evaluate what factors correlate 21 with colour elaboration, colour diversity and sexual dichromatism. Specifically, we test the association 22 between different aspects of parrot colouration and (1) the intensity of sexual selection and social 23 interactions, (2) variation along the slow-fast life-history continuum and (3) climatic variation. We show 24 that larger species and species that live in warm environments display more elaborated colours, yet 25 smaller species have higher levels of sexual dichromatism. Larger parrots tend to have darker and more 26 blue and red colours. Parrots that live in humid environments are darker and redder, whilst species 27 inhabiting warm regions have more blue plumage colours. In general, the variables we considered explain 28 small to moderate amounts of variation in parrot colouration (up to 20%). Our data suggest that sexual 29 selection may be acting more strongly on males in small, short-lived parrots leading to sexual 30 dichromatism. More elaborate colouration in both males and females of the larger, long-lived species with 31 slow tropical life-histories suggests that mutual mate choice and reduced selection for crypsis may be 32 important in these species, as has been shown for passerines. 33 Keywords: plumage colour elaboration, sexual dichromatism, body size, climate, psittaciformes, 34 comparative analyses. 35 36 37 38So far, few studies on plumage colouration have considered multiple variables. Dale et al. (2015) used 131 comparative analyses to explore the effects of multiple traits on plumage colour in passerines. 132Specifically, this study suggests that the evolution of plumage colour and sexual dichromatism are mainly 133 driven by sexual selection and life-history traits, with stronger effects on female than on male colour. 134Both males and females are more colourful in larger species and in species with tropical life histories (i.e. 135 small clutch size, low seasonality habitats), whilst sexual dichromatism was higher in smaller species and 136 in species with male-biased sexual selection. 137Here, we ask what factors affect plumage colouration in parrots.We quantified achromatic and 138 chromatic colour variation among all 398 species of the order Psittaciformes based on colour plates, and 139 computed estimates of colour elaboration, colour diversity and sexual dichromatism. Our study had three 140

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