It has long been suggested that tropical species are generally more colourful than temperate species, but whether latitudinal gradients in organismal colourfulness exist remains controversial. Here we quantify global latitudinal trends in colourfulness (within-individual colour diversity) by collating and analysing a photographic dataset of whole-body plumage reflectance information for >4,500 species of passerine birds. We show that male and female birds of tropical passerine species are generally more colourful than their temperate counterparts, both on average and in the extreme. We also show that these geographic gradients can be explained in part by the effects of several latitude-related factors related to classic hypotheses for climatic and ecological determinants of organismal colourfulness. Taken together, our results reveal that species’ colourfulness peaks in the tropics for passerine birds, confirming the existence of a long-suspected yet hitherto elusive trend in the distribution of global biodiversity.
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All analysis data are available in Supplementary Data 1. In addition, the phylogenetic trees were downloaded from http://www.birdtree.org, the geographic and ecological data were accessed via BirdLife International’s Data Zone (http://www.datazone.birdlife.org), and the global climate data were downloaded from WorldClim (https://worldclim.org/).
The R code is available at https://github.com/christophercooney/Avian-colourfulness.
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We thank M. Adams, H. van Grouw, R. Prys-Jones and A. Bond from the Bird Group at the Natural History Museum at Tring for providing access to and expertise in the collection, N. Brett for assistance with data collection, and F. Babarović, J. Kennedy, L. Rombault and A. Slavenko for helpful discussion and feedback. This work was funded by a Leverhulme Early Career Fellowship (ECF-2018-101) and a Natural Environment Research Council Independent Research Fellowship (NE/T01105X/1) to C.R.C., a Leverhulme Centre for Advanced Biological Modelling PhD studentship to Y.H., a Royal Society Wolfson Merit Award (WM170050, APEX APX\R1\191045) and a National Research, Development and Innovation Office of Hungary grant (ÉLVONAL KKP-126949, K-116310) to T.S., and a European Research Council grant (615709, Project ‘ToLERates’) and a Royal Society University Research Fellowship (UF120016, URF\R\180006) to G.H.T.
The authors declare no competing interests.
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Extended Data Fig. 1 The distribution of sampled species and illustrations of the colouration data workflow and colour diversity metrics used in this study.
a, The proportion of passerine species sampled per grid cell. Grid cell size is 50 × 50 km (Behrman projection) and only cells containing at least 5 passerine species are plotted. b, The phylogenetic distribution of sampled species (blue, n = 4,527) relative to the whole passerine radiation (n = 5,966). c, An example showing the workflow used to extract whole-body reflectance data from specimen images, as applied to lateral (side) view images. An analogous workflow is applied to the other two views of the specimen (dorsal and ventral). The resulting sets of measurements for each view are then combined into a final dataset of 1,500 measurements for each specimen, capturing whole-body plumage colouration. d, The relationship between specimen-level scores of colour loci and colour volume based on a ultraviolet-sensitive (UVS) visual system.
Extended Data Fig. 2 Latitudinal gradients in male and female colourfulness in passerine birds using colour volume.
a, Mean colour volume scores for grid cell assemblages, separately for males (top) and females (bottom). b, c, Distributions of mean species’ colour volume scores for grid cells (b) and ecoregions (c) with respect to latitude, separately for males (top) and females (bottom). Grid cell size is 50 × 50 km for all panels (Behrman projection) and only cells containing at least 5 sampled species are plotted. Colour volumes are based on an ultraviolet- sensitive (UVS) visual system. Note: colour volume values are multiplied by 1000.
Extended Data Fig. 3 Geographic distributions of male and female colourfulness in passerine birds using different datasets.
Grid cell size is 50 × 50 km for all panels (Behrman projection) and only cells containing at least 5 sampled species are plotted. UVS, ultraviolet sensitive; VS, violet sensitive. Note: colour volume values are multiplied by 1000.
Extended Data Fig. 4 Geographic distribution of the proportion of species in passerine assemblages in the top colour diversity quartile using different datasets.
Grid cell size is 50 × 50 km for all panels (Behrman projection) and only cells containing at least 5 sampled species are plotted. UVS, ultraviolet sensitive; VS, violet sensitive.
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Cooney, C.R., He, Y., Varley, Z.K. et al. Latitudinal gradients in avian colourfulness. Nat Ecol Evol 6, 622–629 (2022). https://doi.org/10.1038/s41559-022-01714-1
Nature Ecology & Evolution (2022)