Latitudinal gradients in avian colourfulness

Abstract

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 effect 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. Main text

Carotenoid-dependent plumage coloration is associated with reduced male care in passerine birds

“…The positive association between carotenoid-dependent plumage coloration and carotenoid-rich diets is in agreement with the large number of single-species studies that have shown that carotenoid-dependent plumage pigmentation is closely linked to the dietary availability of carotenoids (e.g. Hill et al 2002;McGraw et al 2003;Navara and Hill 2003;McGraw et al 2004; for an example in fishes see: Kodric-Brown 1989), and with the comparative study of Cooney et al (2022). It was of course to be expected that high availability of dietary carotenoids was a necessary pre-requisite for the presence of carotenoid-dependent plumage coloration, as birds cannot synthesize carotenoids de novo.…”

“…We also analyzed their diet, body size, geographical distribution, and polygyny because these could potentially confound the relationship between coloration and male care. Based on recent results by Cooney et al (2022), we expect a negative relation between carotenoid-dependent coloration and body size and a positive relation with a tropical distribution. We also contemplate the mating system because the social mating system affects male care (o vice versa) hence can be a confounding variable on parental care and the expression of a sexual signal.…”

“…We obtained the species’ breeding distribution from del Hoyo et al (2016) for latitude. Geographical distribution has been suggested to potentially influence plumage coloration because of diverse mechanisms, which differ between tropical and non-tropical zones (e.g., parasitic load, and diversity of foods with carotenoids; Møller 1998; Cooney et al 2022). Because we were mainly interested in the contrast between tropical and non-tropical areas, and geographical distribution was included as a potential confounding factor, species were categorized based on whether they presented a tropical distribution (A: 23° N to 23° S), a non-tropical distribution (B: north of 23° N, or south of 23° S), or the distribution spanned both tropical and non-tropical areas (C: for species breeding in non-tropical and tropical areas).…”

Color in motion: Generating 3-dimensional multispectral models to study dynamic visual signals in animals

“…Recently, multispectral images were captured for nearly 6,000 bird species (Cooney et al, 2022) in a study that revealed that tropical passerines are more colorful (have a higher degree of intra-individual color) than temperate species. In this study, Cooney et al (2022) collected three multispectral images of each study skin-a dorsal image, a lateral image, and a ventral image-resulting in an expansive dataset of multispectral images for 24,345 specimens.…”

“…Recently, multispectral images were captured for nearly 6,000 bird species (Cooney et al, 2022) in a study that revealed that tropical passerines are more colorful (have a higher degree of intra-individual color) than temperate species. In this study, Cooney et al (2022) collected three multispectral images of each study skin-a dorsal image, a lateral image, and a ventral image-resulting in an expansive dataset of multispectral images for 24,345 specimens. In the future, perhaps such studies will be based on 3D multispectral models, which would provide information (e.g., the surface area and shape of distinct color patches) not necessarily obvious from just three (dorsal, lateral, ventral) images.…”