The carotenoid-continuum: carotenoid-based plumage ranges from conspicuous to cryptic and back again

Kaspar Delhey,Mark L Roberts,Anne Peters

doi:10.1186/1472-6785-10-13

Abstract

BackgroundCarotenoids are frequently used by birds to colour their plumage with green, yellow, orange or red hues, and carotenoid-based colours are considered honest signals of quality, although they may have other functions, such as crypsis. It is usually assumed that red through yellow colours have a signalling function while green is cryptic. Here we challenge this notion using the yellow and green colouration of blue tits (Cyanistes caeruleus), great tits (Parus major) and greenfinches (Carduelis chloris) as a model.ResultsThe relationship between colouration (chroma, computed using visual sensitivities of conspecifics) and detectability (contrast against natural backgrounds as perceived by conspecifics and avian predators) followed a similar curvilinear pattern for yellow and green plumage with minimum detectability at intermediate levels of carotenoid deposition. Thus, for yellow and green plumage, colours at or close to the point of minimum detectability may aid in crypsis. This may be the case for blue and great tit green and yellow plumage, and greenfinch green plumage, all of which had comparably low levels of detectability, while greenfinch yellow plumage was more chromatic and detectable. As yellow and green blue tit colouration are strongly affected by carotenoid availability during moult, variation in pigment availability between habitats may affect the degree of background-matching or the costliness of producing cryptic plumage.ConclusionsIncreasing carotenoid-deposition in the integument does not always lead to more conspicuous colours. In some cases, such as in blue or great tits, carotenoid deposition may be selected through enhanced background-matching, which in turn suggests that producing cryptic plumage may entail costs. We stress however, that our data do not rule out a signalling function of carotenoid-based plumage in tits. Rather, it shows that alternative functions are plausible and that assuming a signalling function based solely on the deposition of carotenoids in the integument may not be warranted.

Highlights

Carotenoids are frequently used by birds to colour their plumage with green, yellow, orange or red hues, and carotenoid-based colours are considered honest signals of quality, they may have other functions, such as crypsis
Some blue tit measurements were made on fledglings that had been taken into captivity as nestlings in May 2006 together with their parents which continued raising them until independence in large outdoor aviaries
Taking all three species together the relationship between chroma and detectability followed a curvilinear pattern with minimum detectability at intermediate values of chroma for both yellow and green plumage (Figures 1 and 2). This relationship was very similar for U- versus V-type eyes and for both main types of average natural backgrounds

Summary

Introduction

Carotenoids are frequently used by birds to colour their plumage with green, yellow, orange or red hues, and carotenoid-based colours are considered honest signals of quality, they may have other functions, such as crypsis. It is usually assumed that red through yellow colours have a signalling function while green is cryptic We challenge this notion using the yellow and green colouration of blue tits (Cyanistes caeruleus), great tits (Parus major) and greenfinches (Carduelis chloris) as a model. Yellow colouration in these species (which is based on the deposition of the carotenoids lutein and zeaxanthin obtained directly from the diet [14]) has been intensively studied, a confirmation of its signalling function is still elusive For both adults and nestlings the intensity of yellow colouration depends on environmental factors at the time of plumage development such as food availability [15,16], carotenoid content of food [17,18,19,20], parasitism [21] and pollution levels [22,23,24]. This has led to doubts about the signalling function of yellow colouration in these species [32]

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