Function and occurrence of facial flushing in birds

Abstract

So far overlooked as a pigment involved in visual communication, the haemoglobin contained in the blood of all birds is responsible for the red flushing colours in bare skin areas of some species. Our aim has been twofold: (1) to study sub-epidermical adaptations for blood circulation in two flushing species: the crested caracara (Polyborus plancus) and the hooded vulture (Necrosyrtes monachus), and (2) to provide the first compilation of avian species with flushing skin. The bare facial skin of both the caracara and the hooded vulture contains a highly vascularised tissue under the epidermis that may be filled with blood and would thus produce red skin colours. In contrast, feathered areas of the head show very few vessels immersed in connective tissue and have no potential for colour changes. Species with flushing colours are few but phylogenetically diverse, as they belong to 12 different avian orders and at least 20 families. The majority are dark-coloured, large-sized species living in hot environments that may have originally evolved highly vascularised skin patches for thermoregulation. Bird behaviour as well as sex and age differences within species suggest that a signalling system for condition or status based on haemoglobin may have been super-imposed to the physiological process of heat dissipation.

Color in competition contexts in non-human animals

“…Although birds, like primates, also show both haemoglobin-dependent red skin (Negro, Sarasola, Farinas, & Zorrilla, 2006) and structural blue skin (Prum & Torres, 2003), the majority of studies of bird colour focus on carotenoid or melanin-based plumage coloration (Hill, 2006). Furthermore, while most primate studies have examined colour itself, with the exception of the extent of red in mandrills and drills, badges of status in birds appear to be based more on the size of the colour patch than on colour itself (Senar, 2006).…”

On Showy Dwarfs and Sober Giants: Body Size as a Constraint for the Evolution of Bird Plumage Colouration

“…Surprisingly, the relationship between body size and plumage colour heterogeneity has never been reported. If anything, previous papers tangential to this topic are those by Badyaev (1997a), who reported no effect of body size on overall plumage brightness of cardueline finches, Bretagnolle (1993), who reported that body mass was positively correlated with the proportion of white in albatrosses and Negro et al (2006), who reported an association between large size, dark plumage colour and facial flushing colouration. Furthermore, opposing natural and sexual selection pressures are often expressed by different colour components, the latter mainly favouring brighter colours (Hill 2006).…”

Changes in breeding status are associated with rapid bill darkening in male red‐backed fairy‐wrens Malurus melanocephalus

“…In the American goldfinch Carduelis tristis, bills of males held in captivity showed a significant decrease in brightness within 24 h, perhaps due to physiological stress (Rosen and Tarvin 2006). Rapid and ephemeral color changes in exposed facial skin, including ''flushing'' due to dilation of blood vessels, may also serve as a short-term signal in a number of avian families (Curio 2004, Negro et al 2006). …”