SynopsisBirds have a layer of plumage over most of the body which provides thermal insulation. The vane-shaped feathers are more efficient than fur in decreasing air movement between the skin microclimate and the ambient environment. Around the feathers is a layer of air, the depth of which for optimum insulation is determined by their angles to the skin surface maintained by the follicular musculature. The mean avian core temperature is 41°C which is higher than in mammals, but skin surface temperature is several degrees lower. Over the exposed skin the microclimate is more variable, but the skin is warmed by the dermal blood vessels and so the feet do not freeze even at sub-zero temperatures. Transpiration occurs through the skin and helps cool the body. Some water vapour is absorbed by the feather keratin, which prevents the feathers from becoming brittle in dry ambient air. Maintenance of the feather vane arrangement requires a more rigid keratin than in hairs. Birds have only one skin gland, the secretion of which contains wax and lipids, but the epidermis is also strongly lipogenic. The sebaceous secretion probably waterproofs the skin, prevents the feathers from becoming too dry, and is anti-microbial. The avian skin surface provides habitats for many commensal micro-organisms and a few bacterial, fungal and arthropod parasites. Pathogenic microorganisms can penetrate the epidermal barrier in wounds or indirectly through the bites of animal parasites. Once in the dermis micro-organisms are combated by immune responses.
The Body of Breath: Morphologies of Air Movement
