The mechanism of feather movements: Implications for the evolution of birds and avian flight
Abstract
The feather-bearing skin of birds differs fundamentally from the fur-bearing skin of mammals. Especially within the feather tracts, it contains much structural fat tissue in the dermis and the subcutaneous Fascia superficialis. These are separated from each other by an elastic membrane and are, together with the feather muscles, part of the hydraulic skeleto-muscular apparatus of the feathers. While feathers are raised by erector feather muscles and are returned to their resting position by the resilience of the surrounding fat tissue and an elastic membrane, the depressor feather muscles counteract external forces, such as air turbulences, thereby ensuring a smooth surface of the coat of feathers and reducing drag during flight. The coat of feathers itself creates fusiform body contours, which also reduce drag. Furthermore, subcutaneous fat bodies are strategically placed to ensure an even draping of the skin over the body and, thereby, contribute to streamlined body contours. The subcutaneous fat bodies and the dermal depressor muscles are part of the unique characteristics of the avian integument and have evolved under the selective regime for streamlining of body contours and surface as a precondition for the evolution of avian flight.