Feathers are the epidermal growths that form the distinctive outer covering, or plumage, of birds. They are unique to birds and are different from hairs, scales, and nails in both structure and function. The main functions of feathers include insulation, aerodynamics, and display.
Birds have two types of feathers: contour feathers cover the body and give it its shape, while down feathers are found under the contour feathers and provide insulation. Contour feathers arise from pits in the skin called feather follicles. A single follicle may produce more than one shaft at a time; however, once a shaft has matured, its associated follicle becomes quiescent until that shaft is molted (shed).
Molting is a periodic process whereby birds replace some or all of their feathers. This allows for growth and repair of damaged plumage as well as provides an opportunity for animals to change their appearance seasonally or in response to environmental cues such as temperature changes. The molt usually starts at the head and progresses posteriorly; however, there is considerable variation among species. For example, some waterbirds shed all their plumage simultaneously (eclosion), while others go through a series of partial molts in which only certain feather groups are replaced at any given time (prenuptial molt).
The rate at which new feathers grow also varies among species; some chicks can acquire a full set of adult contours within days or weeks after hatching (precocial), while others take many months to develop functional flight plumage (altricial). In addition to periodic molts throughout their lives, many birds also undergo a complete replacement of all their plumage once yearly during what is typically referred to as the prebasic molt—a molt that occurs just prior to breeding season.
The structure of a feather consists of three parts: the rachis (or shaft), barbs, and barbules (see Figure 1). The rachis is made up mostly of keratin—the same protein substance that makes up our hair and nails—and gives each feather its strength and stiffness. It also contains blood vessels that bring nutrients to growing cells at the base of the feather during formation and growth. Attached to either side of the rachis are flattened structures called barbs; these are connected together by even smaller structures called barbules. At the tips of some barbules are tiny hooks called barbicels that lock neighbouring barbs together much like Velcro®, giving each feather flexibility as well as increasing its surface area relative to its weight—an important factor in providing insulating air pockets within down feathers (Figure 2). Barbule size also influences how light reflects off a feather’s surface—an important consideration in courtship displays where males use colourful plumageto attract mates.(For more on colouration see “Pigments” below.)
Feathers grow incrementally from regions near their bases known as germ zones.(A detailed descriptionof this process can be found in “Biology Of Birds: Development And Maturation Of Feathers” by Jainudeen et al.) Blood vessels carrying oxygen-rich blood supply nutrients necessary for cell division within these regions which resultsin continuous expansionof boththe lengthand widthof eachfeather.(It should benotedthat notallfeathersfollow thistypeof growthpattern–contourfeatherson therumpand thighsof many passerinebirds actuallyreducein sizeprior tobreedingseasonas partofthe prenuptialmoltdescribedabove.)The resulting new cellspush older onesoutwardand eventuallyoffthe tipofthe featherwhere theybecomebarbicelsor hookletsattachedtobarbs(Jainudeenet al., 2001)