HawkTalk Issue 89 Winter 2021

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HAWKTALK WINTER 2021 ISSUE 89

HAWKTALK WINTER 2021 ISSUE 89

Fascinating feathers

FEATHER TYPES

If asked what makes a bird a bird, many would say ‘they fly’. While this is true for the majority, there are 59 species of bird (including ostriches, kiwis, 18 species of penguin, 20 species of rail and crake, five wildfowl, two grebes and a cormorant) which are flightless. Aside from flight then, perhaps the only really obvious features that all (modern) birds share are their feathers. Feathers are fascinating structures and studying them reveals a lot about a bird. Here we’re going to take a deep dive into this intriguing world and explore some key questions about them.

Down feathers on a Tawny Owl chick

HOW MANY FEATHERS DO BIRDS HAVE?

The total number of feathers a bird has depends on how big it is, where in the world it lives, and whether it is winter or summer (many birds have more feathers during the winter). A medium-sized songbird such as a Blackbird has between 2,200-2,600 (contour) feathers, whereas some of the smaller hummingbirds have as few as 940. The species with the most feathers is thought to be the Tundra Swan which has 25,216. In many songbirds the weight of all feathers combined accounts for about 7 percent of total body weight, however, for some species it can be much greater; for example, one Bald Eagle weighing 4,082g had feathers that weighed 677g (that’s 17 percent!).

FLIGHT

CONTOUR

DOWN SEMIPLUMES FILOPLUMES BRISTLES

HOW ARE FEATHERS FORMED? Feathers are made from keratin, a structural protein also found in claws, fingernails, hair, hooves, horns and skin of other animals. On a typical contour feather, each feather vane is kept together as a single aerodynamic surface by having hundreds of interlocking parallel barbs. Each of these barbs is divided into tiny branches called barbules, each of which is further divided into dozens of tiny projections which, collectively, are called barbicels. The two edges of each barbule have different projections, those facing towards where the feather is attached to the wing have a small flange whilst those facing the feather tip have four or five small hooklets (hamuli). These two areas lock together, a bit like Velcro, to keep the surface of the feather neatly held as a solid surface. During preening birds are often attempting to re-join any of these barbules that have become separated. Although quite strong, the fine structure of feathers means that they are subject to abrasion and wear as a result of rubbing against other materials and individuals, and damage through exposure to ultraviolet radiation. Each of these can break barbicels, barbules, barbs and even the rachis (main shaft) which will affect how well the feather vane stays together. Because full-grown feathers are dead structures which are not continually renewed, they must be replaced totally to maintain good flight efficiency and insulation. This replacement happens during a process called moult which, in most birds, happens usually at the same time each year. During this process, newly growing feathers push out and replace old feathers to leave the bird with a set of fresh plumage. The timing, extent and duration of moult (the strategy) varies between different species and families of bird according to the environment they inhabit, their life history (how they live their lives) and how big they are. This is quite a big subject, and we will explore it in more detail, along with other characteristics of bird feathers, next time.

STRUCTURE OF A FEATHER (Kestrel wing feather)

Bristle feathers around eyes and beak of a Little Owl

ANTERIOR VANE

POSTERIOR VANE

WHAT TYPES OF FEATHER ARE THERE? With such a wide of variety of roles, there are six different types of feather to meet these requirements. The most familiar of these are the contour feathers, all of which have a vane (a flat area of interlocked barbs and barbules). Altogether the contour feathers form the outer layer across the body of a bird, and include the flight feathers of the wings, and the tail feathers. Down feathers lay beneath the contour feathers and help provide insulation. They do this by trapping air between the contour feathers and the skin. Down is also present as the first feather type of many birds after hatching. After these, the remaining feather types are likely to be less familiar to most people. First there are semiplumes which are similar to down and help provide insulation and buoyancy for waterbirds. Next come the filoplumes which are sparsely-distributed, longer, hair-like feathers which stick out through the contour feathers and are thought to help the bird assess flight speed. Lastly, there are bristles and powder down (or pluviplumes). Bristles have very few barbs and can serve as guard hairs (protecting the eyes and nostrils), as sensory organs (like a cat’s whiskers) or as a net to help direct prey items towards the beak, for example in swifts and nightjars. Powder down are feathers whose tips disintegrate into a fine powder to provide a waterproofing coating, such as in parrots and herons.

PERCENTAGE OF OVERALL WEIGHT MADE UP BY FEATHERS

RACHIS

PENNACEOUS BARB

WHAT ARE FEATHERS FOR? Birds are obviously heavily reliant on their feathers (which, altogether are called the plumage) which serve many functions. The lightweight and protective layer that feathers provide also helps in many other ways such as for visual display and signalling, protecting against the sun, providing waterproofing and insulation, keeping an individual camouflaged and, of course, to provide an aerodynamic shape to help (most) birds fly. Aside from these functions, feathers also help with more unusual roles in some species, such as to aid buoyancy in waterfowl, to produce sound for birds such as snipe, to deaden sound from owls or to transport water by sandgrouse.

PLUMULACEOUS BARB

CALAMUS