Fabulous Feathers

I've talked on many occasions before about feather colours - look on the right of this page under Labels, then Colours in Nature for some links - but I haven't actually looked at the feathers themselves, and they are truly wonderful. They are a key, perhaps the key, to the extraordinary success of birds, and they help determine how and where it lives. A measure of their importance is that, despite their proverbial lightness, they weigh two to three times what the bird's skeleton does.

Cinereous Harrier Circus cinereus, Los Glaciares NP, Argentina.

Feathers, or at least filamentous scales, evolved well before the development of flight and in at least two other dinosaur groups - the certatopsid group, the poster species of which was triceratops, and the flying pterosaurs - as well as the running carnivorous theropods, a branch of which became birds. The purpose of these was doubtless insulation, in animals which were controlling their internal temperatures, or becoming 'warm-blooded' as we might carelessly put it. (Consider that a 'cold-blooded' lizard on a rock at 40 degrees has warmer blood than the mammal passing by.)

More complex branched feathers arose, as far as we know, only among the theropods, but were possessed by many non-flying species of them. It seems that display was an important function before various small dinosaurs adapted long arm and leg feathers to assist in gliding, which ultimately evolved into true flight.

A feather is a marvellously complex structure. A 'basic' feather - and we'll get to the different types of feather in a while - comprises a shaft with barbs projecting along it on each side. The base of the shaft is hollow, where it is embedded in the skin; this is the calamus. The rest of the shaft – the rachis, or quill – is solid. Along each side of the barb, in turn, are rows of barbules. The barbules on one side of the barb are hooked, those along the other side are looped, so that each barbule ‘zips’ onto the next one, like velcrose.

Basic parts of feather (of Australian Bustard - found dead by the road!).

Enlargement of part of the same feather - still far too small to see the barbules, though we can
see on the right where some barbs have become 'unzipped'.
Take a feather, pull the barbs apart (feel the resistance to your pull),
then rezip them by running your fingers along the barbs.
I love how the intricate patterns (formed by melanin pigments) run across all the barbs!

As well as keeping warm air in, a bird (other than some diving birds like cormorants and darters) needs to keep water out – waterlogged feathers are heavy and cause chilling by expelling the insulating air layer. Preening oil can be part of this waterproofing, but so is the structure of the surface feathers. Barbs on the outer surface are more widely spaced, which has the effect of causing the water to form drops, which are readily shed.

Wing and tail feathers are very enlarged and strengthened, shaped to give lift and steering. 

Flight feather (of Australian Pelican) - very stiff and strong.

The wing feathers, the flight feathers, are called the remiges, singular remix, and are divided into the primaries and secondaries. The primaries are longer, and are attached to the outer bones of the wing, the modified hand and wrist bones. The secondaries are shorter, and grow from the larger inner bone, the ulna. The tail feathers, or retrices, play a role in steering, balancing and braking. Here's the Cinereous Harrier again to demonstrate.

Wing and tail coverts overlie the flight and tail feathers above and below (at the front of the wing and the base of the tail) to protect them from damage and to provide a smooth surface to promote aerodynamic efficiency.

Contour feathers, the other visible body feathers, cover the surface of the body and also aid flight by streamlining. Most contour feathers also have a downy base, for insulation. 

The extensive downy base is evident; down feather barbs have no barbules, so are loose and woolly.

Many adult birds also have a layer of down feathers (with no barbules at all) under the contour feathers; these grow from different follicles from those from which contour feathers sprout. Many chicks however are covered in a layer of 'natal down'. These insulating feathers grow from the same follicles which will later produce their adult feathers.

Natal down on Swallow-tailed Gull Creagrus furcatus chick, Galápagos (above),
and on Darwin's Rhea Rhea pennata chicks, Torres del Paine NP, Chile (below).

 

These chicks are still substantially covered in natal down, but are also developing flight feathers.
Red-tailed Tropicbird Phaethon rubricauda, Lady Elliott Island, Queensland, above;
Australian Darters Anhinga novaehollandiae, Canberra, below.

 

As suggested earlier it is not of course the feathers per se which insulate, but the layer of air they trap - the same principle as double glazing. We've all seen a small bird in winter looking twice as big as normal, with its 'doona fluffed up'. The feathers are controlled by tiny muscles which can raise them, or indeed squeeze them flat in hot conditions. No sleeping bag manufacturer has ever produced a synthetic substitute for feather down which is anything like as efficient.

Contour feathers often have a woolly ‘aftershaft’, at the base, though it is mostly very small and not easy to see among the downy base. It is believed to be primitive remnant trait, based on who does and doesn't have them - passerines, the most modern Order of birds, don't. However in emus and cassowaries, among the most ancient of living birds, it is the same length as the rest of the feather.

Emu feather with aftershaft.
The barbs have no barbules, hence the characteristic 'haystack' appearance of emus, and the
hairy look of cassowaries.

Then there is a range of specialised feathers which are not present in all birds.

Single shafts of bristles around the eyes or base of bill are believed to be sensory, but probably also play an important role in protecting the eyes from struggling prey; they are primarily observed in birds which catch insects. 

Tawny Frogmouth Podargus strigoides (on nest), Canberra

Paperbark Flycatcher Myiagra nana, Nitmiluk NP, Northern Territory.

Filoplumes are also single shafts with abundant nerve endings at the base. They are scattered among other feathers, especially flight feathers, and apparently convey information about the movement of other feathers, so they can be adjusted if necessary. We know them as the ‘hairs’ on a plucked chook (ie 'chicken' to my non-Australian readers). 

Powder down is a particularly remarkable feather type which occurs among ordinary down; the tips of the feathers disintegrate into a powder of keratin, used for preening and conditioning - like talc powder, though chemically quite different. This remarkable feather type has evolved independently among many bird groups; the down feathers may be scattered throughout the body, or concentrated in patches. Powder down feathers are not moulted with other feathers, and grow continually.

Mealy Parrots Amazona farinosa at Blanquillo Clay Lick, Peruvian Amazonia.
The floury (ie 'mealy') appearance is due to an abundance of powder down.

White-necked Heron Ardea pacifica, Grenfell, New South Wales.
All herons have powder down which they collect with the bill and pass to the foot to apply to the feathers.

Next time you pick up a feather, have a good look at it and admire it with the wonder due to a truly remarkable product of nearly two hundred million years of evolution.

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