Birds have up to five feather types:
1. Contour
feathers cover the body, wing (remiges) and tail (rectrices).
2. Down feathers - these soft and fluffy feathers trap air and
create a layer of insulation next to the bird's body.
3. Semiplumes function to fill in between contour and down feathers.
4. Bristles are small feathers with a stiff shaft and barbs only on
the base, or often not at all. Bristles occur most commonly around the base
of the bill, around the eyes, and as eyelashes.
5. Filoplumes are hairlike feathers that consist of a very fine shaft
with a few short barbs at the end. They are typically covered by other
feathers, and may function as pressure and vibration receptors - they sense
the location of other feathers so they can be adjusted properly.
An Owl
has very few down feathers, but has downy barbules on the parts
of the contour feathers closest to the skin.
Many of the Owl's feathers are specially designed - around the
face there are the stiff facial disc feathers or ruff, crown feathers,
ear-flap feathers and also bristles around the bill.
The feet and bill have Filoplumes that work somewhat like feelers, to help
the Owl react to things they touch, such as prey.
The most unique adaptation of Owl feathers is the comb-like
or fimbriate (fringe-like) leading edge of the primary wing feathers referred
to as "flutings" or "fimbriae". With a normal bird in
flight, air rushes over the surface of the wing, creating turbulence,
which makes a gushing noise. With an Owl's wing, the comb-like feather
edge breaks down the turbulence into little groups called micro-turbulences.
This effectively muffles the sound of the air rushing over the wing
surface and allows the Owl to fly silently. There is also an alternate theory that the flutings actually shift the sound energy created by the wingbeats to a higher frequency spectrum, where most creatures (including prey and humans) cannot hear.
Silent flight gives Owls the ability to capture prey by stealth, and also
allows the Owl to use its hearing to locate
potential prey.
This adaptation is not present on some Owl species that hunt in the
daytime.
Pattern and Colour
In general, an Owl's cryptic
colours and pattern allow it to blend in with its surroundings, hiding it
from potential danger. This is especially important for the nocturnal
owls, as they need to remain hidden when roosting in the daytime.
When threatened, an owl will often take up a
concealing posture, with closed eyes, raised ear tufts, and compacted
feathers.
The ear tufts are actually nothing to do with hearing, they are display
feathers, used to indicate moods, such as fear, anger and excitement. They
also help with camouflage.
It is also interesting to note that although owls of the same species look
alike, each individual owl within a species has slightly different markings.
Preening
All birds frequently clean and
groom their feathers in order to remove dust, dirt and parasites. Owls,
like most other birds, use their beak and talons to do this. The two outer talons on the owls feet are the "feather combs". The sharp
medial edge of these outer talons enables them to clean their heads.
Flight feather barbs have tiny barbules that lock the barbs together,
making the feather into a single continuous surface. These barbules often
become unhooked during harsh flying conditions or collisions. A bird will
use its beak to realign the unhooked barbs and restore the feather to peak
condition.
There is small gland called the uropygial, located at the base of
the tail, that produces a thin oily liquid. This gland is stimulated by
the beak, which is then used to transfer the liquid to the feathers to
provide them with a protective coating.
Molting
When an Owl hatches, it has no
flight feathers, but is covered with downy feathers that keep it warm.
This down is gradually replaced with feathers as the Owl grows. Juvenile
plumage is similar to adult's, but often paler, and sometimes with
different markings.
An adult bird's colour is
derived from mature feathers. During the normal course of the bird's life,
these feathers suffer from damage caused by abrasion, flexing and even
collisions.
Like other birds, Owls regularly replace their feathers in a process known
as molting. This usually happens once a year, beginning after the
parent birds have raised a brood that has fledged and can care for
themselves.
The process takes up to 3 months, during which feathers are shed and
re-grown over the
entire body in a regular pattern. In order to minimise the impact of the
molt on the Owl's flight and hunting skills, this molting pattern only
allows a few of the primary or secondary flight feathers to be shed at
time.
With the exception of the Barn Owl, molting of wing feathers is from the
inside out. Barn Owl wing feathers are replaced from the middle of the
wing out (in both directions). Tail feathers also drop out a few at a
time, except in some smaller Owl species, who loose all the tail feathers
at once.
When birds molt, new feathers
grow to replace the ones that have fallen out. The new feathers immerge
from the skin tightly bound in a thin shaft of tissue. These are called
pin feathers. The shaft splits shortly after, allowing the new feather
to unfurl and grow to its full size.
Flight
Most owls have relatively large,
rounded wings. The wings are broad, with a large surface area relative to the weight of
the bird i.e. a low wing loading. This allows them to fly buoyantly and effortlessly,
without too much flapping and loss of energy. They can glide easily and fly slowly for
long periods of time. Many species use this slow flight to hunt
ground-dwelling prey from the air.
References:
Anderson, George W.. 1973. "An Experimental Investigation of a High Lift Device on the Owl Wing". Air Force Institute of Technology Air University
Campbell, Wayne. 1994. "Know Your Owls (CD-ROM)". Axia Wildlife
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