If clothes make
the man as the old saying goes, then feathers certainly make the bird!
It is easy to take
for granted the amazing structure and function of feathers and their incredible
diversity, which has enabled birds to inhabit every part of the globe from the
tropics to the Polar Regions and from the skies to the oceans.
This post briefly
considers the basic stages of feather development from chicks to adult birds.
The various
plumage types or plumage stages over a bird’s life can make identification of
some bird species a challenge at times, especially juvenile, immature and non
breeding plumage. However this adds greatly to what makes birds and birding
interesting and it reflects the great diversity of our feathered friends.
Most birds start
out life after hatching from the egg as featherless chicks. They quickly grow a
covering of downy feathers which no doubt helps to keep the young birds warm
while the parent/s are away gathering food. As the chicks grow the next set of
feathers start to emerge through the down. The process of moulting natal down
and growing the first covering of true feathers is known as fledging and birds
at this stage of development are often referred to as fledglings.
Once a bird has
its first set of true feathers it is ready to fly and is generally referred to
as a juvenile. During this time the juvenile must learn to fly and fend for itself
so it can gain independence from its parent/s.
Parent/s is used
because in some cases the female raises the young birds with no help from the
male parent – for example male Satin Bowerbirds play no role in raising their
young – they are too busy displaying at their bowers. For some species the male
is the sole parent, for example emu and cassowary. For most species however
both male and female play a role in raising their young.
Young birds
dependent on their parents are very vulnerable to predators while they spend
many hours perched waiting for parent/s to return with food and as they take
their first tentative flights. It is therefore not surprising that the first sets
of true feathers are often very mottled and nothing like adult plumage – just
what is required to enable them to blend in to the background and make them
less obvious to predators. The following
photos show a number of juvenile bird species and their cryptic plumage
together with photos of the adult birds for comparison.
A juvenile Jacky Winter perched waiting for its parents to bring food. |
The cryptic feather pattern and colours helps this juvenile Jacky Winter blend in with the background. |
An adult Jacky Winter |
Juvenile Dusky Woodswallow. |
Adult Eastern Yellow Robin. |
Following the
juvenile stage, birds moult their juvenile plumage and grow sub adult plumage,
during which stage the young birds are sometimes referred to as immature. The
birds become full adults when they grow their breeding plumage. This is
obviously more apparent for male birds in dimorphic species (species where the adult
males and females have different plumage).
Satin Bowerbirds are
an example of this where the male looks like a female until three or four years
of age when it develops a green throat, breast band and other subtle plumage
changes. However it is not until the sixth year that the blue/black feathers
start to emerge with full adult plumage achieved in the seventh year.
In a species
where the male and female adult birds both have similar plumage, for example
White-bellied Sea-Eagles, first year birds are juveniles and then over a number
of years, when they are referred to first as immatures and then as sub-adults,
they gradually lose the juvenile plumage, gaining full adult plumage at about
five years of age when they are ready to breed.
These photos of
Satin Bowerbirds and White-bellied Sea-Eagles illustrate the immature to mature
plumage stage.
This Satin Bowerbird could be a female or an immature male? |
This is an immature male Satin Bowerbird - note the green throat and fine white spots and streaks, the green band on upper breast and tip of bill starting to turn white. |
While some birds
retain their adult breeding plumage throughout the rest of their lives and
through numerous moults to grow new fresh feathers, others grow and then lose
their breeding plumage each year. For example male Fairy-wrens put on very
colourful plumage for the breeding season but shed it and grow more somber
plumage for the rest of the year – no doubt this is an effective survival
strategy as bright plumage makes the adult males very visible targets for
predators.
An adult female Superb Fairy-wren - note the reddish brown bill, lores and brow and brown tail feathers. |
An adult male Superb Fairy-wren in non breeding plumage - note the black bill and blue tail feathers. |
Adult male Superb Fairy-wren in breeding plumage. |
Many of the wader
species also grow and shed/moult breeding plumage each year,
There are many
more fascinating aspects to bird feathers and their function and maintenance
during the course of bird’s lives – this post has just touched on the subject.