Friday, 24 January 2014

Feathers make the bird - a look at juvenile, immature and breeding feathers

If clothes make the man as the old saying goes, then feathers certainly make the bird!
Feathers have developed for insulation and flight however evolution of extravagant male breeding plumage is probably the main reason why we find birds so attractive? This is a male Variegated Fairy-wren in very obvious and spectacular breeding plumage.
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 Dusky Woodswallow at nest.
Juvenile Eastern Yellow Robin.
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.
This male has not quite reached full maturity - it does not have a full compliment of blue/black feathers however its bill seems to have turned fully white. This bird must be six going on seven years old.
This is a juvenile White-bellied Sea-Eagle.
This is a young adult White-bellied Sea-Eagle. It is about four years old and has passed through the first and second immature stages however it still has some light patches on the flight feathers so is not a full adult.
A full adult White-bellied Sea-Eagle.

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,
This is an adult Double-banded Plover in breeding plumage. They breed in New Zealand and fly across the Tasman Sea to spend winter in Australia. This bird was photographed in late July at Byron Bay - it has developed breeding plumage ahead of its return to NZ.
A Double-banded Plover in non breeding plumage.
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.

1 comment:

  1. It really is a fascinating topic, and interesting too. Thanks for another informative post.