‘Birds of a Feather’ – Everything you ever needed to know about feathers

When will the chicks start to grow their feather?


Our two little chicks began their journey covered in a fine coat of downy feathers. The cute, fluffy stage however doesn’t last long. At around 10-12 days they enter what is referred to as the ‘reptilian stage,’ where the start to grow in their second down which can be much darker and scaly, reminiscent of tiny velociraptors!

At around two weeks the first pin feathers will be visible on their neck and heads, followed by their bodies. At around three weeks (the current age of our chicks), their primary and secondary feathers will start to grow on their wings. Once a month old the chicks will usually have acquired their full set of feathers, with perhaps a few primary and secondary feathers still to fully grow.


What are the different types of feather?


Birds have seven different types of feathers which all have different forms and functions.


Wing feathers are for flight and have a windproof surface which is created by an interlocking micro structure. Wing feathers are made up of different types of feathers such as primaries, secondaries and contour. Ospreys have ten primary feathers and nineteen secondary feathers per wing. The longest primary feather can be up to 38cm in length, which is a lot! That’s longer than a standard-length ruler! The outer primaries are referred to as “fingers” as they are quite spread out, showing the individual feathers quite distinctly like a human hand with fingers. Secondaries can be up to 21cm in length and are strongly dark barred.



Tail feathers are used for precision steering. They can also be ornamental and have no function in flight such as the tail feathers of a peacock. However, this is not the case in osprey, their tail feathers are incredibly useful in flight. Osprey have twelve tail feathers that can be up to 23cm in length. The tail feathers are dark barred like the secondaries, but this can be hard to see unless the tail is fanned.


Contour feathers cover a bird’s body in an overlapping pattern and help to streamline its shape. The feathers are made up of waterproof tips which are exposed to the elements, and closer to the body are the fluffy bases which help birds keep warm. Contour feathers on the wing are called coverts, which help to smooth over where the wing feather attaches to the body. This ensures that there is an efficient airfoil created (lift and drag as in an aeroplane wing which helps the plane to fly).


Semiplume feathers are mostly hidden beneath other feathers, providing form and contribute to making the birds more aerodynamic in flight. They also provide padding, creating a fluffy insulating structure.


Down Feathers are quite short and fluffy. They are the feathers closest to the body and are responsible for trapping body heat ensuring birds are well insulated and warm on a cold night. Geese and duck down feathers are used in jackets and bedding to keep us warm.


Filoplume are very long and thin, almost hair-like feathers mixed in between contour feathers. Although the function of these feathers is not well understood, it is said they function like the whiskers of mammals. They are used to sense the position of the contour feathers and possibly adjusting them in response to air pressure.


Bristles are even simpler than filoplumes, and are generally found on the head. It is thought that bristles are thought to have both a sensory and protective function, protecting the bird’s eyes and face.




Light as a feather…

Although feathers are light, hence the expression, the plumage of an osprey weighs more then its skeleton. This is because ospreys bones, like many other birds, are in fact hollow, containing air sacs! An Ospreys wingspan can be in the range of 152-167cm, that’s equivalent to a 5’5 tall human! The smallest of the wingspans would most likely be a male and the largest a female. Female ospreys can be up to 20% larger than males.

Below is a picture of NCO’s feathers which clearly shows the primary and secondary wing feathers (regimes). These are the main feathers that enable the birds to fly, and are attached to the bone, unlike other feathers. On the wing, the primaries are the longest of the feathers, providing forward thrust, and can be controlled and rotated like rigid fingers.

Secondaries provide most of the lift by overlapping to form an efficient airfoil, and cannot be controlled as precisely as the primaries. Tail feathers (rectrices) can also be classified as flight feathers. However only the two most central feathers are attached to the bone. Tail feathers are important for precision steering. Below is a picture of NC0’s huge wingspan showing the wing feather placement.



How do feathers work?

When you look at the structure of a flight feather, the calamus (quill) extends into the central rachis (shaft) which is easy to see. We need to look quite closely to see that the central rachis branches into barbs, and then into barbules. On each of the barbules we would need a magnifying glass to see the small hooks which cover the barbules. When you see birds preening, they are keeping their feathers tidy by ‘zipping up’ the barbules which hook together, with its bill. Once ‘zipped up’, the feathers are more streamlined which aids with flight, insulation, waterproofing and buoyancy.



What are feathers used for?

Apart from the obvious to aid with flight, feathers have quite a few different functions. Feathers can be used for streamlining the body. All birds bodies are in essence shaped like a teardrop, most noticeable in flight. Specially arranged contour feathers are responsible for reducing friction during flight, which would otherwise act as drag.


LM12 in flight – Scottish Wildlife Trust Webcam


Feathers can help provide insulation, for example our osprey chicks hatched with a fuzzy coat of natal down, which will be replaced by adult down in a few weeks. While the chicks are still young and exposed to the elements, they will huddle together in the nest and  NC0 will continue to do most of the brooding to keep them warm as they would get very cold if left unattended for long. Whilst NC0 was incubating the eggs, some of the feathers on her belly would have fallen out creating whats called a brood patch. This patch is heavily infused with blood vessels and this allowed NC0 to transfer heat to the eggs, which in turn speeds up development of the embryo.


Snuggling for warmth – Scottish Wildlife Trust Webcam


Feathers are also used for waterproofing, this is important in Scotland, especially this year when we did not have the best start to the season weatherwise! Continual wet weather is bad for vulnerable chicks as they have not developed their waterproof feathers yet. Furthermore, both our adult ospreys can succumb to drenchings over a period of time. This can be combated however by preening, which maintains the waterproofing of the feathers. Below is a picture of NC0’s lovely waterproof feathers which is shown by the waterdrops sitting on them.



Ospreys have a large preen gland on their back at the base of their tail feathers which secretes an oily substance. Ospreys pick up the oil by rubbing their bills against the preen gland which they use to coat their feathers with when preening shown below by NC0. This coating insulates the interlocking barbules in their feathers, as water cannot penetrate through the oil coating, therefore making the feathers waterproof. Ospreys also lack a small extra feather which is attached to larger feathers called an aftershaft, this helps them to lose water droplets after fishing.



Feathers can be used for display in order to strengthen the pair-bond between an already established pairing, or in order to attract a mate in a new-pairing. Ospreys are monogamous, meaning they will mate for life. If one of the pair fails to return from migration, only then would they find a new mate. A male osprey will display to a female using a display called “sky dancing”. It is also used to warn off other osprey that this is his territory and to stay away.


Osprey © Ron Walsh


Feathers can also be used for camouflage. This is especially important for nesting females and also juveniles to stay hidden and avoid possible predation. For example, female ducks are often quite brown and speckled in order to blend in with the nest, whereas males are more colourful and flashy. Both male and female ospreys take turn to incubate the eggs, with females doing around 75% of the incubating. Perhaps this could explain why there is little difference in their plumages than in other species where only the female incubates, such as ducks.

Young chicks also blend into the nest extremely well, with the white streak down their back mimicking the pale colour of the sticks on the nest, often ‘decorated’ with pale, splashes of excrement, also known as ‘mutes’.


Spot the chicks – Scottish Wildlife Trust webcam


Our female NC0 has less uniform brown feathers than our male LM12, with quite noticeable pale fringes to her upper body feathers, visible in particular when she is sitting on the nest. Whilst these are not as marked, they are similar to the scalloped edged plumage of juveniles. Perhaps this could be down to the fact that NC0 still is quite a young bird at around 5 years old, whereas LM12 is older being at least 13 years of age.

The juveniles have an overall brown look to the upperparts like adults, but more white scaling to the feathers. This mottled plumage ensures they are inconspicuous from any predator lurking above. Their underparts have a light brownish yellow or buff tinge to them, not as striking white as adults are.


2017 osprey chicks ringed PH1 & PH2 at Loch of the Lowes © Keith Brockie
2017 osprey chicks ringed PH1 & PH2 at Loch of the Lowes © Keith Brockie


Of course the brilliant white underside of ospreys is also a form of camouflage, but from below – making them harder to spot when hunting fish. Even their famous black eye strip plays a role, thought to help diffuse the reflection of the water, a bit like American footallers who use black make-up under their eyes to reduce the glare of the stadium lights.


When do ospreys moult?


Would you believe me if I told you feathers were made of the same material as our fingernails? This also means that bird feathers are dead, like the hair on your head. Feathers are made of a lightweight material called keratin, and the base of a bird’s feathers is attached to muscles, allowing birds to move them. Just like the hair on our head, bird’s feathers fall out. This process is called moulting, and it happens every year. A birds feathers have to go through a lot of wear and tear, so need to be replaced with a new set of feathers each year. Below is a picture of LM12’s worn wing and tail feathers!



Birds still need to fly or stay warm, so they moult their feather in stages. Ospreys moult their wing feathers in pairs, which helps them to remain aerodynamic when in flight. It can take around a month for an osprey to regrow a large primary flight feather.

Moulting is an energetically expensive business; therefore ospreys will moult their feathers throughout the year, except at energetically-stressful times such as when on migration and during the breeding season. Most ospreys will molt their primary feathers when on their wintering grounds in Africa, and will resume molting once they are on their breeding grounds in summer and have finished breeding, before the long migration back to Africa. Below is a picture on NC0 and what I suspect is a moulted secondary flight feather at the edge of the nest.



The chicks are currently growing at an incredible rate, with changes visible from one day to the next. Both chicks currently have a thick coat of dark secondary down with a healthy head and neck of orangey-brown pin feathers. The first signs of feathers sprouting on the wings can also be observed. If you look carefully as the chicks stretch their first primary blood feathers can be seen pushing through.

First signs of primary feathers on the wings – Scottish Wildlife Trust webcam


It won’t be long until the chicks are almost full size and begin the task of mastering the art of flight, beginning with vigorous flapping to build up flying muscles and even helicoptering’ above the nest.

Be sure to follow our live webcam to follow their incredible journey.



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When will the chicks start to grow their feather?   Our two little chicks began their journey covered in a fine coat of downy feathers. The cute, fluffy stage however …

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