Let's start with the structure. At it's most basic, imagine a feather as a tree - a trunk-like hollow shaft in the middle. The branches, or thinner shafts, come out of each side and are called barbs. These barbs have 'twigs' called barbules. As we'll see later, not all feathers are the same. Those close to the body, down feathers, are fluffy and loose. These are overlaid by contour feathers and, whilst there's a bit of fluffiness at the base of these, they're much smoother and the barbules lock together to make them weatherproof.

Feathers are different depending on both the bird and where they are on the body. Flight feathers, for example, are longer and stiffer than down feathers. But even flight feathers aren’t necessarily the same - if a robin flies past you, you’ll hear it; if a barn owl flies past you, you won’t.  For barn owls, it’s vital that their prey don’t hear them coming and so their feathers are loose, a little ragged and the front edges have spikes that move the air in a different way so it doesn’t ‘whoosh’ over them. 

Tail feathers differ too.  It's obvious if you stop to think about it; a woodpecker or treecreeper would be lost if they were trying to cling to a tree with a duck’s tail!  These boreal birds need really stiff tail feathers to help them balance against a tree as they search for food or make their home. 

Like our hair and fingernails, feathers are made from keratin. And, like our hair, feathers grow and are replaced (although it takes more energy than growing our hair). Some birds moult many feathers at the same time – this can ground some birds for a short while – whereas others moult them bit by bit.  It’s why many garden birds disappear from late July; they’re hiding away, trying to keep out of the way of predators whilst they’re at their most vulnerable.

Take a look out of your window at this time of year and you may well see birds as round as the baubles you'll have recently removed from the Christmas tree. All birds have downy feathers close to their bodies that they fluff up to keep warm. The body heats the air between the feathers, enabling them to keep out some of the winter cold.

The larger feathers, the ones that are visible at a distance, can be used to appeal to the opposite sex.  Take great crested grebes as an example.  Most of the time you may wonder what the ‘crest’ in their name is for as they often look rather sleek. When they want to attract a mate or ward off a rival, though, they fan out their crest and look most impressive; this is a behaviour that will become more common over the next couple of months.  Goldcrests, our smallest birds and a species that you may see in your garden if you're lucky, do the same with a thin strip of gold along the top of their heads. 

All of this and we've not looked at colour yet! We're still learning lots about feathers. Take blue tits, for example, to our eyes they look pretty much the same but the birds themselves take their cues from the colouring of the crest. When seen through the ultra-violet vision of blue tits, the feathers in their blue crests can be startling. Research shows that females prefer males with brighter crests. And what about the beautiful iridescent feathers that change colour as the light hits them? The black of magpies, and other corvids, changes through greens and purples as the light changes. In starlings, males are glossier and females spottier but their feathers are iridescent, reflecting rainbow colours as we look at them. But this beauty has a trade-off - iridescent feathers are weaker than their non-iridescent counterparts so the trade has to provide another vital element for the bird or it simply wouldn't be worth it.