Living with Birds Feathers don't come cheap Tweetapedia

Feathers are the defining feature of birds. They fulfil many functions. They give them the ‘superpower’ of flight, of course, and play an essential part in thermoregulation, camouflage and communication.

Made from a substance called keratin - the same protein in our hair and fingernails - they are dead as soon as they’ve finished being ‘built’ by the bird.

While our hair and nails continue to grow from the base at the follicle, a bird’s feathers don’t. They will not be replaced as long as long as they sit in the follicles. This means, over time, they start to become damaged and show signs of wear and tear, which has implications for all of the jobs that feathers are required to do.

After a long and busy breeding season, most birds’ feathers, and the birds themselves, look tired, faded, battered and threadbare. They have to do something about it and the solution is moulting: the process of replacing old feathers with new ones.

Moulting costs the birds lots of energy. To minimise this, and other impacts on their lives, they often lose their feathers in a regular and predictable manner, usually between breeding and the harsh weather conditions of the winter or the rigours of migration.

Feathers are lost in a bilaterally symmetrical way too, which makes total sense: you don’t want to lose some feathers on one wing and not the other. Otherwise, while you might not fly in circles exactly, you'll be unbalanced.

Feathers don’t come cheap

The first consideration when birds replace feathers is cost. To produce new feathers, birds must be well-nourished. This is why most species, especially the perching birds commonly seen in gardens, undergo their moult during the summer when food is plentiful and breeding or migration is over. This sweet spot in the birds’ calendar typically falls between the end of June and August. 

Flight, along with feathers, is the most defining attribute of birds. Wings enable a bird to move, migrate, and avoid predators, so special consideration must be given to how and when they moult their flight feathers.

The most important feathers for flight are the largest, representing a significant investment by the bird. Despite the vast diversity of birds around the world, they all have between 9 and 11 primary flight feathers, with most species having 10 on each wing. These ‘primaries’ are moulted in sequence after breeding, starting from the inner primaries out to the wing tips. Species that are single-brooded or finish breeding early, begin their moults first. Blue tits start earliest, with some kicking off as early as June, while Bullfinches moult later, from August even into September.

Speed moult

Those that migrate long distances need to ‘express check out’ of our country. As you might expect, they complete their moult the fastest - replacing all of their 20 primary feathers in around 2 months (that’s 3.5 feathers per wing at any one time). After all, it is these feathers that have to power a long-distance migrant, such as a Whitethroat, over 9,000 miles to their sub-Saharan wintering grounds!

In contrast, our slowest moulters are the local residents. Take House Sparrows for example. They are in no rush and so can afford to be a bit more leisurely, taking nearly twice as long to complete their primary feather moult.

Another interesting aspect of the moult is that its speed and timing can vary even within a species. For example, a blackbird in Shetland might start and finish breeding later in the year but still has to complete its moult before winter arrives. These Northern birds moult faster and in a shorter time frame than their contemporaries down South.

Feather in the nest

Now, what about the class of 2024? You may have noticed some of your garden birds seeming to change colour almost before your eyes! Given what you now know about feathers, this is only possible if they are replacing them. Let’s take your Garden Robin, for example.

They leave the nest having replaced their fluffy nestling plumage with ‘proper’ feathers and appear as big-eyed, brown and speckled apparitions. They have their parents’ familiar silhouette, poise, and posture, but lack the defining ‘red’ breast (plus all the other adult plumage palettes). However, as summer progresses, you might notice they start showing the adult breast colouration as, one by one, they swap out their first feathers for adult plumage. 

Being a newbie has its disadvantages., They’re not very good at looking after themselves yet or finding food, and their first feathers are often poor quality because they grew quickly in the nest. Their first moult is a partial one. They’ll swap out the vital and insulating body feathers and just a few of their wing feathers and tail feathers. It’s like budget clothing to see them through their first winter.

Out for a duck

Another variation on a theme is the moult of wildfowl, such as ducks. Take the Mallard, for example. Female ducks moult twice a year, often using their moulting feathers to line their nests while they gradually replace their flight feathers. Males, or Drakes, on the other hand, seem to vanish after breeding! However, this is an illusion; they are still present but look more like females. 

This is because they undergo a ‘catastrophic moult’ during which they lose all of their flight feathers simultaneously. This makes them highly vulnerable, so they avoid predators and bird watchers by also replacing their bright breeding colours with dull body feathers that make them look more like females for a short period of time. This plumage is known as ‘eclipse’ plumage, as it masks the birds during this risky time in their calendar.