What are feathers? What is molt?

Eaglets go through two molts and three feather stages in the nest: natal down (and molt), followed by thermal down (and molt), followed by juvenile feathers. As of this blog, the Decorah North eaglets are shedding the very last of their natal down and their thermal down is rapidly being replaced by juvenile down and feathers. We thought we would blog a little more about feathers to celebrate! 

When we think about feathers, we tend to think about their qualities (light, soft, fluffy, hard, flat, and horny are all included in dictionary definitions) and how they help birds fly, stay warm, shed or retain water, build nests, and so much more! But what are feathers? It sounds like a silly question. We all know what feathers are, right? And we all know what a bird’s nest looks like, how to describe the color and shape of an egg, and when bald eagles lay their eggs. Maybe we don’t know as much as we think.

April 15, 2024: Natal down mohawks, thermal down bodies.
April 15, 2024: Natal down mohawks, thermal down bodies.

So again, what are feathers? Like hair, fingernails, and scales, feathers are growths produced by epidermal cells in the outer layer of a bird’s skin, which makes them part of the largest organ system in a bird’s body. Derived from the Latin integumentum, which means “covering”, a bird’s integumentary system includes its skin, feathers, scales, feet, beak, and the glands in its outer ear canal and at the base of its tail. Mom’s integument keeps her insides in and protects her from pathogens while allowing her to exchange wastes, react to stimuli (think of how she fluffs her feathers in the cold), and produce important organic compounds like uropygial (preen) oil.

Feather Follicle
A feather follicle

Where do feathers come from, and how do birds grow them? Birds grow feathers throughout their lives, but feather development begins in the egg. Seventeen to eighteen days after Mom lays an egg, interactions between the embryonic bald eagle’s outer and middle layers of skin form feather buds – columns of epidermal cells that dimple its developing body. The cells differentiate into three layers as they grow downward into its skin to create a tiny feather-producing organ that resembles a small pit or tube – the feather follicle. The cells at the base of this tube grow, divide, and die, leaving behind small masses of keratin that are pushed upward through the tube to form a feather. The outermost layer of cells form a temporary sheath that protects developing feathers, while the middle and inner layers form the feather’s rachis and barbs. The developing feather has a blood supply that extends through its central pulp to nourish it as it grows – hence the name ‘bloodfeather’ to describe a feather still connected to its blood supply. When the feather is mature, the blood supply recedes.

Bald Eagle Nestling Molts
From 2023: Natal down (white), thermal down (grey), emerging juvenile feathers.
From 2023: Natal down (white), thermal down (grey), emerging juvenile feathers.

Our eaglets go through two molts and three feather stages in the nest. What we call natal down – the adorable fluffy white plumage that covers hatchling eaglets – is known as neoptile feathers to people who research feather development. All feather follicles develop in the egg, but in altricial birds like bald eagles, most follicles don’t activate prior to hatching. Those that do produce natal down, which is symmetrical, uniform, and general in purpose. In altricial birds like bald eagles, follicles that become active prior to hatch can only grow down.

What we call thermal down – the greyish plumage that begins to replace neoptile feathers at about eight days of age – is known as mesoptile feathers to the same people who call natal down neoptile feathers. While we don’t see mesoptile feathers right away, mesoptile feather germs ‘wake up’ and begin the work of assembling feathers shortly after hatch. As the name suggests, mesoptile feathers are intermediate feathers – more complicated than the natal down they replace and simpler than the juvenile feathers that will replace them. Thermal down is somewhat waterproof, but can’t zip together like feathers, which is why the eaglets spent much of April 16 – a very rainy day – trying to stay uner Mombrella.

What we call juvenile feathers – the fully-formed feathers that begin to replace mesoptile feathers between roughly 20 and 28 days of age – are also known as juvenile feathers to people who research feathers, although any given species of bird may have more or fewer stages between natal plumage and juvenile feathers than the one stage that bald eagles have. Most juvenile feathers emerge from the same pores that produce mesoptile or thermal feathers.

More about molt and feathers

Feather follicles, which are nourished by blood and grow via the division and enlargement of cells, are alive and will continue to produce feathers throughout a bird’s life. But non-living things like feathers, crystals, icicles, and bald eagle nests grow via accretion: the addition of new material on top of old. Remember N1? Once Mom and Dad stopped replenishing it with branches, it began to disintegrate. The same is true of feathers. They have no blood supply, no cells, and no way to repair or replenish themselves. So how do birds replace feathers, especially flight feathers, without impacting flight? Once an eagle has juvenile feathers, its molt cycles are regulated by hormones. During molt, older feathers are pushed out of their follicles by newer feathers in a genetically programmed, orderly replacement that can take years to complete in larger birds of prey such as bald eagles.

Like so much else in a bird’s life, the cue for molt initiation is day length, which effects the hormone levels that control molt progression. In the temperate zone – the part of the earth’s surface lying between the tropics and arctic and sub-arctic circles – most breeding birds of prey molt in the summer, after they have finished raising young, and in winter or early spring, as the breeding season begins. Producing new feathers is a costly affair. Molting birds replace 20-40% of their mass through the molt, drawing on protein and energy reserves to create new feathers and offset the effects of reduced insulation and flight ability. Their ‘down time’ – the space between laying eggs, endlessly feeding hungry nestlings, and migrating or enduring the winter cold – is a good time to engage in the energetically expensive task of producing new feathers. Molt is suspended during periods of intense flight activity (say, when Mom and Dad are feeding young, or falcons are migrating) and food scarcity.

Daylight length and photoperiod change in Decorah, Iowa
Photoperiod change, Decorah Iowa

In short, photoperiod changes initiate hormone production. Among many other things, these hormones trigger the cells at the base of the feather follicle to start growing, dividing, and dying. New feathers are produced and old feathers are pushed out. The molt cycle does not take feather condition into consideration – feathers are replaced whether they need it or not. While this might seem wasteful given the amount of energy that molt takes, it is better to replace feathers automatically during ‘down time’ than to develop a replacement on demand system that could leave a molting bird vulnerable to the weather or impair its flight during nesting season.

Wing anatomy
Wing anatomy

In birds of prey like bald eagles and peregrine falcons, molt progresses from the front to the rear of the bird. Unsurprisingly, flight feathers molt symmetrically. In falcons, primary feather molt begins with the fourth innermost primary (P4) and works its way inward and outward, but in bald eagles and most other birds of prey, the innermost primary (P1) is molted first and molt proceeds outward. Tail molt (bald eagles have 12 tail feathers) usually begins with the third and fourth feathers on either side of the bird’s central tail feathers and proceeds simultaneously outward and inward. The growth of individual flight feathers takes 2-3 weeks in a smaller bird like a kestrel, and 2-3 months in a larger bird like a bald eagle.

Sharp-eyed watchers noticed adult eagles at all of our nests shedding adult down and body feathers during incubation. The production of sex hormones triggered body feather molt at a time when we might expect birds to be conserving insulation, not regrowing it! However, this is a very good time for them to replace feathers. They aren’t as active during incubation, which requires long periods of sitting on eggs – a great time to replace feathers. They also need to transfer body heat to their eggs, which is done through bare or mostly featherless skin. I wasn’t able to find much about an eagle’s brood patch and molt, but we know that hormones cause breast feathers to loosen and fall out, creating the brood patch. Like flight feather molt, brood patch molt is optimized to perform a specific task.

In his book Raptors, the curious nature of diurnal birds of prey, Keith Bildstein concludes his section on molt by stating “We still know little about feather molt in the overwhelming majority of diurnal birds of prey, including that of many abundant and widespread species.” Reading and writing about feather growth and molt left me with more questions than answers. Do sex hormones influence sub-adult plumage patterns and colors? How do hormones change plumage signaling at maturity? Do regional populations of birds experience different molt patterns? Specifically, how might the nearly tropical bald eagles at Fort Myers experience molt when compared to deeply temperate Mom and Dad, or to nearly arctic eagles in northern Canada? Do long-time territorial birds experience different molting patterns than birds that migrate every year – say, Mom and Dad versus Brett’s migratory Canadian eagles? If sex hormones help regulate molt and feather production, why don’t bald eagles and peregrine falcons have sexually dimorphic plumage? For that matter, why do American kestrels have sexually dimorphic plumage? The more I learn about birds, the more questions I have!

Things that helped me learn and write about this topic:

Did you know?

Why is molt so energy intensive? After all, feathers are made of dead material and don’t weigh very much, right? Feathers don’t weigh much collectively, but the entire plumage of a bald eagle makes up about a sixth of its total weight, or roughly three times that of its skeleton. Molt involves the replacement of a huge area of a bird’s body, and the plurality of its mass. Altogether, a bird’s plumage weighs more than any other part of its body.

Birds replace their feathers more or less annually, depending on the bird. Do humans really replace their cells every seven years? Nope – it is a lot more complicated than that! Watch this video to learn more (and check out the awesome skunkbear science tumblr here: http://skunkbear.tumblr.com/)

If we replace skin cells, why do scars and tattoos persist? https://www.scientificamerican.com/article/if-the-cells-of-our-skin/

I’ve never thought much about the integument before. Read this to learn more about our integument and marvel at the similarities and differences between feather follicles and hair follicles: https://oli.cmu.edu/jcourse/workbook/activity/page?context=90d2a06680020ca600bc63bf9b37a68f.

Photo credits

R.B Ewing was a science illustrator who drew the awesomely detailed feather follicle that I found all over the place, although I was not able to source the publication. It may have come from Ornithology in Laboratory and Field by Burgess Publishing, edition unknown. Here is a link to just one place I found it: http://ncsce.org/pages/home.html
The wing came from wikicommons.