Thursday, February 22, 2018

How long does it take Mom to lay an egg?

We've talked about how long it takes bald eagle eggs to hatch after they are laid (an average of 38 days from first egg to first hatch in Decorah), how long hatch takes once pip starts (it can take upwards of 24 hours), and how long it takes Mom to lay each egg (she usually lays the second egg about two to three days after the first, and the third egg roughly four days after the second egg). But how long does it take Mom to make and lay an egg?

We'll start with copulation. It is believed that female bald eagles begin laying eggs five to ten days after productive mating begins. Mom laid her first egg at 7:28PM CT on February 21st, or about eleven days after copulation went from casual to frequent...and very determined on Mom's part.  Between February 10th and February 18th, Mom and Dad copulated nine times that we saw and had an additional 2 failed attempts. Mom took the lead five times - beak biting Dad, footing him, loudly vocalizing her intentions, and mounting him while wagging her tail ( You didn't need to be a male bald eagle to know that Mom meant business!

So we know that bald eagles ensure fertile eggs by copulating regularly, storing sperm in storage tubules, and concentrating sperm at the infundibulum, or site of fertility. But how long does it take Mom to lay an egg once her first yolk has been fertilized? The short answer: approximately two days (48-50 hours). It goes through a lot of changes along the way! Note that our oviduct times were derived using the ratio between bald eagles (~48 hours) and domestic chickens (~25 hours). The actual times could be slightly different.

Step one: I'm ready!
Diagram of the egg-laying process
According to Tim Birkhead, there are no known cases of copulation-induced ovulation in birds. As we've seen in Decorah, female birds signal their fertile status by their interest in (or their insistence on) copulation. In response to an internal clock and the presence of a male - say, Dad - one of Mom's yolks swells until it ruptures the follicle that produced it, releasing a ripe yolk into her oviduct.

How long does it take for her yolk to swell? Our observations suggest about eight to ten days. For comparison, it takes four to five days in small birds like great tits and white-crowned sparrows, six to eight days in larger birds like ducks and pigeons, 10-13 days for large gulls, and up to 16 days in some penguins.

How many follicles are swelling inside Mom each season? In domestic hens, each follicular cohort (follicles that will become eggs) numbers six to twelve follicles, and follicles are selected roughly every 24 hours. We know that Mom's annual cohort measures at least three. Since it takes roughly 48 hours for her to lay an egg, follicle number two must rupture about the time egg number one is laid. Follicle number three experiences a delay of about 48 hours, giving Mom's 'egg-machine' a chance to rest and resupply!

Step two: The infundibulum! Length of stay: ~15 minutes (T - 47+ hours)
While the name infundibulum (funnel) suggests a passive process, the microscopic yolk stays in one place while the infundibulum flows around it. Since Mom and Dad have been copulating regularly and Mom has been storing sperm, hundreds or thousands of Dad's sperm are waiting inside to fertilize it! Fertilized or not, the infundibulum seals the yolk after 15 minutes, and it proceeds on its journey down her oviduct.

How does the yolk stay in the center of the egg? The infundibulum's seal, or chalaza layer, forms the egg's first layer of albumen, aka egg white. The ends of this dense chalaziferous zone twist with other proteins to create two filaments as the egg spirals through the magnum. These filaments will eventually anchor the yolk to the egg's hard calcareous shell, keeping it in place. 

Step three: The magnum! Length of stay: ~5 hours (T - 42.5 hours)
Her yolk makes its next stop in the magnum, where it receives another coating of albumen. The albumen is secreted by special cells in the magnum wall that absorb water and proteins from Mom's bloodstream. The albumen will cushion the developing embryo and provide much of the protein needed for its development. Remember Dad's food gifts? Mom needs all the protein and calcium she can get to produce viable eggs!

Step four:  The isthmus! Length of stay: ~3 hours (T - 39.5 hours)
The yolk moves into the isthmus next. It receives a little more albumen and its inner and outer soft shell membranes, which (like Mom's feathers, beak, and talons) are made of keratin - another protein! The inner shell membrane provides a point of contact for the chorioallantoic membrane that develops in the first three to four days of an embryonic eagle's life. Both membranes help protect the porous egg from bacterial contamination and keep water from escaping too quickly. The membranes sit closely together during the egg's trip through the oviduct, but separate after the egg has been laid.

An egg in cross section, modified from Romanoff and Romanoff, 1949
Step five: The shell gland! Length of stay: ~39 hours (T - 15 minutes)
Mom's yolk, wrapped snugly in its jacket of albumen and shell membranes, moves into her shell gland or uterus. Water and minerals are pumped into the developing egg and a hard calcareous shell is formed around it. 

Mom is removing a lot of calcium from her body to produce the egg shell. While I couldn't find figures for bald eagles, hen chickens remove about 25 mg of calcium from their blood every 12 minutes during active egg shell formation. Since bald eagle eggs are significantly larger than chicken eggs, it seems very likely that she is removing more calcium than that. Anything that can't be derived from dietary sources will be obtained from her skeleton - another reason that food is extremely important right now! 

Step six: The vagina and egg labor! Length of stay: roughly 15 minutes!
We finally have an egg! Mom's initially microscopic yolk has been wrapped in membranes, plumped up, and surrounded by a hard shell. While we don't know whether Mom was aware of the egg before, she is certainly aware of it now! Contraction of a powerful sphincter muscle causes the egg to rotate in her muscular vagina and enter her cloaca pointed-end first. Mom's powerful vaginal muscles and full-body contractions eject the egg through her cloaca and into the waiting egg cup. Two days after Mom's first egg was started, it emerges after her brief egg labor and she lays down for a well-deserved rest! Her second egg is just beginning its journey!

Does Mom feel pain during egg labor? 
We often get asked whether Mom experiences pain or discomfort, especially in the final phase of laying. Whether or not she knows her follicle burst, egg laying is an energetically expensive process and her behavior changed substantially since she started chasing Dad around the nest! Two or three days before she laid egg number one, we started seeing her on and around the nest more. Dad brought several meals to her, and she spent a long period of time in the nest the morning before the egg was laid. While her soft chirps were lovely to hear, she was obviously experiencing some discomfort. It was a relief to all of us when she finally laid the egg!

Mom will incubate her first egg between 35 to 37 days before it hatches. While Dad does his share of incubation, incubation gives Mom the time she needs to rest and build up her reserves. Sweet eagle dreams, Mom - you've earned them!

What happens when a bird loses its nest and mate with an egg in the pipeline? 
We also get asked what happens when a bird's season is interrupted. We haven't seen it at any of our bald eagle nests, but we have seen it in several peregrine falcons. Last year, an interloper laid an egg in the nest box at Dairyland Alma before the returning female ousted her, and Newman spent a lot of time courting and copulating with St. Louis Girl before Michelle showed up and kicked her out. While there isn't much information about it, I would guess that any eggs in the pipeline get laid somewhere. But if follicle stimulation is about more than just lengthening daylight hours - which it appears to be, at least in wild birds, which don't tend to lay eggs absent a male, copulation, and a nest - losing a mate and territory may shut follicle stimulation and yolk production down. This would prevent additional eggs from forming: a wise strategy given how energy intensive egg production is! 

Does it feel like the parts of a female bird's reproductive system were named by committee? Technical language is daunting enough when it fits together nicely! But people have studied birds, especially domestic chickens, for a long time. Not everything we are describing now was named at the same time by the same people, which can lead to an odd combination of words. 
  • Infundibulum is derived from a16th century Latin word for "pour in" (or funnel): infundere.
  • Chalaza is derived from a Greek word that means "small knot": Khalaza 
  • Magnus is a Latin word that means "great". The magnus is the largest part of the oviduct. 
  • Isthmus is derived from a Greek word that means narrow neck of land: isthmos.

Things that helped me learn and write about this topic!
Image Credits
  • Diagram from the paper Sperm storage in the female reproductive tract in birds, Sasanami T, Matsuzaki M, Mizushima S, Hiyama G - J. Reprod. Dev. (2013). Taken from Open i, US Department of Health and Human Services,
  • Egg cross section derived from The Avian Egg: Alexis L. Romanoff, A.J. Romanoff, 1949. Image credit poultryhub, although it appears in other places as well: If you own this image and do not want it posted or need the credits changed, please contact me:

Friday, February 09, 2018

What are feathers? What is molt?

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! (See this blog, for example.) 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.

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. Click to enlarge
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.

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? Hormones regulate molt cycles, periods when 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.

Annual light cycles and rate of change in Decorah
In short, day length (or changes in day length, or the rate of change of day length) initiates 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.

Components of a bird's wing
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 have recently noticed Mom and Dad shedding adult down and body feathers. 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:

If we replace skin cells, why do scars and tattoos persist?

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:

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:
  • The wing came from wikicommons.