Friday, April 13, 2018

Eaglet Growth and Development, Week Two

It is April 13 as I write this, and our eaglets are growing rapidly! In Decorah, D29 is twelve days old, D30 is eleven days old, and D31 is nine days old.

Left to right: D30, D29, D31
In their second week of development, the eaglets will gain roughly two pounds between their 7th and 14th day of life. They will experience rapid growth in features like beaks, culmens, and footpads, start replacing their white natal down with thicker grey thermal down, and begin exploring the nest. Although they aren't yet standing on their toes, they are able to sit up - way up! - for feeding and shuffle around on their metatarsi. Their eyes are wide open and fit more comfortably in their eyesockets, features like brow ridges are beginning to appear, and their legs and footpads are yellow, not pink. Gary Bortolotti wrote that bald eagles might gain more weight per day than any other north American bird, although the majority of their weight gain occurs within the first 30-40 days. This rapid weight growth is fueled by their nutrient-rich diet of meat. Over the past week or so, we watched the eaglets chow down on fish, rabbit, squirrel, ground squirrel, coot, groundhog, and unidentified birds. D29 became proficient enough at shooting poop to christen the Poopcasso tree on April 9th, although it was quickly joined by its siblings on April 11th and 12th. While babylet battling hasn't entirely subsided, it has become less intense as pecking orders are established and eaglet crops are repeatedly stuffed until they look ready to burst! The eaglets are alternately hitting, submitting, and quitting to cuddle in the nest cup, grow, and wait for more food to arrive. We haven't gotten to see much of them given the cold, but D29 did finally escape the nest cup during warmer weather Wednesday!

So what did we see in the nest this week?


April 8: Eaglet explorers. Despite the cold weather, the eaglets are busy popping out from under Mom and Dad and starting to nibble at nest materials, exploring  the world with their sensitive beaks and tongues.

April 9: Preening! When birds preen, they remove dust, dirt and parasites from their feathers and align each feather in its optimum position. D29's exploratory downy nibbles today marked an important first step on the road to feather care!
April 10: Thermal down begins to emerge! A hatchling eaglet's fuzzy white natal down doesn't assist thermoregulation, aka controlling one's temperature. Denser thermal down provides more insulation and helps nestling eaglets keep their body temperatures at a relatively constant 105'ish degrees. 

April 11: Eaglet escapee! D29 took its first sojourn out of the egg cup today! At ten days of age, D29 weighs between about 2 and 2.5 pounds. It can't stand on its feet, but it can sit upfor feeding and shuffle around on its metatarsi. Its feet and toes are yellow, but its tiny talons are still clear.

April 12: Cropzillas! D29 has just gorged itself on more food than it can stomach! It will store the food in its crop until it is ready to transfer it to its stomach for digestion. This mini 'pantry' helps assure that the rapidly growing eaglets get the nutrients and calories they need, when they need them.

Painting the Poopcasso tree! On the morning of April 9th, the eaglets weren't quite shooting poop as far as the Poopcasso tree. D29 christened it that day, and all of the eaglets were hitting it by the 13th!
In the week to come, we can expect (continued) rapid growth in footpads, talons, and legs. Beak growth will rapidly slow as the eaglets' beaks approach adult size and we may see dark juvenile feathers start to sprout from their grey down. Overall weight and height gain will continue, most likely reaching their steepest curves some time this week. By the end of their second week of life, our little bobbleheads at Decorah will be almost a foot tall! Enjoy eaglet earholes and egg teeth while you still can - their earholes will soon be covered by down and their egg teeth are wearing away.

The general stages of eagle development are:
  • Stage 1 - Structural growth. In their first thirty-five to forty days of life, eagles grow very rapidly, gaining weight and building bones, muscles, tissue, and features like tarsi, footpads, toes, and claws. This phase of development slows down about halfway through an eaglet's time in the nest, even though individual features might continue some level of growth.
  • Stage 2 - Feather and flight-related growth. Eagles grow four sets of feathers - natal down inside the egg, thermal down, juvenile feathers, and adult feathers. Thermal down starts growing at about ten days, juvenile deck feathers at about 20-23 days and juvenile flight feathers at about 27 days, but feather growth doesn't overtake structural growth until thirty-five to forty days after hatch. Flight muscles also begin growing as eaglets wingercize, flap, hover, and eventually branch and fledge.
Neurological coordination occurs throughout an eaglet's time in the nest. During week two, their eyesight and basic coordination skills are improving as they grab food from Mom and Dad, sit up tall for feedings, shuffle around on their tarsi, and explore the world with their sensitive beaks. As they grow, they will become more adept at controlling beaks, legs, wings, and feet. They will learn to stand on their own feet, tear food, self-feed, and flap their wings, going from cute but clumsy clown clompers to graceful young eaglets poised at the edge of fledge.
I'm not sure how familiar many of you are with the cortical homunculus, an image-based tool that maps tactility. While useful and extremely cool, most cortical homunculii are static - that is, they reflect just one phase (usually adult) of an organism's life. But an eaglet's cortical homunculus will differ from an adult's as body parts and associated skills are gained and neural pathways developed. Our eaglets' brains and bodies are rapidly growing and changing as they gain the skills they need for life outside the egg! I'd tend to think that visual acuity and eaglet beak and tongue sensitivity  suddenly 'lit up' this week, leading changes in coordination as the eaglets began sitting up, grabbing food, moving around, and preening and nibbling at things.

Things that helped me write this blog, with a few considerations:

Tuesday, April 10, 2018

Bald eagle tongues and beaks!

This blog was inspired by RRP Director John Howe, who was reviewing footage of Mom and Dad feeding eaglets and was delighted by the dexterity of their beaks and tongues. I had never blogged about that topic and it was very fun to learn about just how specialized some beaks and tongues are!

We know that bird beaks are specialized for feeding. Birds of prey have strong, curved beaks with sharp edges to help them tear meat. Falcons specialize even further, adding a tomial tooth to help them kill prey. Dabbling ducks have tiny, comb-like structures on their beaks to strain small animals, insects and plants from water and mud, while piscivorous ducks have saw-like structures to help them hold on to struggling fish. But what about bird tongues or, more specifically, bald eagle tongues? Are they specialized as well?

Let's start by taking a look at Mom and Dad's tongues!
Dad's Tongue














Mom's Tongue


At first glance, bald eagle tongues look somewhat similar to ours. They are pinkish, relatively narrow, fit nicely between the sharp ridges of their beaks, and are flexible. They are short enough that eagles can't easily bite their tongues, although they can stick them out. Unlike us, bald eagles have two barbs, or rear-directed papillae, to help lift and pull food items to the back of their long mouths: a relatively common feature in bird tongues.

Although birds of prey don't have specialized tongues, their tongues (and beaks) are packed with mechanoreceptors that respond to pressure, distortion (think of an eagle stretching its tongue or biting into bone), temperature, texture, and vibration. Merkel cells are sensitive to very fine changes in touch, pressure, and temperature. These tonic cells keep providing information to tell Mom and Dad exactly where their beaks and tongues are and what they are doing. Phasic Herbst and Ruffini cells sound an alarm to let Mom and Dad know that something has just happened. Herbst cells are sensitive to vibration, pressure changes (I grabbed or dropped an object), and texture changes. Ruffini cells are sensitive to stretching, distortion, and temperature (I just bumped something warm and hard - must be an eaglet's beak and mouth).

These sensors aren't evenly distributed throughout a bird's beak. While I couldn't find any studies on birds of prey, studies have been done on quail and ducks. Sensory cells in a quail's beak are distributed like this:

Cadual BeakMiddle BeakCranial BeakBeak TipCell Type
.95%014.28%3.8%Herbst
1.9%051.42%2.89%Ruffini
0012.38%12.38%Merkel

Bald eagles have considerably different diets and lifestyles than quail, but both ducks and quail concentrate sensors in the cranial areas of their beaks (think of a falcon's tomial tooth or Mom and Dad's tomial ridge). Once a bird has allowed something past the tip of its beak, it has a lot of decisions to make about how to respond to it and what to do with it. Given that quail and ducks are preccocial omnivores and bald eagles are altricial carnivores, I would love to see a study on bald eagle beaks.

Thanks to their beak blinders, bald eagles have a blind spot directly in front of them, which means they can't always see their young during feeding. So how do they select food, find the eaglets, and get food into their mouths? Immediate and ongoing sensory feedback helps parents and young align beaks, and tells new hatchlings to open their mouths wide for food delivery! Once both beaks are in the correct position, Mom or Dad use their sensitive, flexible tongues to carefully push food into the eaglet's waiting mouth. Changes in pressure, temperature, and texture help the eagles change course in mid-delivery, detect dropped food, find eaglets, and pull back as needed. Mom and Dad also use their tongues and beaks to detect and avoid feeding young hatchlings chunks of bone, large pieces of fur, or sharp fins that could cause them to choke.

How do Mom and Dad manipulate food so well? Check out the top photo, which shows a side view of Dad's tongue. We can clearly see that its fleshy tip rests on a muscular stalk. As described in Laura Erickson's excellent blog on bird tongues, this stalk controls Dad's tongue to manipulate food items. Dad can stretch and tip his tongue forward to feed eaglets, pull and tip it backwards to move food into his mouth, or move his tongue to one side or another as needed. His flexible two-stage tongue and rear-directed papillae give him incredible control, and the sensory cells packed into the tip of his beak and tongue provide the feedback he needs to feed eaglets...even when he can't quite see where they are. Annoyed with Mom or Dad over dropped food or missed beak-loading opportunities? Imagine trying to feed eaglets using your just tongue and nose! The video below shows just how delicate they are when it comes to feeding hatchlings.



Did you know? 
Bird banders can use the color of a bird's mouth and tongue to help age birds. Younger birds may have spots or bands to help parents with food targeting. Colored tongues in some adult birds can aid mating displays and/or signal a warning to other birds.

Understanding a species' behavior and diet helps us understand how its tongue and mouth evolved. Check out Laura Erickson's blog More About Bird Tongues Than A Normal Person Would Want To Know for a great look and description of some bird tongues! You know you want to learn more!

Things that helped me learn and write about this topic

Friday, March 23, 2018

The Broken Egg at the Decorah North Nest

We asked our panel of eagle experts about the broken egg at the Decorah North nest. Could egg viability be determined from the video? Did they have any ideas about egg breakage? Was Mrs. North likely to produce more eggs?

In general, the panel believed that the egg’s viability couldn’t be determined from the video, although two of them commented that a darker patch in the draining egg fluid could have been an embryo. At 19 days old, an embryo would still have been quite small and not especially easy to see given the circumstances. Since we couldn’t come to a definite conclusion, our answer is “No” – we don’t know whether the egg was viable or not. We have some information about embryonic developmental stages here: https://raptorresource.blogspot.com/2017/03/whats-inside-those-bald-eagle-eggs.html

So why did the egg break? Female eagles lay down calcium in their shell gland/uterus, and their large eggs take a lot of calcium. Was she not able to lay down enough calcium to protect her egg from breaking, and if so, why? She may not have had enough calcium available during that stage of egg formation, or something else could be going on with her reproductive system. One member of the panel asked about her age. Female bald eagles and peregrine falcons can experience reduced fertility as they age. They might lay fewer eggs, strangely colored eggs, oddly shaped eggs, ‘smooshy’ eggs, thinly-shelled eggs, or infertile eggs that never hatch. It’s too early to know whether that’s the case, but we will be watching to see if any of those things happen moving forward.

Was she poisoned again? We saw no sign that she was poisoned this year, and the poison from two years ago would not affect her shell deposition now.

Is Mrs. North likely to produce more eggs? The answers were mixed. If she does, it should happen around March 30, or about 14 days after she lost the first one. Other birds of prey, including gyrfalcons and barn owls, take 14 days to recycle after losing a clutch. I really went back and forth about this (19 days is a long time to incubate an egg!) but the answer gave me some hope, especially given that Mr. and Mrs. North are still copulating. We’re crossing our talons, hoping for the best, and keeping a very sharp eye on Mrs. North!

Tuesday, March 20, 2018

Why did Mom and Dad lay eggs later this year?

We've been getting questions about egg timing. Why did Mom and Dad lay eggs later this year? Overall, avian egg-timing in the temperate zone is heavily regulated by the light cycle. As the days begin to lengthen, birds' gonads swell and produce sex hormones. Around the end of January, our bald eagle pairs switch from infrequent copulation to frequent, highly enthusiastic copulation, putting us on notice for egg watch! Females chase males, males entice females, copulation takes place on a regular basis, and our eagles start laying eggs in mid-February. We have a general egg outline, but what factors correlate to the onset of egg-laying in any given year?

New mates can equal later eggs
While it doesn't always happen, a new mate (male or female) often pushes egg-laying a little later in the eagles and falcons we watch. Sometimes it happens for very obvious reasons, like mid-season mate replacement. Let's say that female falcon A and male falcon B are nesting together on a cliff in mid-March. Falcon A has begun the process of egg production: her egg follicles are swelling, she's gaining weight, and she's copulating like crazy! Unfortunately, she's killed or displaced by female falcon C, who is producing plenty of sex hormones, but no eggs. B doesn't waste any time courting C. His bowing, gifts of food, and copulation will trigger her to begin egg production, but their egg-laying schedule will be pushed back by as much as or more than ten days. Absent other factors, it should float earlier in the years to come.

Later egg-laying can also happen in new couples that don't experience mid-season mate replacement. Both Mom Decorah and Ma FSV laid the first eggs of their lives in early March (March 2, 2009 for Mom Decorah and March 3, 2007 for Ma FSV) but moved earlier as the years progressed. Mom had just achieved sexual maturity when she bonded with Dad, but we don't know if the same is true of Ma FSV. Is initial egg production - an eagle's first egg - a factor in egg timing? How about receptivity in a new pair versus an already bonded pair? We don't know, but it's something we'll be looking at.

Cold dry weather? Predict eggs on the later side!
Having said that, we didn't have a new eagle in Decorah or at Fort St. Vrain this year. So what happened? We decided to look at weather. Using the nearest weather stations we could find to each nest (especially important in Colorado with its altitude-based weather patterns), we obtained historical figures for temperature and dewpoint from January 30 to February 10. There isn't a long time period between the onset of egg-laying and egg-laying itself, so we started around the time that both sets of eagles usually ramp up copulation and ranked our results from latest to earliest laying date.

FSV: January 30 to February 10
1st Egg DateHigh TempMean TempLow TempDewpoint
2/21/1431 F19 F6 F12 F
2/17/1348 F32 F17 F15 F
2/16/1646 F31 F17 F17 F
2/14/1560 F44 F28 F21 F
2/14/1756 F45 F33 F21 F
2/12/1851 F37 F22 F22 F

Decorah Eagles:  January 30 to February 10
1st Egg DateHigh TempMean TempLow TempDewpoint
2/23/1412 F4 F-3 F-3 F
2/21/1837 F20 F8 F4 F
2/20/1730 F21 F12 F15 F
2/18/1629 F22 F16 F19 F
2/18/1524 F16 F9 F12 F
2/17/1234 F28 F22 F23 F

In general, both sets of eagles laid eggs slightly earlier during periods of unusual warmth and slightly later during periods of unusual cold. But much to my surprise, humidity (as measured by dewpoint) turned out to be a slightly better predictor of egg-timing than temperature! What was going on with that - and with the one interesting exception in Decorah in 2015? Time to do a little reading!

It is widely believed that breeding female raptors gain weight by building fat and storing minerals to provide the energy reserves needed for egg-laying and incubation. But a study of female barn owls found that almost all of their pre-laying weight gain was due to water accumulation! From the study's discussion: "To conclude, the gain in body mass observed in breeding barn owls is not due to an accumulation of energy reserves but of an accumulation of water." Incubating female barn owls undergo a more complete molt than incubating bald eagles, but egg production mechanisms are very similar and it seems reasonable to believe that water retention would be similar in both species. The nutrients required for egg formation can be obtained by routine food intake during egg formation, but if the weather is extremely dry, it might take longer for female bald eagles to accumulate the water reserves they need for egg production, especially give that drier weather results in higher levels of evaporative and cutaneous water loss. It wouldn't make a few weeks difference in any given year, but it might make a few days!

I couldn't find an explanation for the anomaly in Decorah in 2015, but this is a relatively small dataset. As we keep records longer, we may learn more about trends in egg-laying. It also might be worth getting weather stations down below our nests to collect local data about temperature, humidity, and wind at each nest. We still have a lot to learn about the ways in which weather and climate affect the birds we watch.

Did you know?
The barn owl study also had some interesting information about fat/lipid dispersal in a breeding female barn owl's body. Although breeding females had fewer body lipids than non-breeding females, they had more subcutaneous fat in the area of their brood patch. The accumulated fat caused their brood patches to increase in size! So Mom's more visible brood patch and slightly larger appearance aren't just our imaginations. She has gained water weight and shifted fat to her brood patch in preparation for egg-laying and incubation. I'm sure that more than a few of us can relate to that!

While we aren't sure why Midwest peregrine falcons are laying eggs earlier, they are laying significantly earlier than they did at the beginning of falcon recovery in our area. Interested in the topic? Check out Average Nesting Trends!

Why didn't you use data from the North pair?
This was a tough one to leave out, especially given that many watchers think Mr. North was a new mate in 2016 - a year of later egg production. However, we only have three years of production here, the eagles have not established a regular laying pattern (2018 was also late), and we have no weather history anywhere near the nest, so I'm not comfortable including their data. We'll see what happens in the years to come.

Things that helped me learn about this topic

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 (https://youtu.be/I4xlu9IEItY). 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, https://openi.nlm.nih.gov/detailedresult.php?img=PMC3944358_jrd-59-334-g001&req=4
  • 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: http://www.poultryhub.org/physiology/the-avian-egg/. If you own this image and do not want it posted or need the credits changed, please contact me: ries93@gmail.com.

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: 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.



Saturday, December 30, 2017

A trip down memory lane: Favorites from 2017, Decorah North Edition!

Cowghetti...yum!

It's time for a look back at 2017! We asked the Decorah North Mods for some favorite moments from Decorah North. We hope you enjoy a trip down memory lane!

2017 marked the 8th year that eagles have nested in the valley of the Norths! We don't know whether the same eagles have been here all along. The first nest (DNN0) was built in a pine tree. The branches collapsed after the second nesting season and the eagles moved to a dead elm tree. They nested there for just one year before moving to their current location in late 2013. 2017 will mark their fourth season in the nest we are watching them in now! Although we refer to it as DNN, it is technically DNN2.

Sherri Elliott said: "Seeing Mr. North's new maturity and almost 50/50 partnership in purveying provisions this year. Hearing Mrs. North 'teakettle' for the first time this year with new mic, and knowing she can easily set it to 'steam' like Mom Decorah. Observing both parents in a deep dream state, vocalizing, jolting, and in the case of Mr. North drowsy scolding at night. The North's dexterity in footing to roll their eggs."

Her favorite videos included:

12/21/17: Coyote Howling  & Chorus - https://youtu.be/tB_TU-hFuIE
It's not eagles, but it is very cool! The chorus made the hair on the back of my neck stand up!

12/03/17: Perch party! Not 1, not 2, but 7 Eagles - https://youtu.be/q7P00h8vJ6o. Sherri said: To me this ranks as one of the top observations of BE sociability and reminds me of last seasons “Pool Party” with multiple BE’s bathing in the stream.

11/30/17: Struggling to Lift Stick from stream pasture - https://youtu.be/_61VDr4OqJc 
Sherri: This was Mrs, who later brought the stick to the nest, but to me the observation seems a lot more like stick play rather than attempts to lift and carry.

11/29/17: Keep Off Our Nest - Dad attacks J/SA - https://youtu.be/19hhzDKRgLA
If you watch the North Nest, you know all about the Super Flyway. The North's accept visitors across the field, but don't tend to appreciate them at the nest tree. It isn't uncommon to see hungry juveniles and subadults attempting to steal a little breakfish during peak migration.

11/24/17: Splashdown, Parent Catches a Fish - https://youtu.be/txA1XfSmHqc
Sherri: I love to see them fish!

11/07/17: Sub Adult Chase - https://youtu.be/Y7Z5riTs_tA

10/30/17: Eagle With Fish & One In Pursuit! Perching & Nice Fly-Off  - https://youtu.be/YTsASh1c3K8

10/25/17: Intruder wants Dad's Fish - https://youtu.be/caQq6uqZRvY.

10/11/17: GHO Visits - https://youtu.be/cpE63ODKquw. We've heard GHO at Decorah North, but we've never seen them nest shopping like this before!  

7/26/17: Sighting of DN4 and DN5 - https://youtu.be/Jto4hhl-eDg. We believe this was the last sighting of both caught on cam.

7/22/17: 2:35 p.m. CDT: Round the Pasture We Go - https://youtu.be/IBXVO-2FQJUTwo fledglings/juvies and both parents seen flying in the pasture. 

6/15/17: DN4 & DN5 Rumble in the Nest  - https://youtu.be/qMxXrznXJsM. A lot of you watched and commented on this. DN5 had a very strong response to DN4's return, although they quickly accepted one another once the initial surprise wore off. 

6/11/17 - DN4 Accidental Fledge (knocked off by DN5) - https://youtu.be/SsY0WYgqM7E.
What do you mean? I didn't notice my sibling was there! We got a lot of email and comments about this as well. 

5/30/17: Small owl attack on DN4 - https://youtu.be/rkvCp8XyVpQ.
I initially thought this might have been a barred owl, but it is really small! We're going to share this video with Karla Bloem of The International Owl Center and see what she thinks! 

4/17/17: Dad North's cowghetti delivery gets hung up on the rails, and Mrs fetches it. Hmmmm....cowghetti! https://youtu.be/apRlJEl84uk

4/08/17: Dad North Alerts & Defends from Intruder (with DN4 copycatting Dad's wingspread) - https://youtu.be/IQAAZi7Tu4g. I saw this exact same behavior when we banded falcons at MPL's Cohasset plant this year. It was charming and fascinating to see "Big Sis" copying Mom's defensive behavior!

3/11/17: Mom Home, Drowsy Dad Refuses To Leave - https://youtu.be/9EQ2mIzZh5U.
This was so cute!

River Eagle also liked Mom's late evening arrival. I had completely forgotten about "Stranger Danger". Mrs North's late arrival startled Mr. North but scared chatters who thought a stranger was invading. The "Juvie in Pasture Runs Parent off of Prey" video was another favorite. I think I'd rather face a hangry human than a juvenile bald eagle intent on eating, and I can't tell you how much I enjoyed seeing the green pasture again!

6/22/17: Juvie in Pasture Runs Parent off of Prey - https://youtu.be/AUNyG__scVI

ModV brought up many of the same moments, but added two new ones:

5/08/17: Goose at the nest - https://youtu.be/cDv_sBFwyyQ
One of the most interesting sights was early one morning when a goose landed on the babysitting branch.

3/07/17: Mrs. North dreaming - https://www.youtube.com/watch?v=voNRDmpde1A
Mrs. North dreaming was so cool to watch!
(Blog here: https://raptorresource.blogspot.com/2017/03/sweet-eagle-dreams.html)

Mod7 and BeautyofEagles both brought up a personal favorite of mine - a peregrine falcon visits DNN! Note that this video also contains some cows! https://youtu.be/HyVFISzaqOQ (Blog here: https://raptorresource.blogspot.com/2017/05/a-peregrine-falcon-at-decorah-north-nest.html).

Mod7 said: "Of course the OOPS PS (there's always a few!) of DN5 on Mrs. North. it's almost like DN5 aimed at her. lol Don't PS on the eagle that feeds you!" https://youtu.be/9sCoTpJpK2g

"And watching Mr. North feed the little ones...you can really see the change in him and he's grown so much. It melts my heart when he takes care of his eaglets." This video is really lovely: https://youtu.be/JIRm2Oh6qjE

Thank you so much for the trip down memory lane!<3 p="">