Showing posts with label eagles. Show all posts
Showing posts with label eagles. Show all posts

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.



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Friday, April 01, 2016

Eaglet Growth and Development: Week One

Newly hatched D25, March 31st 2016
D24 is three days old today, sibling D25 is not quite a day old as I write, and D26 is still inside the egg. What can we expect in the first week of watching?

Weight Gain!
Like humans and other animals, growing nestlings have developmental milestones. The eaglets spend roughly the first week of their life gaining weight. They aren't able to thermoregulate yet, so depending on the weather and temperature, they may spend a lot of time under Mom and Dad. We'll see them eat, sleep, scuffle, and grow stronger as they interact with one another.  They will go from roughly 3.2 ounces - about the weight of 18 nickels - to roughly 16 ounces or one pound, increasing their weight five times over in just seven days.

Many structural features, including foot pads, tarsi, and hallux claws, won't start rapid growth until 10-15 days after hatch. But the hatchlings' mid-toes and culmen - the dorsal ridge of the upper mandible - are already growing longer! Food is the root of all else besides, so it isn't surprising that the culmen achieves maximum growth in the first ten days. I suspect that the mid-toe aids balance, a crucial element of sitting up and exploring the nest. While our eaglets won't truly stand on their feet until they are roughly four weeks old, they will begin to shuffle around the nest on their tarsi long before that.

Enjoy the downy bobbleheads this week! By next week, they will already be growing their longer 'wooly' second or thermal down and alternately worrying and thrilling us with their interactions and sojourns around N2B.


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. While thermal down starts growing at about ten days and juvenile flight feathers at about 27 days, 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.

Stage 3 - Neurological Coordination. Eagle watchers know how ungainly eaglets can seem! As they grow, they become more adept at controlling beaks, legs, wings, and feet. They 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. We discussed it very briefly in this blog and I'll include links below. 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!

Things that helped me write this blog, with a few considerations:
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Tuesday, March 29, 2016

Do bald eagles delay incubation?

2014, Egg #2. Temp with windchill: -40F.
Do bald eagles delay incubation? It wasn't an a question we've thought about much, since bald eagles in Iowa usually lay eggs in temperatures under - sometimes well under - freezing! However, this year was quite a bit warmer, and the eagles in Decorah and Fort St. Vrain seemed to spend more time off the first two eggs than we are used to.

The question seems pretty cut and dried. After all, peregrine falcons delay full incubation very consistently until the third egg in a four-egg clutch, and the fourth egg in a five-egg clutch. In cold years (2013, 2014), we tend to experience higher losses than in warm years, since eggs one and two can freeze and die prior to the beginning of full incubation. However, it isn't that simple with bald eagles and many other birds.

2016, Egg#2: Temp no windchill: between 25-40F.
According to Professor Jim Grier, birds often start incubation slowly or gradually when temperatures aren't in the freezing range. Freshly laid eggs can spend a lot of time in the zone of suspended development (roughly 28.4 to 80.6°F or -2°C to 27°C) with no harm to the egg or embryo. While I don't have storage times for bald eagle eggs, Wikipedia tells me that the freshly laid eggs of domestic fowl, ostrich, and several other species can be stored for about two weeks when maintained under 5 C. That is quite a bit longer than the time between eggs one and two in a bald eagle nest!

Temperature aside, why do some birds begin incubating immediately? In a blog on the subject, David Hancock discussed two theories of immediate incubation: one, to protect eggs from marauding ravens and other predators, and two, to assure the survival of at least some young during periods of food scarcity. In the last case, the 'strategy' assumes that it is better to have one or two surviving children than no surviving children at all.

However, there are more benefits to delaying incubation so eggs hatch closer together: indeed, the survival of many waterfowl depends on it since adults leave the nest 24-48 hours after hatch begins, whether all of the eggs are hatched or not. Perhaps most importantly, delaying incubation helps assure that rapidly growing young are in the same stages of development. In the presence of an abundant food supply, more young will likely survive when incubation is delayed, since there will be less variation in size and ability between older and younger siblings, and food should be shared more equitably as a result.

Professor Grier pointed out that the nature (if not essence!) of biology is variation. This caused me to think back on eagles and monogamy. When we first started this adventure, everyone 'knew' that eagles were monogamous and mated for life. A new mate would be taken only if the old mate died. Remember when that was true? But once we really started looking, eagle relationships turned out to be more complicated. While many eagle couples appear to be monogamous, we've also documented extra-pair copulations, eagle 'divorce', polygyny, and polyandry.

The same is undoubtedly true for incubation. Professor Grier wrote that he would expect variation among different pairs of bald (and other eagles) in how much time they spent off the egg(s) both at the start and as incubation proceeded, as well as under different conditions such as temperature, humidity, and precipitation. We've certainly seen that in Decorah. In 2014, February's mean temperature was 9.4F - the coldest in 20 years of recording! Under those conditions, Mom and Dad incubated almost constantly. But in 2012, February's mean temperature was 27.8F - not the warmest ever recorded, but quite close to it. As indicated by video footage and all of the blogs and posts we did on the subject, Mom and Dad spent much more time off the eggs in 2012 than they did in 2014. While weather is clearly a factor, David Hancock noted timing variability between rural and urban bald eagles, and speculated that the presence of predators might make a difference in whether incubation was delayed.

The world is not a static place. Animal populations fluctuate as food supply, weather, and disease cause cycles of booms and busts. High populations and/or low food supplies can result in massive dispersals or irruptions as birds compete for territory and food. Populations may adopt different behaviors at different levels of density and food abundance, perhaps becoming less aggressive in a situation where neighbors and food are both in high supply. Everything changes over time: given a large enough timescale, the continents themselves flow like water. In that light, it is no surprise that eagle incubatory behavior might vary, or that eagles might adopt different behaviors as their population and/or the world around them changes.

So philosophy aside, can we really answer the question of delayed incubation in bald eagles and other birds? That's a tough one. The onset of incubation doesn't seem to involve timing in most species of birds, since they appear to be responding more to external factors than to an internal clock. This makes it hard to define what exactly 'delay' is for most birds. If I'm staying off my eggs because it is warm out, am I really a delayed incubator? If I almost always incubate right away because I lay eggs in sub-zero temperatures, am I really an immediate incubator?

Short answers are often necessary in our instantaneous and highly-connected world, so here is mine: "Bald eagles may or may not begin incubation immediately after the first egg is laid, depending at least in part on local factors that include temperature, humidity, precipitation, and predators. Apart from that, bald eagles exhibit variability: that is, not all pairs act the same way, even given the same or similar situations. There is more to learn, so keep watching, keep documenting, and trust the eagles!"

I'm going to close by quoting Professor Grier again: "The nature (if not essence!) of biology is variation." In my experience, almost every time we say a species always does this, or never does that, the species proves us wrong! Onset of incubation and mating systems are just two more examples of that.


Our ideas about birds have been shaped by how we observe them. Professor Grier pointed out that in the past, normal incubation details for various birds were studied from blinds, long-term and time-lapse photography, and with electronically telemetered artificial eggs, nest thermometers, treadles or scales to measure adult presence on the eggs. He wrote: "Eagle cams provide a whole new opportunity to observe and study traits such as incubation behavior (by individual parent, gender, total time, and under various environmental conditions) on increasing sample sizes of birds under undisturbed conditions." In other words, nest cams give us an unparalleled opportunity to observe large populations of birds in the wild, relatively free from human interference. Who knows what we will find, or what theories and beliefs will be challenged? We've got the cameras in the field. The next step is to develop appropriate methodologies and tools to enable large scale data collection across species and populations.

Thanks to the following for helping me learn about this subject.
  • Personal communication, Professor James Grier. Any mistakes I made in interpreting his email are my own. 
  • Personal communication, Bob Anderson. Bob used to tell us that eagles were pretty smart, but peregrine falcons were only about as smart as garter snakes. Even though he loved falcons, he believed that their lower intelligence made them less variable in their response to external stimuli. While that was outside the scope of this blog, I will return it later on.
  • David Hancock's blog on Bald Eagle Laying and Hatching Sequences

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Thursday, August 27, 2015

Decorah North Nest

We surveyed a new nest last week. 'Decorah North' is a very large nest in a secluded valley north of Decorah. A few statistics from our trip:
  • The nest is 56 feet off the ground 
  • It is nine feet long at its longest point
  • It is seven feet wide at its widest point
  • It has a total area of 49 square feet
  • It is about 5.5 feet high
  • The eagles began building it in 2013.
Using the method listed here (http://goo.gl/WajKSb), I calculated the nest's weight at roughly 2100 pounds (using 37 pounds per cubic foot for wood weight). Good thing it is located in a sturdy white oak tree!

Kike, a professional photographer and climber, found the following bones in the nest:
  • Adult male turkey leg and and ilium
  • Turkey polt leg. 
  • Fawn leg
  • Adult raccoon skull
  • Young raccoon skull
We are looking at adding cams to this nest and will keep everyone posted on our progress. Given the nest's size and prey remains, these must be industrious, hungry eagles!

Quick note: The nest is located on private property and can't be seen from the road. We are asking everyone to respect the landowner's privacy (and cattle) and refrain from visiting the nest in person. Thank you.
From above. Kike used his climbing helmet for scale

Another view. More than one person could fit quite comfortably

The nest from the side

The fawn's leg. Note how tightly woven the nest is. 

The nest from above. Kike's shadow helps provide a little scale.
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