Archive for the 'Bird Anatomy' Category

Jan 03 2014

Staying Warm

Black-capped chickadee, common raven (photos from Wikimedia Commons and Shutterstock)

On this very cold morning everyone’s working hard to stay warm but some have an easier time than others.  Who loses heat faster, the chickadee or the raven?

Just like us, birds burn calories no matter what they’re doing.  However, birds have higher metabolic rates than mammals and require more calories for everything they do.  Any activity, from sleeping on an empty stomach to a burst of rapid flight, burns more energy than in vertebrates of a similar size.

Small birds have higher metabolic rates than large ones because of the relationship of surface to volume.  Heat dissipates from the surface of an object so the more surface there is, the greater the heat loss.  So, yes, the chickadee loses heat faster than the raven.  That’s why northern animals are often larger-bodied than those who live in warmer climates.  Even among chickadees the black-caps in Maine are noticeably larger than the chickadees in Pittsburgh.

To stay warm the chickadees will look fatter today because they’ll fluff their feathers to raise the loft of their down coats.  They’ll also cover their legs to reduce heat loss and they will eat — a lot! — to replace the calories they’re rapidly burning.

“Eat like a bird?”  Today all birds, and especially the little ones, are chowing down to stay alive.


(photo of Carolina chickadee from Wikimedia Commons. photo of raven from Shutterstock. Today’s Tenth Page is inspired by page 150 of Ornithology by Frank B. Gill.)

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Nov 18 2013

Not Born Like This

Black Skimmer (photo by Steve Gosser)

Speaking of pied shorebirds as I did yesterday …  when I see American oystercatchers I’m reminded of black skimmers (Rynchops niger).  Both have bold black-and-white plumage and long beaks but their differences are striking.

Unlike oystercatchers, skimmers have very short orange legs and a beak whose mandibles are two different lengths.  They use their long lower mandible — 2-3 cm longer than the upper — to skim food from the ocean’s surface in flight.  Click here to see.

Black skimmers aren’t born like this.  At hatching their beaks are normal but by the time they fledge four weeks later their lower mandibles have grown 1 cm longer than the uppers, halfway to this striking adult appearance.

One more amazing beak fact:  Black skimmers’ beaks look fat from the side but if you see them straight on they are knife-thin like this.

The better to skim with, my dear.


(photo by Steve Gosser)

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Sep 19 2013

Fancy Feet

Published by under Bird Anatomy,Quiz

Snowy egret feet (photo by Chuck Tague)

Monday’s blog about identifying white wading birds got me thinking about snowy egrets’ black legs and fancy yellow feet.  Wow!

Are there other birds in North America whose legs and feet are different colors?

The immature blackpoll warbler has them.  Adult blackpolls have bright orange-yellow legs and feet but the youngsters have black legs.  Their contrasting feet are a good identification tip during fall migration.  This one is wearing orange slippers.
Immature Blackpoll warbler (photo by Marcy Cunkelman)


Beyond the blackpoll I was stumped.  I searched my field guide page by page and discovered that golden-crowned kinglets have dark legs and pale yellow feet.  Who knew?  I never looked at their feet before.
Golden-crowned kinglet (photo by Shawn Collins)


Do any other North American birds have fancy feet?  I don’t think so, but maybe you know of one.

In the meantime I’ll leave you with this thought …

Have you ever seen a Eurasian Coot?
Eurasian coot (photo from Wikimedia Commons)


(photo credits:  Snowy egret feet by Chuck Tague, immature blackpoll warbler by Marcy Cunkelman, golden-crowned kinglet by Shawn Collins, Eurasian coot from Wikimedia Commons)

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Aug 23 2013

House Sparrows Bulk Up

House sparrow molting, Sept 2008 (photo by Remi Jouan via Wikimedia Commons)

Have you noticed that a lot of birds are molting now?  On the extreme side I’ve seen a bald male cardinal and Mary DeVaughn reported a bald blue jay, both of whom shed all their head feathers at once.

Less extreme-looking but still ragged are the house sparrows.  Ten or more of them line up at my bird bath to splash wildly and loosen their old feathers.

Birds must molt to replace worn feathers but house sparrows, who don’t migrate in North America, have an additional reason.  In August they put on heavier plumage that will keep them warm over the winter.

According to Ornithology by Frank B. Gill, the plumage on house sparrows weighs 0.9 grams in August.  By the end of September they’re wearing 1.5 grams of feathers.

Our house sparrows are bulking up.

(credits: photo from Wikimedia Commons. Click on the image to see the original. Today’s Tenth Page is inspired by page 154 of Ornithology by Frank B. Gill.)

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May 03 2013

Incubation Chamber

Egg illustration by Stuart Lafford from Michael Walters Birds' Eggs, published by Dorling Kindersley

Last week we examined a newly laid bird’s egg.  This week things get more complicated.

Eggs are tiny incubation chambers with all the tools needed to transform an embryo into a baby bird.  The right temperature gets the process rolling.

As an egg is incubated the embryo changes and the membranes take on the critical functions of respiration, circulation and excretion.   The yolk and albumen shrink as they’re consumed and the shell participates in respiration and bone construction.

This illustration by Stuart Lafford, from Birds’ Eggs by Michael Walters, shows what’s going on inside.

  • The embryo, surrounded by the amnion, floats in a fluid cushion.
  • The yolk is attached to the embryo’s belly and shrinks as its food is consumed.
  • The allantoic sac acts like a sewer collecting excretion from the embryo.  It also functions in respiration because it’s pressed against the chorion for air exchange.
  • The chorion supports all the embryonic structures and acts like a lung, exchanging oxygen and carbon dioxide through the shell’s pores.
  • The shell thins as the baby bird takes up calcium to construct its bones.  The thinning allows for increased air exchange so the growing embryo receives more oxygen.  It’s also easier to break the thinner shell at hatch time.

In a matter of weeks the egg contains a baby bird … and then he breaks the shell.

The egg has fulfilled its role as an incubation chamber.


(illustration by Stuart Lafford from Birds’ Eggs by Michael Walters, published by Dorling Kindersley, 1994, used by permission. Click on the image to visit Stuart Lafford’s website. This “Tenth Page” article is inspired by page 425 of Ornithology by Frank B. Gill.)

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Apr 26 2013

Eggs’ Potential

Anatomy of an egg (illustration from Wikimedia Commons)

We’ve had eggs on our minds this week while we’re watching them hatch at the Cathedral of Learning peregrine nest.

Eggs start as the familiar objects we see every day in our refrigerators and miraculously become baby birds.  The process is so amazing that I’m devoting two Tenth Page articles to it.

Shown above is the un-incubated egg we know so well.  If fertilized before it’s laid — and then incubated — it becomes a bird.  Each component plays a part.

  • Blastodisc or germinal disc:  Potential embryo.  If fertilized and incubated this small circular spot on the yolk becomes a chick.
  • Yolk:  Food for the embryo.  The female’s ovary deposits layers on the yolk to increase its size before ovulation.  Yellow layers are laid on during the day, white ones at night, so the yolk has rings like a tree.  It’s housed in a yolk sac which is why you have to “break” the yolk when cooking.  The yolk is ovulated with the germinal disc attached (cradled by the yolk) so the food is next to the potential embryo even before fertilization.   As the embryo develops, the yolk shrinks.
  • Albumen = Egg White:  Food, water, shock absorber, and insulation from sudden temperature changes.  The albumen makes up 50% to 71% of the egg’s total weight.  It’s laid on after fertilization while the yolk-with-germinal-disc rotates gently in the oviduct.  As the embryo develops the albumen shrinks too.
  • Chalazae:  Because the yolk is rotating during albumen deposition, twists form in the albumen.  Chalazae act like springs and stabilizers to keep the yolk and embryo in place inside the egg.  They’re the white twisted bits in the egg white.  (Totally amazing!  Shock absorbers, insulation, springs and stabilizers!)
  • Inner Shell Membrane:  the first of two membranes that hold the embryo-yolk-albumen together
  • Air Space:  Between the inner and outer shell membranes the air space acts as a condenser for moisture exchange.  This is where the baby bird takes its first breath before hatching.
  • Outer Shell Membrane:  The final packaging before the shell is laid on.  It’s attached to the shell when you crack open an egg.
  • Shell: The female’s uterus deposits calcium on the outer shell membrane to make the hard enclosure for the egg.  The shell has microscopic pores to allow air exchange for the developing embryo.
  • Cuticle:  A thin layer on the shell that adds protection.  The cuticle has caps on top of the pores that close when necessary to protect the embryo.

Eggs have the tools and potential to become baby birds.  Next week I’ll show you how.

(illustration from Wikimedia Commons; click on the image to see the original. Today’s Tenth Page is inspired by page 420 of Ornithology by Frank B. Gill.)

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Apr 19 2013

How Do They Hatch?

Chicken hatching in incubator (photo by grendelkhan on Wikimedia Commons)

In the next few days the peregrine eggs at the University of Pittsburgh are going to hatch, so now’s a good time to explore…

How does a baby bird get out of the egg?   It’s a strenuous one to two day process in very tight quarters.

  1. When a chick is ready to hatch, he pulls himself into the tucking position with his beak sticking out between his body and right wing.  This gives him the leverage he needs to whack at the shell.
  2. The chick then breaks through the membrane at the large end of the egg that isolates the air sac and he breathes for the first time.
  3. Next he starts to bump the shell with the curved ridge of his beak where he has a calcified egg tooth that’s sharp enough to crack the shell.
  4. His strenuous hammering is aided by the hatching muscle on the back of his neck.
  5. While still in the egg he communicates with his parents and siblings by peeping and pecking sounds.  The parents know which eggs are alive because they’re speaking.  The siblings know their brothers and sisters are ready to emerge.  In precocial species, which must all hatch at once, the chicks listen to each others’ tapping to coordinate the hatch.  Elder chicks tap slowly, younger ones tap rapidly so that all of them reach the finish line in a 20-30 minute window.
  6. Finally the chick cracks his shell all the way around.  He pushes with his feet and the egg splits open.  His mother moves the shell away and he lies quietly, waiting for his down to dry.

After hatching the chick’s specialized tools aren’t needed anymore.  The egg tooth falls off (in songbirds it’s absorbed) and the hatching muscle shrinks into just another neck muscle.

Watch the National Aviary falconcam for hatching at Dorothy and E2’s nest.  The streaming cam is blurry but it is broadcasting sound so you’ll be able to hear the chicks peeping inside their shells.  That will be our first sign that hatching is underway.


(Credits: photo of a chicken emerging from its egg from Wikimedia Commons.  Click on the image to see the original.  Today’s Tenth Page is inspired by page 460 of Ornithology by Frank B. Gill.)

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Apr 05 2013

How Can They Sit For So Long?

Dorothy asks E2 to get up so she can resume incubation (photo from the National Aviary falconcam at Univ. or Pittsburgh)

During courtship E2 is very active but now Dorothy has to plead with him to get up off the eggs.  Dorothy herself is able to sit for 12 hours in a snow storm.  How do they do it?

How do birds instantly switch gears from the frantic activity of courtship to sitting on eggs all the time?

They’re cued by hormones.  Here’s how:

  1. As day length increases after the winter solstice, a bird’s hypothalamus releases LHRH (luteinizing hormone releasing hormone).
  2. LHRH triggers the pituitary gland to release LH (luteinizing hormone).
  3. LH increases production of testosterone in males and progesterone in females.
  4. Testosterone triggers aggression, territoriality and sexual behavior.  It’s good at the start of breeding but doesn’t help raise a family.
  5. Progesterone is the “pregnancy hormone” that induces egg production.  It’s only needed for a short time since female birds are only ovulating and pregnant until they lay the eggs.
  6. On the day before incubation begins the hormones switch.  Prolactin, the hormone that promotes incubation behavior, rises sharply while the other hormones suddenly decrease.  In females, LH and progesterone drop off.  In males, testosterone has been dropping since egg laying began.  If the male shares incubation he has a sharp rise in prolactin, too.  On a graph this hormone switch looks like a sine curve.  There’s a moment where all these hormones are low, then prolactin takes off.

In peregrines, both parents have to be ready to incubate at the same time.  Their courtship rituals help get the couples’ hormones in synch.

This whole process may sound as if birds are at the mercy of their hormones but in every species reproduction is chemically tuned for success.  In humans for instance, progesterone and prolactin switch after delivery so that the mother’s body produces milk to feed the baby.  Individual animals whose hormones malfunction do not have live offspring.

So how do birds incubate so nicely?  In a word, prolactin.


(photo of Dorothy and E2 from the National Aviary falconcam at the University of Pittsburgh.  Today’s Tenth Page is inspired by page 448 of Ornithology by Frank B. Gill.)

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Feb 05 2013

Occupational Hazard

Barred owl at Crooked Creek (photo by Steve Gosser)

How do owls turn their heads this far without killing themselves?

Trauma experts know that when humans turn their heads too far or too fast the arteries to the head are stretched or torn, cutting off the blood supply or producing blood clots that can kill.

Why doesn’t this happen to owls?  A team at Johns Hopkins decided to find out.

Led by medical illustrator Fabian de Kok-Mercado, they used imaging technology on barred, snowy and great horned owls who had died of natural causes. The researchers found four adaptations that make the owls’ wide range of movement possible:

  1. As in humans, the major arteries that feed the brain go through bony holes in the vertebrae but in owls these holes are 10 times larger than the arteries, allowing them to move within the hole without pinching.
  2. The owls’ vertebral artery enters the neck higher up than in other birds — in the 12th vertebrae rather than the 14th.  This provides more slack.
  3. When an owl turns its head the arteries at the base of the head balloon to take in more blood.  In humans the arteries get smaller and smaller.
  4. Owls also have small vessel connections between the carotid and vertebral arteries so if one path is blocked the other still works.

A simple turn of the head that’s so hazardous to us is all in a day’s work for an owl.

Read more about the study here in Science Daily.

(photo of a barred owl by Steve Gosser)

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Feb 01 2013

Slots Help Me Fly

Turkey vulture (photo by Chuck Tague)

A bird’s lifestyle is written in its wings.

Birds who fly fast and maneuver quickly, such as peregrines and swifts, have narrow pointy wings built for speed and agility.  They need this equipment to capture prey in the air.

Birds who soar slowly in search of food, such as red-tailed hawks and turkey vultures, have broad wings with a lot of surface area.

Broad, blunt wings create a lot of wingtip turbulence (remember those vortices?) so soaring birds have feather slots at their wingtips.  This confers two flight advantages.

First, each feather stands alone like a tiny pointy wing with a high aspect ratio (ratio of length to breadth) that’s more like a peregrine’s wing.   The winglets create less turbulence and therefore less drag.

The second advantage is in the gaps. As air is forced upward between the feather slots, it expands on the upper side creating low air pressure on top and therefore more lift.

Turkey vultures are masters of slow speed flight.  They turn and teeter without flapping — not even once!

The slots help them fly.


(Photo by Chuck Tague.  Today’s Tenth Page is inspired by page 120 of Ornithology by Frank B. Gill.)

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