Category Archives: Bird Anatomy

You Can See Her Egg!

Bird Anatomy

Gravid female northern cardinal, held for banding by Bob Mulvihill (photo by Kate St. John)

We learned a lot about bird anatomy, at the Neighborhood Nestwatch banding on Saturday.  Did you know that …

  • When you blow on the belly feathers of a songbird during the breeding season you see bare skin underneath.  This is the brood patch for incubating eggs and keeping nestlings warm.
  • Songbirds have translucent skin.  The red color is muscle under the skin, yellow is stored fat.
  • You can see the egg under the skin of a “pregnant” bird!

Even before he checked her belly, Bob Mulvihill could tell this female northern cardinal was gravid when he held her.   When he blew on her belly feathers we saw the white oval of the egg near her tail, circled below.

Gravid female northern cardinal, egg under skin (photo by Kate St. John)

This lady must have come out for breakfast before laying her egg and was delayed by the mist nets near the feeders.  She needed to get back to her nest soon(!) so Bob quickly banded, weighed and released her.  She immediately flew to the big maple and disappeared.

She weighed about 47 grams — 5+ grams heavier than normal because she was carrying the egg.  That’s a significant load to carry.

I hope she deposited it safely and that her morning turned out better than it began.  😮

 

(photos by Kate St. John)

Bird Skeletons As Heavy As Mammals’

Bird Anatomy

Skeleton of wood grouse, Museum of Toulouse (photo from Wikimedia Commons)

A misconception that began with Galileo in 1683 was debunked four years ago but I’ve only just learned it now.  Perhaps this is news to you, too.

In the March 17, 2010 issue of Proceedings of The Royal Society B Biological Sciences, bat researcher, Elizabeth Dumont of the University of Massachusetts Amherst published her bird, bat and rodent bone density study and concluded:

Bird skeletons do not actually weigh any less than the skeletons of similarly sized mammals.  In other words, the skeleton of a two-ounce songbird weighs just as much as the skeleton of a two-ounce rodent.(1)

Elizabeth Dumont reached this conclusion by measuring the density of skulls (crania), upper arm bones (humeri) and thigh bones (femurs) of 20 families of perching birds, 11 families of rodents and 13 families of bats, all of them less than 400 grams.  The birds’ bones were the densest.  Then she ran the numbers on skeletal bone mass and volume.

She remarked, “The fact that bird bones are denser than bones in mammals not only makes them heavier for their size, but it may also make them stiffer and stronger. This is a new way to think about how bird skeletons are specialized for flying and solves the riddle of why bird skeletons appear so lightweight and are still relatively heavy.

“This has never been explained fully and so has never gotten into the textbooks. I’d like to see that change.”

So now we know.  Sorry, Galileo.

Read a (1)summary of the findings in Science Daily or the full article here in the Proceedings of The Royal Society B Biological Sciences.

 

(photo (flipped) of wood grouse skeleton at Museum of Toulouse by Didier Descouens, Creative Commons license, Wikimedia Commons. Click on the image to see the original. )

Staying Warm

Bird Anatomy, Tenth Page

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.)

Not Born Like This

Bird Anatomy, Water and Shore

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)

Fancy Feet

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)

House Sparrows Bulk Up

Bird Anatomy, Tenth Page

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.)

Incubation Chamber

Bird Anatomy, Nesting & Courtship, Tenth Page
Egg illustration by Stuart Lafford from Michael Walters Birds' Eggs, published by Dorling Kindersley
(illustration by Stuart Lafford from Birds’ Eggs by Michael Walters, published by Dorling Kindersley, 1994, used by permission)

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.)

Eggs’ Potential

Bird Anatomy, Nesting & Courtship, Tenth Page
Anatomy of an egg (illustration from Wikimedia Commons)
(illustration from Wikimedia Commons)

Eggs are familiar objects that 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.  Click here to learn how the chick develops.

(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.)

How Do They Hatch?

Bird Anatomy, Nesting & Courtship, Peregrines, Tenth Page
Chicken egg hatching (photo from Wikimedia Commons)

Now that the peregrine falcon eggs are ready to hatch: How does the baby bird get out of the egg?   How long does it take from pip to hatch?

Hatching is a strenuous process in very tight quarters. In all bird species it is the chick’s responsibility to get out of his shell, the first big test of his life. His parents do not assist. (Read more here.)

Peregrine falcon chicks take up to 72 hours from pip to hatch with a lot of resting between activities. Here’s what happens:

  1. Absorb the yolk: Just before hatch day the chick fully absorbs the remaining yolk inside the egg. The yolk was its food during incubation.
  2. Get into position: Inside the egg the chick pulls himself into a tuck with his beak sticking out between his body and right wing.  This gives him the leverage he needs to whack at the shell.
  3. Communicate with parents and siblings: The chick peeps inside his shell and his pecking makes noise. His parents know which eggs are alive and his siblings can hear that a chick is ready to emerge. 
    1. From Birds of the World, peregrine falcon account: An observer in a nearby blind, “Nelson could hear chick peeping inside egg before hatching began, becoming louder during hatch; initial pip of shell occurred >72 h before chick broke free completely. In 500 artificially incubated eggs of F. p. anatum, mean time from pip to hatch was 47.8 h. During this period, chick periodically works to break up area around initial pip but rests most of time. Human imitation of parental chip call stimulates chick to vocalize.”
  4. Take the first breath: The large end of the egg has an air sac beyond the inner membrane. The chick breaks the membrane and breathes for the first time.
  5. The pip: The chick bumps the shell with his egg tooth, a curved ridge of his beak that’s sharp enough to crack the shell. He has a hatching muscle on the back of his neck to help him along. In this way he creates a pip hole.
  6. Resting: This is hard work! The chick rests a lot between activities.
  7. Making a seam: After the pip, the chick taps and cracks his shell all the way around. 
  8. Hatchling emerges: When the seam is complete, the chick pushes with his feet to split open the two pieces of the egg. His mother may move the shell away.
  9. He rests and dries off: The chick is tired from this activity so he lies quietly, waiting for his down to dry. Ta dah!

In the photo below, Dorothy examines her fourth chick of 2010. His opened egg shell lies nearby. He is still wet and pink.

Dorothy examines her fourth chick, 22 April 2010 (photo from the national Aviary falconcam at Univ of Pittsburgh)

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.

(Credits: photo of a chicken emerging from its egg from Wikimedia Commons.  Click on the caption to see the original.  Dorothy examining her 4th chick in April 2010. Today’s Tenth Page is inspired by page 460 of Ornithology by Frank B. Gill.)

p.p.s. In precocial species, such as ducks, 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.

How Can They Sit For So Long?

Bird Anatomy, Nesting & Courtship, Tenth Page

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.)