Category Archives: Tenth Page

He Stoops To Conquer

Peregrine falcon, Mo, tucks his wings in a stoop (photo by Chad+Chris Saladin)

Peregrines are famous for speed when diving on their avian prey.  The dive was named a "stoop" because the word means "to bend the head or body downward and forward."

The stoop is amazing in many ways:

  • Peregrines dive at a 30 to 60 degree angle.
  • They may start the stoop 5,000 feet away from the prey and drop 1,500 to 3,400 feet in altitude.  These distances are exceeded when a falcon sky-dives with a falconer.
  • Land-based speed calculations have clocked them at 100 to 273 miles per hour.  Falconer Ken Franklin sky dives with his falcon at 242 mph.
  • Peregrines can accelerate from 100 to 200 mph in eight seconds according to Ken Franklin.
  • At 150 mph they tuck their wings tight and extend their shoulders, making their bodies into a diamond shape.
  • At 200 mph peregrines pull in their shoulders and extend their heads to become extremely streamlined.
  • Because their acute vision is at a 40 degree angle, they reduce drag and keep an eye on their prey by not diving straight at it.  Instead they spiral downward keeping the prey to the side so they can see it.  Their logarithmic spiral is rarely noticeable from the ground.

Here are three examples of diving peregrines, thanks to Chad+Chris Saladin.

Above, Mo is tucked into an arrow shape in Canton, Ohio.

Below, Rocky at Cuyahoga Valley National Park shows how peregrines hold their wings slightly open at the shoulder.  If he was going faster his shape would be more angular.

Peregrine falcon, Rocky, in wing-tuck stoop (photo by Chad+Chris Saladin)


And finally, Dorothy and E2's son Henry shows off his flying prowess at Tower East in Shaker Heights, Ohio.   His angle of attack is dramatic but he's not traveling so fast that he has to tuck in his wings.

Peregrine falcon, Henry, stooping in Shaker Hieghts Ohio (photo by Chad+Chris Saladin)


He stoops and conquers.


(photos by Chad+Chris Saladin. Today’s Tenth Page is inspired by page 122 of Ornithology by Frank B. Gill.)


p.s.  She Stoops To Conquer is a play by Oliver Goldsmith first performed in 1773.

Special Gear For Young Fliers

Juvenile peregrine falcon at University of Pittsburgh (photo by Colette Ross)When young peregrines fly for the first time they're specially equipped for their big adventure.

Like many raptors, peregrines' tail feathers are longer in juvenile plumage than in adults.  In peregrines it averages more than a centimeter.  In red-tailed hawks the difference is even greater but the effect is the same.  Longer tails give the birds more lift "by improving airflow over the wings, especially at slow speeds, and by reducing turbulence as air passes over the body." (1)

The added lift makes the juveniles' flight more buoyant than their parents' and is a great help as they learn to fly and hunt.

By the time they molt into adult plumage a year later, young peregrines have mastered the skills they need and are ready for speed.  In the meantime they have special gear to help them fly.

Think of their tails as "training wheels."


(photo by Collette Ross.  Today’s Tenth Page is inspired by and quoted from page 131 of Ornithology by Frank B. Gill. (1))

Feather Atlas

Mothers’ Work

Mallard with ducklings (photo from Wikimedia Commons)

We tend to think that birds with precocial chicks have an easier time as parents than those whose nestlings are naked and blind at birth, but this isn't necessarily so.

Ducklings can walk, swim and feed themselves shortly after they hatch but their mobility is problematic.  They have no idea where to find food nor how to stay safe.  All they know is "Stay with Mom!"

Mother leads them to feeding areas and shows them what to taste.  The ducklings peck in the vicinity until they find good food.

Her hardest responsibility is protecting them from danger.  Baby ducklings are tasty morsels for raptors, minks, cats, dogs, large fish and snapping turtles.  If you watch a mallard family day to day you'll notice the number of ducklings decreases over time.  Mom does her best but danger lurks.

This mother mallard has had pretty good success so far.  Out of 8 to 13 eggs she still has six chicks.

Until they can fly she has mothers' work to do.


(photo from Wikimedia Commons. Click on the image to see the original. Today’s Tenth Page is inspired by page 483 of Ornithology by Frank B. Gill.)

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.)

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.)

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.)

Turning The Eggs

Dorothy turns her eggs (photo from the National Aviary falconcam at the Univesity of Pittsburgh)

During incubation there's not a whole lot of activity at a bird's nest except for this:  Mom (or Dad) periodically stands up, stares at the eggs and draws each one toward her with her beak.  She's not just rearranging the eggs, she's turning them.

Other than a few notable exceptions, all birds turn their eggs because it's required for the embryos' survival.  For instance:

  • The temperature in the middle of a clutch is warmer than the edge.  Birds move the outer eggs to the middle to keep them evenly heated.
  • In the early days of incubation, it's important that the embryo floats inside the egg while the membranes that support its life are growing and developing.  Turning optimizes membrane growth.
  • Eventually the chorion and allantoic membranes will be pressed to each other and to the shell.  If these membranes adhere too soon the chick will not be able to move into the hatching position later and get out of the egg.  Turning prevents premature adhesion.
  • The albumen (the egg white) is the embryo's fluid cushion and water supply.  Turning the egg optimizes the fluid dynamics of the albumen so the chick can absorb it properly.

Egg turning is so important that it's a wonder some species don't do it.  One notable exception are the megapodes who lay their eggs in compost heaps and let the heat of the decomposing vegetation incubate them.  No turning there!

I'd rather watch a peregrines' nest where things are happening, if only a bit of egg turning.


(photo of Dorothy turning her eggs from the National Aviary falconcam at University of Pittsburgh.  Today’s Tenth Page is inspired by page 460 of Ornithology by Frank B. Gill.)

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.)

When Will The Robins Nest?

American robin on nest (photo by William Majoros on Wikimiedia Commons)

Spring is moving north and so are the robins.  This week a big wave arrived after Monday's snow.  Now that they're here, how soon will they nest?

Robins nest later the further north you go.  In 1974 Frances James and Hank Shugart were curious about the conditions that governed their nesting times throughout the U.S.  Using climate data and Cornell nest watch information from 8,544 robins' nests they developed a model that predicted when robins would nest in a particular region.(*)

The model shows that robins cue on weather.  Hatching is timed to occur when local humidity is 50% and temperatures are between 45 and 65 degrees Fahrenheit.  By April 23, Pittsburgh's highs and lows are exactly in that range so our birds are getting ready.  Here's what they're up to:

  • Robins spend 5-7 days building their first nest of the season. 
  • Egg laying begins 3-4 days after first nest completion.
  • Eggs are laid one per day for a clutch of 3-4 eggs.
  • Incubation lasts 12-14 days.

From nest building to hatching, the first nest takes 26 days. (Subsequent nests take less time.)

Our robins should be nest building right now except for one thing:  Do they have enough mud to begin construction?   Has the mud been frozen?

Watch the robins in your neighborhood to see what stage they're in.   Join Cornell Lab's Nest Watch program and your data can become the basis for studies like James' and Shugart's that broaden our knowledge of birds.


(Credits: photo by William Majoros on Wikimedia Commons.  Click on the image to see the original.
Today’s Tenth Page is inspired by page 260 of Ornithology by Frank B. Gill, portions of which are quoted(*) in this article.

Who’s My Nearest Relative?

Peregrine falcon, Dorothy (photo by Jessica Cernic Freeman)

Remember the first time you were puzzled by the arrangement of birds in your field guide?   Why were loons at the beginning of the book?  Why did kingfishers come after hummingbirds?

It took me a long time to get used to taxonomic order but I finally mastered it and could thumb to the right place every time.

Not anymore!  DNA testing has revealed new relationships.  The old order is shaken up.  Ducks are first, kingfishers follow motmots, falcons have moved to be near their closest relatives.

So here's a quiz:
Of the four birds shown below, which two are most closely related to peregrines?

Red-tailed hawk?                                                      Red-crowned parrot?
Red-tailed hawk by Bobby Greene, Red-crowned parrot from Wikimedia Commons


Red-legged seriema ?                                      Yellow-crowned night heron?
Red-legged Seriema (Wikimedia Commons), Yellow-crowned Night-heron (Chuck Tague)


Amazingly, parrots and seriemas are the falcons’ closest relatives. Seriemas, from South America, are actually an older species than falcons and peregrines.

The evidence first surfaced in 2006. In 2012, a proposal was made to the AOU (American Ornithological Union) to change the taxonomic order of falcons, moving them away from hawks and near parrots.  Here's a wealth of information on the move.

  • Paul Hess blogged about this in 2012 at Breaking Up The Hawks on the ABA blog.
  • The AOU Checklist is in the new taxonomic order.
  • And this link has a chart of the new relationships and descendants. Click here for a large version of the chart where the most ancient species are at the bottom, the newly evolved at the top. Falcons are a relatively new species, third from the top. Evolution saved the best for last.  🙂


(photo credits: Peregrine falcon (Dorothy) by Jessica Cernic Freeman, Red-tailed hawk by Bobby Greene, Red-crowned parrot by Roger Moore Glandauer via Wikimedia Commons, Red-legged seriema from Wikimedia Commons, Yellow-crowned night-heron by Chuck Tague.
Inspiration for this Tenth Page comes from a conversation with Dr. Tony Bledsoe, Dept of Biological Sciences, University of Pittsburgh)