Any parent can tell you that raising kids is hard work and even harder if there are multiple infants the same age. (Think triplets!)
Most birds experience this multiple effect every time they nest. In fact, the work is so exhausting that having “extra” kids beyond their normal clutch size decreases the parents’ life expectancy in some species.
This was shown in studies of common kestrels in Europe in the 1980s.
A team led by Cor Dijkstra artificially lowered and raised brood sizes of common kestrels by removing eggs from some nests and adding them to others. Kestrel parents whose brood size of five remained normal or was reduced to three experienced the typical winter mortality of 29%. On the flip side, adults whose broods were augmented were much more likely to die the next winter. 60% of the kestrels who raised two extra chicks were dead by the following March.
For thousands and thousands of years the clutch size of the common kestrel has been honed by the deaths of those who raised too many. The birds settled on the number five. More than that can kill them!
(photo of common kestrel nest in Germany from Wikimedia Commons. Click on the image to see the original.
Today’s Tenth Page is inspired by page 521 of Ornithology by Frank B. Gill.)
Why do peregrines nest on buildings and bridges instead of cliffs?
“Raptors imprint on their natal nest sites. Consequently, they choose a similar situation several years later when they reach maturity.”(1)
This explains why they’ve chosen to nest at the Tarentum Bridge, pictured above. The adult female, nicknamed Hope, was born on the Benjamin Harrison Bridge in Hopewell, Virginia. That bridge is such a dangerous place to fledge that Hope was hacked in the Shenandoah Mountains, but she remembered where she was born and picked a bridge when she chose a place to nest.
There are exceptions to the natal imprint rule. Though Dorothy’s daughter Maddy was born on the Cathedral of Learning, a 40-story Late-Gothic Revival building, she chose the I-480 Bridge in Valley View, Ohio. I can’t think of anything less like the Cathedral of Learning than this. (The nest is at a broken patch of concrete on the bridge support.)
The exceptions have saved at least one species from extinction.
Mauritius kestrels used to nest in tree cavities but monkeys were introduced to the island and ate the eggs and young. By the 1960′s the kestrels were down to two pairs — almost extinct — when one of the pairs decided to nest on a cliff ledge where the monkeys couldn’t reach them. That nest was successful, their youngsters nested on cliffs, and the species rebounded.
The exceptions benefit the rule.
(photo of Hope at the Tarentum Bridge (blue structure) by Sean Dicer. Photo of Maddy’s nest site at the I-480 Bridge at Valley View (busy highway) by Chad+Chris Saladin.
Today’s Tenth Page is inspired by and includes a quote(1) from page 444 of Ornithology by Frank B. Gill.)
Though Pitt’s peregrines, Dorothy and E2, are courting again today’s solstice will change that.
All living things have endogenous biological clocks that can run without light cues but we get out of synch with each other and the seasons in the absence of our external timekeeper, the sun. Today our clocks struck twelve and began to head down again.
For peregrines in northern mid-latitudes the summer solstice ends their breeding cycle (initiated by the winter solstice) and triggers molting and preparation for migration.
Molting is a chilly and energy intensive activity in which birds replace all their feathers. Since feathers provide warmth it’s cold to lose them. Growing thousands of new feathers requires protein, increased blood to the feather sites, and changes in the birds’ calcium distribution. And while flight feathers are being replaced flying is somewhat less efficient, an important consideration for precision-flying peregrines.
It makes sense to schedule this activity for a time when food is abundant and temperatures are warm. Dorothy and E2 molt their flight feathers in July and August. Good timing!
Our peregrines don’t migrate but arctic peregrines face an additional challenge. They begin their molt in the arctic but don’t have time to complete it before they must leave on migration. Their bodies have adapted by starting the molt in the arctic, pausing during migration, and resuming at their wintering grounds in South America. Very ingenious!
So when the sun paused this morning our birds got in synch.
We did too, we just don’t realize it.
(photo of a peregrine falcon tail feather from Shutterstock. Today’s Tenth Page is inspired by page 262 of Ornithology by Frank B. Gill.)
June is “air show” month for our local peregrines. Where the nests have emptied the action is in the air.
After they fledge, young peregrines are dependent on their parents for six to ten weeks while they learn to supply their own food.
Fortunately, as with all predators, they’re born with an instinct to hunt. Kittens instinctively stalk and pounce. Peregrines are programmed to chase. This means they can develop hunting skills without much parental assistance — which is why hacking works.
In their first weeks after fledging, juvenile peregrines chase anything that flies: their parents, their siblings, butterflies, even turkey vultures.
After two to three weeks they begin to focus on prey the right size. Eventually they capture something, almost by surprise.
In the meantime they play at all the right moves: chasing, mock dogfights, roll-overs, talon grappling and prey exchanges.
Above a juvenile in Wilmington, Delaware chases his sibling who won the prize.
Keep looking up and you’ll see the air show.
(photo by Kim Steininger. This Tenth Page is inspired by and quoted from page 501 of Ornithology by Frank B. Gill.)
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.
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.
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.
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))
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.)
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.)
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.)
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.
- 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.
- 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.
- 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.
- His strenuous hammering is aided by the hatching muscle on the back of his neck.
- 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.
- 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.)