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.
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.)
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.
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.)
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.
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:
As day length increases after the winter solstice, a bird’s hypothalamus releases LHRH (luteinizing hormone releasing hormone).
LHRH triggers the pituitary gland to release LH (luteinizing hormone).
LH increases production of testosterone in males and progesterone in females.
Testosterone triggers aggression, territoriality and sexual behavior. It’s good at the start of breeding but doesn’t help raise a family.
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.
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.)
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.)
After a week near western gulls in San Diego I got pretty used to seeing individual gulls perched high, watching the others fly by. Inevitably, the lone gull would throw his head back and give the long call when other gulls flew over. What did he mean?
The “long call” is used in many contexts, as a greeting between mates or a statement about territory. In this video two great black-backed gulls give the long call when they fight over a fish. Watch the video and I’ll tell you what I think about their interactions.
Their gestures tell the tale.
The hungry gull (HG) approaches, bowed low in a threatening gesture.
The eating gull (EG) sees the threat and opens his wings, “Back off!”
HG turns away and gives the Long Call: He hunches over, bows his head, then lifts it high leaning his body at an oblique angle and calling loudly. You might think he’s not talking to EG because he’s not looking at him. Far from it! By turning away he’s avoiding direct confrontation. Perhaps he’s trying appeasement.
That didn’t work. HG walks past EG without looking at him directly. As he approaches EG’s tail he gets an idea.
Tail pulling didn’t work at all, so the hungry gull bows low (a threat) and walks to the front of EG. Facing him and opening his wings (again, a threat), he tries to steal the fish.
Finally the eating gull has had enough. The two fight. EG quickly wins. Hungry Gull retreats while EG gives the long call in triumph, and then resumes his meal.
What’s the relative stature of these gulls? My guess is that EG (the eating gull) outranks HG (hungry gull), but HG is willing to test the limits.