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.
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.
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.
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 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.)
Though I haven’t seen his mate there must be a nest because he defends the area from all potential threats. Yesterday morning I was pleased to see a second vote for the wetland when he had to chase off the competition — another male red-winged blackbird.
Shortly thereafter one of the resident red-tailed hawks flew in to perch on a dead snag. Mr. Red-wing was on him right away!
Though I didn’t record this video, it shows exactly what happened. The blackbird perched above the hawk, shouting and flashing his red epaulettes. He repeatedly dive-bombed the hawk and pecked its back.
At first I thought the red-tail would ignore the red-wing but he could not be ignored. The hawk whined and flew to shelter under the roadbed of the Panther Hollow Bridge.
Persistence pays off. In the match-up between Red Wing and Red Tail the blackbird wins.
(video on YouTube from Illinois’ Lake County Forest Preserve District)
p.s. The red-tailed hawk in this video is a juvenile so he whines a lot more than the adult at Schenley Park yesterday.
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.)
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.)
For 30 years Charles Brown and his wife Mary Bomberger Brown have studied cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska. They’ve meticulously monitored, measured and banded the birds at their nests under bridges and overpasses and they’ve counted and measured the road killed birds.
Their attention to detail has paid off in an unexpected way.
Cliff swallows attach their mud nests to cliffs or bridges. In Nebraska where there are few cliffs, the swallows use busy highway overpasses. If the swallows aren’t quick to fly up out of traffic they become road kill.
When the Browns began their study in 1982 they typically found 20 road killed cliff swallows per season, but since 2008 they’ve usually found less than five. The traffic has remained the same while the swallows’ population has more than doubled, yet the road kill numbers dropped dramatically.
What changed? The swallows changed!
The Browns’ data reveals that thirty years ago Nebraska’s cliff swallows had longer wingspans. Today’s shorter wings allow the birds to maneuver more quickly and turn away from oncoming vehicles. In fact, the few road killed birds they find today have longer wings than the rest of the population.
The shorter-winged birds survive to breed, the long-winged birds do not. In only 30 years, traffic’s unnatural selection has forced cliff swallows to evolve.
If traffic can do this to cliff swallows, I wonder what it’s done to Pennsylvania’s white-tailed deer.
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:
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.
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.
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.
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.