At the end of the year I’m at the end of the alphabet, but these letters are actually about the sex chromosomes of birds.
Mammals have sex chromosomes called X and Y which determine the sex of the individual. A mammal embryo is born female if it has two of the same chromosomes: XX. It’s male if it has two different chromosomes: XY.
Birds are similar but very different. Like mammals they have two sex chromosomes but the structure and origin of these chromosomes are so different that they’ve been labelled W and Z. They also combine in the opposite way to determine the sex of the individual. Female birds have two different sex chromosomes: ZW. Male birds have two of the same: ZZ.
In birds, unlike mammals, nearly every cell has its own sexual identity so if an aberration occurs during the first cell division of a bird’s fertilized ovum, the resulting individual can be half-male and half-female, neatly divided down the length of its body. These unusual individuals are called “bilateral gynandromorphs.”
Pictured above are three evening grosbeak specimens from the Smithsonian*. One is male, one is female and the third (at the top of the photo) is a bilateral gynandromorph. It’s right half is dull like the female. Its left half is bright yellow like the male. This sexual difference continues inside its body where its organs are female on the right and male on the left.
Gynandromorphs are rare but have been documented in a variety of bird species. It’s not seen in humans because most of our embryonic cells are sex-neutral. Hormones, not the individual cell, govern our sexual characteristics.
Click here to see more photos of bilateral gynandromorphs.
(photo from Flikr by ap2il, licensed under the Creative Commons License 2.0. Click on the image to see the original where one of the keywords is Smithsonian *hence my assumption on the location of these specimens.)
How do birds survive when it’s cold or food is scarce? They live off their fat reserves.
In cold weather, warm-blooded animals burn more energy to maintain their body temperatures. Pound for pound, fat’s the best to burn because it provides twice the energy of protein and carbohydrates. Polar explorers know this, so when they have to travel light they carry fat for food: butter, chocolate and nuts.
Birds prepare for scarcity and cold by eating more and storing fat under their skin. At first the fat is in discrete patches but as the bird gains weight the fat comes a continuous subcutaneous layer. You’ve probably seen this on the chicken you buy at the grocery store.
Shown above is a magnolia warbler in late fall with bulging yellowish fat reserves under its belly skin. This bird was banded and photographed at Powdermill Avian Research Center where the bander blew on its belly feathers to assess the bird’s fat reserves and fitness for migration. (Fat reserve information is noted for all banded birds.)
Large birds can store more fat on their bodies and go longer without eating. A warbler might not survive a day without food in 33o to 50oF temperatures but an American Kestrel with a fat supply can last five.
The champion of fat storage is the male Emperor Penguin who fasts for two to four months during the Antarctic winter while incubating his lone egg and waiting for his mate to return from the sea. He prepared for this feat by nearly doubling his body weight. Good thing he did!
This week it’s been quite cold so we’re all stoking up our fat reserves. That’s why the birds — and we — are so hungry right now.
(photo linked from Powdermill Avian Research Center. Click on the photo to see the original. And my thanks to Frank B. Gill’s book, Ornithology, which supplied much of this information.)
When I was a kid I would try to fly by holding my jacket open on windy days. This didn’t work because I was too heavy and my “wings” were too short for the wind to lift me.
Weight is clearly a disadvantage if you want to fly. The more you weigh the bigger your wings have to be and, as we learned a year ago, there is a limit to how big you can be and still replace your flight feathers in a reasonable amount of time.
To adapt for flight, birds lightened their skeletons by evolving hollow bones. This sounds fragile but the bones are strong because they are braced internally by tiny trusses. You can see these trusses as a network inside the outer edge of the bone pictured above or click here to see a drawing that shows how engineers borrowed this structure to strengthen bridges.
Not all birds have hollows bones. Loons, for example, dive deep underwater for their food. For them buoyancy (air inside hollow bones) would be a disadvantage, so their bones are solid.
(photo of a bird’s hollow bone linked from Renn Tumlison’s Nature Trivia at Henderson State University. Click the photo to see the original and Dr. Tumlinson’s description of hollow bones.)
original was at www.hsu.edu/uploadedImages/Biology/hollow%20bone.jpg
One of the most fascinating things about birds is that they can perch while asleep and not fall off the branch.
We know from experience that our hands can grasp things while we’re awake but when we fall asleep our hands relax and drop what we’re holding.
Why doesn’t this happen to birds?
Songbirds’ feet work quite differently than our hands. Perching birds have a long tendon that starts at the calf muscle, extends around the back of the ankle and travels down the insides of each toe. When the bird squats the tendon is pulled tight and it, in turn, pulls the toes closed. When the bird stands tall, the tendon relaxes and the toes open.
In the illustration above I’ve drawn the calf muscle and tendon in red. The “ankle” is the sharp bend in the bird’s leg shown just under its wing. According to Frank B. Gill’s Ornithology, songbirds also have a special system of ridges and pads between the tendons that assist the natural locking mechanism.
So that’s how they do it. When a songbird relaxes, its feet grasp more tightly.
That’s how they sleep without slipping.
(Image altered from Chester A. Reed, The Bird Book, 1915. In U.S. public domain via Wikimedia Commons. Click on the image to see the original.)
Today’s anatomy lesson was inspired by Michelline who asked why she sees only peregrines’ neatly folded talons when they fly. Where do the rest of their legs go?
The bones in birds’ legs are of nearly equal length and the hinges are opposite like an accordion. This has two advantages: They can lower themselves straight down to sit on their eggs without tipping over and they can retract their legs to a nearly flat position in flight.
To illustrate this I’ve highlighted the legs in red and numbered the joints:
- From the body to joint #1 is the thigh (femur)
- Joint #1 to #2 is the shin (tibiotarsus) and calf (fibula)
- Joint #2 to #3 is the foot (tarsometatarsus)
- Joint #3 to the end are the toes.
On peregrines it’s rare to see all those segments. Their legs are much longer than we think!
The blue arrows show how birds fold their legs when they fly. In step (a) the thigh and shin fold up flat to the body and are hidden in the body feathers. In step (b) the foot and toes can do several things:
When you see only a peregrine’s yellow toes in flight it’s because his feet (which we call “legs”) are extended backward and covered by his body feathers.
Aeronautical engineers learned from birds. Watch a jet take off and you’ll see it retract its “legs” under its wings.
(bird skeleton by W. Ramsay Smith and J S Newell, 1889, via Wikimedia Commons, altered to illustrate the leg. Click on the image to see the original.)
Six days from now most Americans will get to see a bird skeleton. After Thanksgiving dinner is over many will save the wishbone, dry it out, and pull it to make a wish.
What is this bone?
The real name of the wishbone is the furcula or “little fork.” It’s actually the bird’s collar bones fused together in the center. It acts as a spacer between the bird’s shoulders, strengthens its skeleton for flight and may even help it breathe.
Because the furcula is U-shaped, it works like an elastic spring when the bird flies. On the downbeat the U opens wide, on the upstroke it returns to the resting position.
Open, rest, open, rest. Imagine how fast the furcula vibrates inside a hummingbird!
Save the furcula from your turkey next Thursday and before it dries notice how flexible the U is and how well the fused center holds.
Dry the furcula for three days. Then find someone to pull it with you.
I hope you get your wish.
(photo from the blog wheniwas8. Click the photo to see the blog where it appears.)
Any visit to a crow roost focuses one’s mind on the subject of bird poop. How do I to avoid it? Will it stain? Do they time this so they’ll hit me? So in today’s anatomy lesson I’m skipping over the next logical topic in bird digestion and jumping directly to the back end. Hold on to your hats!
Unlike mammals, birds have a single opening for both urinary and digestive excretions so their poop is made of two components: output from their kidneys and from their intestines.
Birds’ kidneys are a miracle of water conservation. Instead of passing urea and water their kidneys produce uric acid, a white, crystalline, semi-solid that’s not water soluble and is full of nitrogen. This makes it a good fertilizer that’s easy to collect because it doesn’t wash away.
If you were on the receiving end of bird poop and it was only made of uric acid, it wouldn’t immediately stain your clothes. You could probably scrape it off. Unfortunately the second component of bird poop — digestive waste — is disgusting and it can stain. Pokeberries make purple marks no matter whether you daub them on your clothes yourself or receive a little “gift” from a bird. Need I say more?
You may not have noticed, but birds poop just before they fly to lighten their load. I sometimes amaze my friends by remarking, “That hawk is about to take off,” and then it does. They don’t know I just saw it poop.
Some people think birds are aiming for them. “Do they time this so they’ll hit me?” Not exactly. If you stand below a flock of birds and give them the creeps they’ll get ready to fly. When they lighten their load you might get hit, but they weren’t aiming for you.
Which brings me full circle to the crow roost and a word to the wise: The absolute worst time of day to be on the ground under the roost is just when the flock disperses at dawn.
Wear a hat. Better yet, wear a rain slicker and carry an umbrella.
(photo of the Ruskin Avenue sidewalk below the crow roost, by Kate St. John)
Last week we learned that the components of birds’ digestive tracts are in a different order than ours so that the heaviest parts are at the center of gravity while they fly.
Birds chemically digest their food (proventriculus), then “chew” it (gizzard). If they swallow something indigestible and bulky, they regurgitate it as a pellet. Some species even get nutrition from normally indigestible substances, a talent that has further modified their digestive systems. The yellow-rumped warbler is one example.
Wax is impossible to digest for most animals and birds. Its description as a “saturated long-chain fatty acid” even sounds dangerous (saturated! fatty!) yet the yellow-rump depends on wax for its winter food. This makes it unique among warblers, most of whom eat insects and must leave North America by September to survive. The yellow-rump sticks around because it switches its diet to wax-coated bayberries.
How has the yellow-rump’s digestive system adapted to do this? They have higher levels of gall bladder and intestinal bile-salt than other birds and their digestive system absorbs the food more slowly. They probably even process it for a longer time, possibly moving it back and forth so the gizzard can grind it again.
The yellow-rumps’ love of bayberry myrtle also gave them an alternate name. The eastern subspecies is called the “myrtle warbler.”
So now you know why yellow-rumped warblers are here in the winter: They’re wax eaters.
(photo of a springtime yellow-rumped warbler by Chuck Tague)
As I said last week, birds have the same basic internal equipment that we do but the location and shapes of their body parts are modified because they fly.
So here’s a puzzle. Where are their teeth?
Millions of years ago the ancestors of birds had teeth but modern birds don’t have even a vestigial tooth. Yet they eat food that ought to be chewed: meat, nuts, and entire mice and fish swallowed whole.
Birds do indeed “chew” their food but not in their mouths. Teeth are heavy equipment for the front end of flying animals and if they had to escape suddenly while chewing a big meal, the food would add extra weight to their heads, a real challenge to flight.
Birds’ bodies have an elegant solution to these two problems. The chewing mechanism and main holding compartment are the same organ, the gizzard, located at the center of gravity under the wings.
The gizzard is a muscular stomach that breaks up food by grinding it with the grit birds eat to aid digestion. The gizzard grinds and turns the food among the grit, breaking it into smaller bits the same way we chew with our mouths.
As you can see from the diagram, the gizzard is the third digestive organ in most birds. The first is the crop, a bulge in the esophagus where food waits to be processed. The second is the glandular stomach or proventriculus where enzymes break down the food before passing it to the gizzard where it’s “chewed.”
So now you know. Birds’ teeth are on the inside.
(image of a chicken’s digestive tract, linked from “Pluck & Feather” an urban farming blog. Click on the image to see it in its original context.)
I thought my anatomy lessons were nearly over because I was running out of material. Then last night Chuck Tague presented an excellent program at the Wissahickon Nature Club on how bird anatomy is adapted for flight. Now I’m inspired.
What most impressed me is that birds have the same basic internal equipment that we do — lungs, backbone, arms, toes, etc. – but the location, proportions and shapes of their body parts are altered because they fly.
For instance, human heads can afford to be heavy (and they are!) because we walk upright and easily balance our heads at the top of our bodies. Birds’ heads cannot be heavy unless something equally heavy balances them horizontally at the other end. Their solution is to have lightweight heads and alter the shape of their bodies to change the weight distribution.
Which leads me to the keeled sternum or breastbone. It provides the anchor for the flight muscles. Notice that it’s huge and sticks out! When a bird flies its keel is positioned in the air the same way a boat’s keel is positioned in the water — one of many reasons why the sternum takes this shape.
If humans had keeled breastbones we’d tip over as we walk. Instead our sternum is flat and positioned vertically.
There are a few birds who don’t have keeled breastbones and they are… can you guess?… birds that don’t fly. Ostriches, emus, cassowaries, rheas and kiwis all have flat sternums. A keel would get in their way and possibly throw them off balance as they walk. Their unusual sternum (for a bird) gave their group a name. Ratites means “raft-like sternum.”
(photo from Wikimedia Commons. Click on the photo to see the original.)