Archive for the 'Bird Anatomy' Category

Jan 14 2011

Dominance


I usually reserve Friday’s blog for an anatomy lesson but today’s topic on bird behavior does have anatomy in it.  Dominance among birds, as among humans, is expressed in both behavior and outward appearance.

If you’ve watched birds at your feeder for any length of time you know some birds are dominant over others, not only between species (blue jays rule!) but among the same species (some cardinals are bossier than others).

The dominant birds tend to be physically larger than their subordinates and sometimes they’re marked differently.  This is especially true of male house sparrows who wear their status on their chests. 

Among researchers, the bib on a house sparrow is called a “badge of status” because it’s a clear outward sign of dominance.  All house sparrow bibs become fainter in winter but at any given time of year the bigger and darker the bib, the more dominant the bird.  In a contest between the two birds pictured above, the one on the left wouldn’t even attempt to challenge the one on the right.  Mr. Big Bib wins, just by showing his chest.

Because they’re unevenly matched, these two are unlikely to fight at all.  However, males with similar badge size fight more often between themselves perhaps because it’s not obvious who’s in charge.  Eventually the contests work themselves out and everyone knows his place.

Jays can avoid contests altogether by figuring out the hierarchy from afar.  Here’s a hypothetical story showing how they do it:

Two jays, Charlie and Bob, are in the same flock where Charlie knows he’s subordinate to Bob.  One day Arnold shows up.  From a distance Charlie can see that Bob is subordinate to Arnold so Charlie knows, even before he meets Arnold, that Arnold is dominant over him.  This saves a lot of trouble in the long run!

It sounds almost human.  ;)

(photos from Wikimedia Commons)

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Jan 07 2011

Anatomy: Musing On Rear Toes

Published by under Bird Anatomy


One of the fascinating things about birds is that each species is specialized and it’s expressed in so many ways, even in their feet.

A couple of days ago fellow birder Bill Parker sent an email in which he mused on the length of birds’ rear toes with photos to illustrate.  He said, “I was noticing in one photo that the Snow Bunting has really long rear toes.”

As you see in Bill’s pictures the rear toe, or hallux, on the golden-crowned sparrow (left) is normal for a perching bird, it appears to be missing on the sanderling (middle), and it’s very long on the snow bunting (right).  I’d blogged about the position of the toes but I’d never thought about their length so I did some research. 

It turns out that rear toes are highly variable.  Many wading and water birds have a vestigial hallux that’s so high on the metatarsus and so short that it doesn’t touch the ground.  That’s what happened to the sanderling. 

But there are exceptions.  On cormorants the rear toes face (vaguely) forward and are webbed with the other three.  On kittiwakes the fourth toe is gone.

Birds’ toes indicate their lifestyle.  Sparrows perch a lot so they need a grasping hallux.  Sanderlings walk on the beach (a lot!) so they don’t need rear toes.  And snow buntings are perching birds who wear snowshoes.

Even we could use a hallux sometimes. “When I’ve been on a ladder painting, I’ve wished for a rear toe like the Snow Bunting,” said Bill.

Check out the jacana’s toes.  They’ll certainly keep you on a ladder!

(photos by William Parker)

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Dec 31 2010

Anatomy: W and Z

Published by under Bird Anatomy


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.)

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Dec 17 2010

Anatomy: Fat Reserves

Published by under Bird Anatomy


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.)

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Dec 10 2010

Anatomy: Hollow Bones

Published by under Bird Anatomy


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

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Dec 03 2010

How Do They Sleep Without Slipping?

Published by under Bird Anatomy


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.)

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Nov 26 2010

Anatomy: How do birds fold their legs in flight?

Published by under Bird Anatomy


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: 

  1. From the body to joint #1 is the thigh (femur)
  2. Joint #1 to #2 is the shin (tibiotarsus) and calf (fibula)
  3. Joint #2 to #3 is the foot (tarsometatarsus)
  4. Joint #3 to the end are the toes.

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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:

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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.)

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Nov 19 2010

Anatomy: Furcula

Published by under Bird Anatomy


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.)

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Nov 12 2010

Anatomy: Excretion

Published by under Bird Anatomy


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)

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Nov 05 2010

Anatomy: Wax Eaters

Published by under Bird Anatomy


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)

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