Several weeks ago I declared an end to my Friday bird anatomy series but I found this interesting diagram so here’s an unscheduled lesson.
Field of view is the angular extent of vision at any given moment. It’s basically “all you can see without moving your eyes.”
Prey species, like pigeons and robins, usually have a very wide field of view because they need to see danger coming from any direction. To achieve this most of their vision is monocular, like our peripheral vision, with only a narrow angle of binocular vision with good depth perception. It’s so important for them to see what’s coming that some prey species can move each eye independently!
As shown above a pigeon can see nearly 360 degrees around its head, a real advantage when avoiding a peregrine.
Predator species usually have a narrower field of view because they need to have good depth perception in order to capture prey. The owl’s field of view is more like ours with a wide area of binocular vision and narrow bands of peripheral, monocular vision on either side.
Peregrines and people have fields of view similar to the owl’s. Ours is actually wider than the diagram. We can see 180 degrees.
Here’s how you to find your field of view, which is basically a test of your peripheral vision. Hold up your index finger in front of your nose and close one eye. While looking straight ahead, move that finger around your head toward the ear near your open eye. When you can no longer see your finger, that’s where your field of view ends.
Now find out where your binocular vision ends. Open your closed eye, close your open eye (i.e. switch eyes). Look straight ahead and move your finger in the same direction as before. When you can no longer see your finger, that’s where your binocular vision ends.
Of course, these tests only work if you have good vision in both eyes.
Fifteen months ago I started this Friday series on bird anatomy as a project for the winter of 2009-2010. Now, 67 entries later, I’m declaring that winter is over and freeing up my Fridays for time to write about spring flowers, bird migration, and peregrines. This is the last regularly scheduled anatomy lesson but it’s not the last blog I’ll ever write on anatomy. Who knows? I might change my mind and restart the series in July. 😉
Today’s topic was suggested by Tony Bledsoe when I posted a photo of a warbler whose belly feathers were blown aside to expose its fat reserves. We can see birds’ belly skin because their feathers are arranged in tracts.
Unlike mammals whose hair sprouts uniformly from the skin, most birds’ feathers sprout in tracts called pterylae with bare patches of skin between them. Pterylae are like forests of feathers and that’s what the word means: pteron is “feather,” hulé is “forest.” The bare patches between them are called apteria: “without feathers.”
Shown above are the pterylae of a rock pigeon. You can see from the illustration that there are apteria on the neck and belly but we rarely see a bird’s bare skin because their feathers fan out to cover their bodies. It’s interesting to realize that the bare spot on the belly is a good beginning toward a brood patch for incubating eggs and brooding young.
Some birds are exceptions to this rule. Penguins’ feathers sprout uniformly across their bodies. You can’t blow on a penguin’s belly and see its skin.
I’ll bet the lack of pterylae explains why penguins don’t use their bellies to incubate their eggs. They use their bare feet!
Scientists who study birds’ brains long ago discovered that, just like humans, birds can be right-handed or left-handed.
In humans, dominance on the left side of the brain results in right-handedness and vice versa. Birds’ brains have functional lateralism too and can show behavior that indicates they favor one “hand” over the other.
An easy way to tell this is on birds whose eyes face sideways (instead of straight forward) because they obviously use one eye or the other for important tasks. What eye do they use to scan for predators? In 2001, Franklin and Lima found that most dark-eyed juncoes use their right eyes.
Crossbills take “handedness” one step further. Their bills cross either to the right or the left and they walk the pinecones on which they feed in a clockwise or counterclockwise direction depending on the “handedness” expressed in their bills.
So, what do you think? Is this crossbill right-handed or left-handed?
(photo of a white-winged crossbill by Raymond Barlow. Inspiration and information from Ornithology by Frank B. Gill)
Preening is very important to a bird’s health and well-being. If her feathers aren’t in top condition she’ll lose warmth and perhaps some flight ability.
Birds spend hours every day methodically preening their feathers, using their bills to spread oil from their preen glands, align the feathers and remove parasites.
Many tiny parasites have evolved to eat feathers. Chewing lice eat the down and barbules, leaving the vane and barb structure. This gives damaged feathers a thin, almost see-through appearance.
Since their bills can’t reach their head and neck feathers, birds use their feet to vigorously scratch away the parasites. This works so well that the reverse it true. Those who are crippled or have lost a foot carry a heavy parasite load.
If you look closely at this female goldfinch you’ll see that the feathers on her scratching side look thick and normal but those on her non-scratching side look thin. I wonder if this is feather damage. Poor dear.
Even though this turkey’s chin is scruffy, that’s not where his beard is.
The “beard” on a wild turkey is that cluster of long hairlike feathers sticking out of the center of his chest. They average nine inches long.
Generally only male turkeys have beards but 10 to 20 percent of female turkeys grow them as well. This poses a problem for those ladies during Spring Gobbler hunting season when only bearded (i.e. male) turkeys can be hunted.
Don’t worry about this turkey, though. He’s probably safe all year long because he’s a regular in Cris Hamilton’s back yard.
A few days ago Steve Gosser posted this video of his grandmother’s talking budgerigar. Amazingly, the bird speaks in complete sentences.
How and why do parrots learn to do this?
Birds sing using their syrinx, a fancy two-sided voicebox with muscles that can control each side independently. This allows the bird to sing harmony with itself, something that wood thrushes are especially good at.
Songbirds, parrots and hummingbirds(!) learn their songs. The rest make appropriate sounds but don’t improve upon them.
Most songbirds learn during a sweet spot of time while they’re growing up. In white-crowned sparrows this is at 10 to 50 days old, and then they’re done. Mockingbirds, on the other hand, learn new songs throughout their lives.
Birds learn by listening to and memorizing the phrases and song traditions of adults in their area, though they don’t practice them at first. After they’ve memorized the audio template, they begin practicing out loud to match it. Studies on the brain waves of zebra finches show that they think about their songs while asleep and practice in their dreams!
Because parrots are social birds, they learn and practice the song traditions of their flock in order to become part of the group. For pet birds, their flocks are the members of their household so they learn the phrases they hear and repeat them when the flock is happy together or when they want attention (as in “Flock, come here!”).
Even so, it’s impressive when a bird speaks in complete sentences. Turn up the sound on your computer and listen to Steve’s video. This bird is a virtuoso!
Did I tell you I live in Pittsburgh where the Steelers are playing the AFC Championship game against the New York Jets this Sunday?
It’s a rare day that bird anatomy is related to the Steelers, but today is that day. Here’s how it happened.
For many weeks I’ve been using Frank B. Gill’s Ornithology to inspire Friday’s anatomy lesson. Rather than read the whole book I open the index at random and with my eyes closed I point to a word. Then I look up that word and find something interesting to write about. Today’s word was “Yellow-throated Brush Finch, page 328.”
Page 328 discusses the advantages of multispecies flocking. Many species form mixed flocks because they get more to eat when there are many eyes watching for danger. In Pennsylvania we often see mixed flocks in winter led by titmice and chickadees. The leader species are dominant, the other species are subordinate.
Some birds go one step further. Ornithology describes how in some mixed flocks “unrelated bird species have similar plumage color patterns that promote flock cohesion. Subordinate species increase acceptance by resembling dominant flock members.”
These distinctive color patterns are called flock “badges.”
The yellow-throated brush finch (bottom right) is a member of one of these unusual flocks in Western Panama. His compatriots are all yellow and black.
As I assembled this photo, I suddenly realized that the brush finch and all his friends are wearing Steelers colors. It’s a whole flock of black-n-gold birds! How cool is that?!
So this is what we look like in Pittsburgh right now.
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!
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!
On this last day of the year we’re at the end of the alphabet with a special significance. W and Z are 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.)