22 October 2010

I thought my blog’s anatomy series was 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.)

Waterfowl are just beginning to migrate through Pennsylvania so now’s a good time to learn the name of an important fieldmark on ducks.

The speculum is a patch of distinctive color on the wing.  It is made up of secondary feathers and is usually iridescent on dabbling ducks, as shown here on this female mallard. 

It’s a useful fieldmark when trying to identify mallards and American black ducks.  The speculum on mallards is blue with white borders.  On black ducks it’s purple without white borders.  Unfortunately mallards and black ducks can hybridize and the result can be confusing!

(photo by Chuck Tague)

An eyering is a ring of color around a bird’s eye.  It can be composed of orbital feathers or bare skin. 

Bare skin eyerings are often sexual cues for the birds who have them.  Among peregrine falcons the eyerings on adults are yellow, on juveniles they’re blue-gray.  This color difference is a cue that the juveniles are not breeding threats.  The juveniles’ brown (instead of gray) plumage and blue-gray cere and eyerings probably save them from being attacked when they pass through adult breeding territories.

Eyerings are a useful fieldmark, especially among species that are otherwise similar.  Connecticut warblers look similar to mourning warblers but Connecticuts have white eyerings.  Many thrushes are similar, but Swainson’s thrushes have buffy eyerings. 

A striking example of eyerings is the bright orange skin around the eyes of killdeer in the spring.  The skin becomes intensely orange when breeding is about to begin and seems to shout, “I’m ready.”

There’s no need to mark up this photo.  The eyering is obvious.  This killdeer is ready to breed.

(photo by Chuck Tague)

If you look closely at this common yellowthroat’s eye you’ll see something like eyelashes surrounding it.  Those “eyelashes” are actually feathers called orbital feathers.

Not all birds have orbital feathers — some have bare skin — but in a beauty contest for long eyelashes ostriches would win the prize.  (Click here to see.)

How amazing that their eyelashes are made of feathers instead of tiny hairs!

(photo by Chuck Tague. Check out his Birds’ Eye Views photo gallery.)

As promised the tall rooster from last week is back again, this time facing the other direction.

Today he has a blue arrow indicating his comb.

Many birds have feathery crests but the heads of gallinaceous birds (turkeys, chickens, etc.) are adorned with a fleshy growth called a comb or cockscomb because the growth is larger on males (cocks) than on females.

The comb on this rooster is quite impressive and helps him stay in charge of the flock.  Power and sex is what wattles and combs are all about.

Would you willingly annoy a rooster with comb like that?

“Not I,” said the little red hen.

(photo by Ron Proctor on Wikimedia Commons.  Click on the image to see the original and its attribution.)

Before I began researching this lesson I thought a wattle was merely the fleshy skin that hangs from the throats of some birds.  But according to Wikipedia a wattle “is a fleshy dewlap or caruncle hanging from various parts of the head or neck in several groups of birds, goats and other animals.”

That means these roosters have wattles (the red fleshy flaps) hanging from their throats and from their cheeks.

The tall rooster has more to tell us.  He’ll be back next week.

(photo by Ron Proctor on Wikimedia Commons.  Click on the image to see the original and its attribution.)

Marcy’s chipping sparrow has agreed to be a model again this week.  Today’s he’s showing off his eyestripe, indicated by the yellow arrow.

The eyestripe is a line of feathers, usually dark, that appears to pass through the eye.  It’s a good field mark for sparrows.

If you’ve been following these anatomy lessons, you now know all the field marks that describe this chipping sparrow’s head:  rusty crown, white supercilium (or eyebrow), black eyestripe, and gray auriculars (or cheeks).

(photo by Marcy Cunkelman)

Today’s anatomy lesson is about eyebrows.

The supercilium, sometimes called the bird’s eyebrow, literally means “above the eyelid.”  It’s a set of facial feathers which extend above the eye from the bird’s beak to the back of its head. 

Often this feature is light colored and, on some birds, it doesn’t extend all the way to the back of the head.  In any case, it’s an important field mark and quite useful when identifying sparrows. 

Here, a chipping sparrow at Marcy Cunkelman’s feeder poses to model his white supercilium (indicated by blue arrow).

(photo by Marcy Cunkelman)

A bird’s chin is exactly where you’d expect it to be — just under its beak — but of course birds’ chins don’t stick out like ours do.

Shown at right are common redpolls whose feathers give them a black chin (indicated by the blue arrow).  From a distance their black faces and chins make their yellow beaks stand out. 

I wish I could have chosen a Southwestern bird, the black-chinned sparrow, to illustrate this blog but I could not find a photo that I had permission to use.  Instead you’ll have to click here to see how stunning a gray bird with a black chin can be.

(photo by Mark McConaughy taken in January 2008 during a Common Redpoll irruption in western Pennsylvania)