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Thursday, July 4, 2013

The King

Sometimes one picture makes you think about something in a completely different way.

I photographed this Caspian Tern (Hydroprogne caspia) in Bodega Harbor today.  Upon reviewing the pictures, I was struck by this head-on view and the tern's amazing wingspan.  I knew Caspian Terns were large, but I hadn't truly considered their wingspan before.  Do you have a guess about the distance measured from wingtip to wingtip?  (The answer is below.)

Caspian Terns have a wingspan of ~50 inches (~4 feet or 127 cm).  That's on par with a Red-tailed Hawk, a Great Egret, and a Pomarine Jaeger.  It's larger than a Red-shouldered Hawk, a Snowy Egret, and a Northern Fulmar.

Because the wingspan was so striking in this picture, Eric wondered about the ratio of the wingspan:body length and how it compared with albatrosses, well known for their long wings.

To help think about this, here are two views of Black-footed Albatrosses from Cordell Bank:

And a Laysan Albatross, also from Cordell Bank:

Somewhat surprisingly, the wingspan:body length ratios are very similar in Caspian Terns and these two albatrosses!  For both albatrosses, it's 2.5 (wingspan 80": body length 32").  For the tern, the ratio is 2.4 (wingspan 50": body length 21").  No wonder the wingspan of the Caspian Tern looked so impressive!

When describing Caspian Terns in Life Histories of North American Gulls and Terns, Arthur Cleveland Bent said it well: "...this king of all the terns may be seen climbing into the air on its long, strong wings, its big red bill wide open, yelling out its loud raucous cry of defiance."

1 comment:

nimbus said...

Fascinating! The other variable is the bird's aspect ratio -- in aerodynamics, the aspect ratio of a wing is essentially the ratio of its span to its breadth (chord). A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings. (More precisely, the aspect ratio is the square of the wingspan divided by the area of the wing planform, since the chord will vary from root to tip.)

Long narrow wings are found on soaring birds, of course, but even birds with relatively low aspect ratios can soar well if they have long enough wings relative to their body mass. The red-tailed hawk's secret is their variable-geometry -- they can stretch their wings for soaring or hovering on the wind, and then instantly fold or fan them for maneuverability (same with eagles and many other birds of prey, of course). Just as in aircraft, the geometry of the bird's tail plays an important role, too.

A shout-out to my friend Alice for turning me on to this very enjoyable blog! :D