On Gulls and Physics

A couple of days of above-freezing temps has turned the ice sheet that was North Point Marina into this:

 When you see this sort of ice/water mix here, it's gull time! Unfortunately, I didn't have much time to look today, and all I could find were various ages of Herring Gull (Larus argentatus), like this one:

Herring Gull (Larus argentatus), N. Pt. Marina, Lake Co, IL 1/12/14

This location has lots of gulls loafing in it for two reasons: first, there's two landfills (and a Wal-Mart parking lot) within easy flying distance, and second, whether it's thin ice like this or the docks, there's places to hang out that a predator can't reach without being perfectly obvious about it.

But this shot shows the sort of thing we never even think about, it's so common:

 Do you see it? All of the birds are facing the same way!

This is something birders often take advantage of -- fact is, gulls are easier to ID from some angles than from others. (Looking at the front of the bird is actually the worst angle -- most of the ID traits are in the back and wingtips.) But why should it be? There's no speaker, band, or movie screen up there, so why not randomly, or in little conversational clusters? (Okay, that last one's a stretch. Still, one would think that some sort of social interaction would cause a different pattern here.)

The answer has to do with how most animals have to live their lives, and with physics. Specifically, the physics of flight. First, the first rule to survival is Don't Get Eaten! When you look at what gulls feed on, you might think they don't taste good enough to bother with, but if you're a Fox or a Peregrine Falcon you can't afford to be picky. So a gull that's awake is a gull that's at least thinking about escaping any predators that might come by.

Second, the physics. Flight depends upon the interplay of two forces -- lift, and gravity. (We'll ignore thrust and drag here.) Gravity's a constant. Lift, though, depends upon the shape of the wing, the way the wing's held, and the speed of the air flowing past the wing. If that air is too slow, gravity becomes stronger than lift, and the bird drops (or simply can't take off, in this case). In still air, a bird the size of a Herring Gull may have to run several steps before they can leave the ground. With a strong enough tailwind, they may not be able to take off at all, since the airspeed would equal their running speed minus the wind speed. On the other hand, facing into the wind, the airspeed equals their running speed plus the windspeed -- which can be a definite advantage on the sort of windy days that beaches are so fond of.

Pennycuick (1997) calculated a minimum power flight speed of approximately 29 miles per hour for Great Black-backed Gulls (L. marinus), closely related to Herring Gulls. (1) That's the speed at which the bird uses the least energy for a given distance of flight, not the slowest speed they can stay aloft at, which would certainly be lower. Sustained winds when I took these photos were 20 mph -- which suggests that on a day like this, gulls might well be able to get airborne simply by raising their wings! (In fact, this is something that's fairly easy to see on a windy day at the beach.) On the other hand, if the birds were to try to take off with the wind, they'd have to run 30 to 40 miles per hour -- as fast as a racehorse!

This has been a fun little diversion into physics, (well, I thought so, anyways), but with a purpose. It's very easy to specialize in a field these days -- in fact, with the huge increase in literature, it's pretty much impossible not to specialize to some extent. But often input from other fields can have surprising impacts on our own. Darwin claimed his key insight in formulating Natural Selection came from reading Malthus -- an economist, which is hardly the field you might expect a biologist to find useful! So whether it's that class the college insists you take, that book that's never quite found it's way off your bookshelf, or that odd little museum down the street you've always wondered about but never bothered to look at, keep finding new things to learn.

Which I guess is quite a leap from an icy harbor mouth full of gulls...., but there's no other way to fly!

(1) C Pennycuick. (1997) "Actual and'optimum'flight speeds: field data reassessed" Journal of Experimental Biology, 200, 2355-2361.