Thanks for all of the comments and ideas. I am toying wth a couple of long-term solutions for providing multiple small shelters throughout the pastures. My instant but temporary solution was to pull one of our giant hay wagons into the current pasture! I also have loads of wooden pallets at my office that I can put into use quickly and fairly easily. I can even utilize an existing stretch of wooden horse fence as the base for my lean-to pallet construction attempts.
One of my plans involves building a living willow structure in the middle of one of the pastures. I have always wanted to attempt one of these and this seems like a good practical application. I am eager to find shelter solutions that work for the chickens and are both attractive and somewhat natural. Until I can establish those, the pallet idea seems to be the most workable.
I do realize, of course, that short of completely enclosing the area of the chicken "run" there is no way to completely protect the flock from winged predators. We keep a few bantams, a slikie or two, and we often have chicks and young birds that are prime targets for hawks. I think that we will establish a large enclosed run for those kinds of birds this spring, although I have no desire to contain the whole flock of standard layers in that fashion. As much as I love my individual birds and HATE to lose them, I think that a life spent as free-roaming as possible and ended quickly by a hawk is still a better life than one spent in a cage. Don't get me wrong... I am all for chicken tractors and the like! But, I have acres of land to explore and my chickens thrive on that exploration... I can't bear the thought of takig that away from them.
So, just how territorial are hawks? I wonder how effective a hawk decoy would be if I moved it around the pasture fences each day? We use this technique to keep herons away from our goldfish and koi ponds with great success. We move the heron decoy every couple of days and the real herons stay away! I think that it is past time to do some serious hawk research...