The first would-be airmen tried to imitate bird flight with negligible success. When aviation took it's own independent path, speeds quickly exceeded that of birds.
Sailplanes are an exception. Their minimum airspeed is not much faster than the birds themselves. Large birds like eagles can glide in formation with us. That they often choose to do so is a matter of great delight for sailplane pilots. Silent sailplanes seem to get a aerial welcome from soaring birds that noisy airplanes don't enjoy.
Watch: http://www.youtube.com/watch?v=oQnGvqXLrJQ The beeping sound is the sailplane's audio variometer indicating strong thermal lift. The pilot is learning he can't outmaneuver an eagle.
Soaring birds work cooperatively by watching each other to detect the thermal updrafts they exploit. One bird soaring upward in tight circles will quickly attract others. A sailplane in their midst is seen as simply a larger, and possibly smarter bird. (After flying a couple of circles with us they tend to abandon that impression.) There is clearly body language communication between the birds - and with humans if we take the trouble to learn their language.
In desert areas you can see large flocks of vultures circling in thermals. They clearly define the thermal updraft serving as markers for sailplane pilots looking for lift. One has to be careful with circling vultures. Sometimes they mark thermals, sometimes a dead cow.
A South African glider pilot told a story of being followed all day by a large flock of African Gray Vultures. "What do you think they were doing?" he asked. "Waiting for you to crash?", suggested a less than generous friend.
I've had eagles fly just ahead of the wing leading edge only a few feet from my cockpit. Looking into those intelligent eyes at 16,000 feet is an unforgettable moment. Most of you will recognize the look of respect exchanged between experienced pilots. I swear, I've seen that look in the eyes of an eagle.
Once I spotted a long line of mountain climbers trudging resolutely up a trail well above treeline on Colorado's Mount Antero. Indulging in a bit of mischief, I approached silently planning my glide so the sailplane's shadow would come up from their rear, freaking them out one-by-one.
As I lined up my 'attack', I noticed motion to my left. A Golden Eagle was pulling into formation with me. Apparently, in a moment of inter-species body language communication, the raptor had interpreted my maneuvering as preparation for an attack on those "ground creatures" below. The eagle seemed to be thinking, "This, I gotta see."
The hikers, a thousand feet below, froze as my shadow passed looking up at a silent white sailplane with an eagle flying in formation. I would have given a great deal to hear what they were saying.
Another time a young student pilot and I were flying over the Pawnee National Grasslands of NE Colorado. We had been cruising for almost an hour following a convergence line of weak but steady lift. My young friend called out, "Bird, opposite direction, level". I looked around him to see the head-on silhouette of a large soaring bird. Since they spend most of their time looking down for prey and not clearing airspace ahead of them, I suspected it hadn't seen us. "Lets drop down 50 feet and let it go over us", I suggested.
As the bird approached, I saw mottled feathers indicating it was a juvenile Golden Eagle - the avian equivalent of a teenager. It probably had not seen a sailplane before. As we passed into it's field of view, it went to a full startle posture with wings cupping the air and talons extended as if to say, "Woah! What the heck is THAT?!" It turned to get a better look at us and we turned with it, circling each other several times before parting on our respective paths. It seemed to have learned sailplanes are just big, harmless things which are fun to play with.
Some encounters are a tiny bit less friendly. I was returning to land with a passenger enjoying his first glider ride when I spotted a Golden gliding purposely, intently looking down at something near the hangar. It was focused on our "apprentice hangar cat" - an orange tabby. I really liked the kitten and thought it an inappropriate lunch for an eagle so I decided a mild intervention was in order.
I nosed the glider toward the eagle slowly overtaking it. The eagle was so intent on the kitten that it didn't see or hear me until the nose of the glider gently nudged its pinion feathers. That got its undivided attention and I was rewarded with a steely glare. Then, with powerful wing beats it was gone. The kitten was never bothered again - it had big friends. The passenger had a story of a lifetime.