Further years ago, while living in Japan, I would slope soar a 7 foot wingspan RC glider in the mountains South and West of Tokyo.   All sorts of birds would line up in formation as I caused the glider to wander back and forth but parallel to the slope of the mountain.   I could stay out there for hours with the right battery packs and these silly birds would fly in formation off either wing as long as the glider was in the air.   When I landed for a lunch break or a battery switch out, the birds would disappear only to come out again when the glider was re-launched.   It was very strange and the only sound that could be heard was the occasional noisy servo making a control surface adjustment.   The birds did not seem to mind the noise at all and seemed perfectly content to just fly in formation even though the glider was much bigger than they were.   I watched the same thing occur when my Japanese friends launched their gliders on the days when I was there with them.   Some of their gliders were a lot bigger than mine and some were very much smaller and faster.   The birds seemed not to care and adjusted their speeds to the one they were following at the time.   Just speculation but it may be the speed of our powered aircraft that freaks them out not the size.

Harry League

Jim Cameron wrote:

    Years ago, a fellow named Pennycuick did a thesis study in Africa on the flight characteristics of large birds, mostly the big vultures like Ruppell's, and large eagles.  He'd fire up a little motorized glider early in the morning to get some altitude, then shut off the motor and soar with the birds for hours, taking notes and shooting video.