Last night we were sitting outside around a little campfire and saw a queer and amazing thing.  It was almost dusk and a flock of swallows were flying overhead in a layer.  They were spread out in a swath maybe a half-mile wide, all flying east-southeast, and they just kept coming for about 10 minutes.  I would guess there were 500 of them in view at a given moment and 30,000 of them in the whole flock.  The only thing I could think of was that the mosquitoes were coming out and it was feeding time, but I could not help but wonder why they were all flying at the same altitude and why they were all going the same direction together and whence they were coming and whither they were going.  It was all rather surreal.