Thanks to everyone who responded to my post about the birds I saw at the
visitors center.

To answer some of the questions:
It was on 3/2/06 about 9am.
I saw three birds.
The description of their flight as randomly criss-crossing their path was apt. 
They also flew in an area up to 10 feet above the water as they fed, not
skimming the surface all the time.  They flaped their wings pretty fast and
there was a lot of banking and random movements in their flight.
They were swallow-like, but definately larger than the tree swallow's 5"-6".
I more likely underestimated their size as 8"-10", but they were not as
large as black skimmers.

Jim Ryon

My original post:

> Hi:
> 
> I'm a novice birder and would like some help in identifying birds
> I saw at the visitors center lake.  In flight they appeared to be gray or
> 
> black on top and white underneath.  They were about 8-10 inches and skimm
> ed
> the water and periodically hit the water to feed on something.
> 
> Thanks,
> Jim Ryon