While attending a child's soccer game, i looked straight up and noticed a shiny mylar balloon float by at about 500ft - 1000ft. observed it for a few seconds, looked down to watch the game, looked back up and it was gone, however in its place was a circling "bird" - same altitude. noticed that it looked like a very large hawk yet there was no wing flapping. just as i was about to look away, it accelerated extremely fast due west on a straight trajectory. it was out of line of sight in a matter of seconds. it made no sound. i would say it was a bird except for the fact that it took off like no bird i have ever seen.