WR... two main suspects, the Sharp-shinned Hawk and the Merlin (a falcon), photos of both can be seen here...
http://www.virtualbirder.com/bkwheeler/photo.htmlBoth are about the same size (small males about like a Blue Jay, big females pigeon size or a bit bigger). Adult Sharp-shins are blue-gray above, barred rusty below. Adult Merlins are variable, often blue-gray above, reddish-brown below. First year birds of both species are brown with barring. Both feed mostly upon small birds cuaght in flight.
The MO sounds like a Sharp-shin, they have relatively short-rounded wings, long legs and a long tail. They hunt down low from ambush sorta like a cat. They use cover to get close and then rush their prey. Aerodynamically they are sort of in a low gear: they can accelerate very quickly but their top speed ain't all that (maybe 45mph). On the other hand they are lightly built and manouverable can do that 45 mph though brush and branches. Usually it is Sharp-shins that stake out bird feeders.
Merlins are falcons, built for speed in open settings; longer, pointed wings. A Merlin typically hunts low over open areas, overhauling its prey by pure speed.
So... when that bird flew, if its wings looked rounded and if it flew off through the trees ya prob'ly got a Sharp-shin. If its wings looked more pointed like a Killdeer or Kestrel and it flew off over the trees or broke for the open more'n likely a Merlin.
Either one, if it didn't actually get hurt it will probably be back, Anytime all the birds at the feeder suddenly bail keep your eyes open.
Birdwatcher