While out owling a couple of nights ago I heard a short series of wailing calls typical of an alarmed Northern Saw-whet Owl. I replayed the wailing call and obtained this response, preceded by a quiet chitter. Shortly afterwards two birds flew just over my head.
(5.07am, Dec, PA)
The most familiar call of saw-whets is the territorial tooting of males, but in fall and winter other calls may be more frequent, including whines, "ksew" calls, wails and chitters. In this case the initial wails may have been a response to my presence, but they also coincided with a short-lived bout of hooting by a nearby pair of Great Horned Owls.
A cautionary note: I've heard wails from Eastern Screech-Owls that I thought might be saw-whets, but these are weaker and more tremulous:
probable Eastern Screech-Owl (11.48pm, Oct, PA)