On Apr 7, 2011, at 1:20 PM, owler at sounddsl.com wrote:

> Males are smaller than females, as is true with most raptors, though he will have the deeper, lower pitched call when the two are dueting.

The female has a pronounced vibrato to the call. It's easy to tell if you hear them calling back and forth. Once recognized it's not easily forgotten.

When I first heard a distant female I took it for a poor human imitation of a barred owl call. The (at the time undiscovered male just a few meters from me) convinced me otherwise.

Donald Kroodsma devotes a chapter to this in "Birdsong by the Seasons: A Year of Listening to Birds".
--
Kevin Purcell (Capitol Hill, Seattle, WA)
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