Questions posed about the ID of the light brown, medium-sized falcon seen at Ardenwood last Saturday being chased off by a Peregrine Falcon prompted me to ask for more expert review. I sent Trent Pearce's photo to Kimball Garrett and Bob Power.
Kimball replies, "I would expect Prairie Falcon to have more linear streaks (rather than chevron-like markings) on the underparts, and a blotch of dark markings on the flanks (that bleeds into the dark axillar markings, obviously hidden in the photo). And a shadow shouldn't make the face of a Prairie look as Peregrine-like as on this bird."
Bob adds, "I don't see the shadow you see on the face. I see a very well lit face with a very wide moustachial. More importantly, the moustache comes straight down from the eye and is connected to the eye. Prairie should have a nice light area surrounding the eye and then the moustache comes down out of that light perimeter. As Kimball pointed out, Prairies show a dark skirt along the flanks bleeding out from the folded wing. This is the outer part of the axillaries that are distinctive in flight. This bird shows no hint of the dark area of the flanks that Prairies show. A lightly streaked Peregrine is my call."
I saw the bird in flight, and I did not notice dark axillaries on its otherwise pale underside.
Sincere thanks to both Kimball and Bob for their help in correctly identifying this bird as a Peregrine Falcon (albeit a pale brown one).