Update: December 30th, 2011; Gracious thanks to everyone for the comments received on this bird. In particular Peter Pyle, Ian Paulsen and Jeffery Davis provided some very useful links on Great Gray Owls, some of these with specific reference to the Sierra Nevada population;
Great Gray Owl - Yosemite, Mariposa Co., CA. September 14th, 2011.
Although I've been fortunate enough to see quite a few Great Gray Owls in North America this bird struck me as being especially small, dark and distinctly brown-toned. The image set was taken at dusk in low light, but even so, the colors are relatively true to what we observed in the field. It wasn't at all what I 'expected' to see in a Great Gray, without doubt the smallest, brownest individual I've seen. Apologies for the poor quality shots digi-scoped at dusk but I'd rather come away with some images than go home empty handed.
Great Gray Owl - Yosemite, Mariposa Co., CA. September 14th, 2010. By way of comparison, this bird was observed at a different site in Yosemite a year earlier and more in keeping with what I'd expect to see in a Great Gray Owl.
Great Gray Owl - Montreal, Quebec, Canada. February 5th, 2005. Images from the incredible incursion of northern owls in the winter of 2004/05. We saw six Great Grays at one site alone and haven't see anything like it since!