Mystery beast

Since November, I've been starting each day with a brisk walk to the top of Skinner's Butte. Several times now, about half-way up, I am greeted by the warble or some creature. It seems like it's coming from high up in a conifer, but I wouldn't swear to it (those of you who have ever walked up the north face of that hill in semi-darkness would understand).

So: anybody have a clue what makes this sound? (250k .wav file)