I threw the tent, entered before the mosquitoes got bad, leaned against my backpack so I could admire the stream through my tent door-screen... and then it began:
A high-pitched, rapid Ee-ee-ee-ee-ee-ee-ee!
Nervous-sounding bursts of 4-5 notes per second, again and again. I tried to think what bird might make a warning call like that but couldn't imagine. I had to go out into the mosquitoes and see who it was.
It looked like a house-rat-size gray squirrel, except that its tail was too scrawny and the white on its belly was bordered with a thin dark line between it and the gray fur above.
It was a Chickaree, TAMIASCIURUS DOUGLASI, a creature so similar anatomically to the North's Red Squirrel that it's in the same genus, Tamiasciurus (Gray squirrels are in the genus Sciurus.) Chickarees occur only in southwestern British Columbia, western Washington and Oregon, and the coniferous forests of northern California. I've not seen them at Fred & Diana's, probably because it's too low there for them. The Sierra Nevada Natural History fieldguide says they're found mainly from 5000 to 11,000 feet so in this protected little valley at 4200 feet they were fairly low.
The fieldguide writer when describing Chickarees couldn't restrain himself from using the adjectives "sprightly" and "talkative," and it was true that these Chickarees put on a show until it got too dark. One squirrel chased another round and round the tree trunk, up and down, occasionally pausing for the chaser to Ee-ee-ee-ee-ee-ee-ee! while shaking his tail and glaring at me. Once during a particularly unrestrained moment the chaser lost his footing, tumbled fifteen feet to the ground, and I do believe he didn't even notice his setback, such was his ferocious attention to the chase.
The curious thing is that they had a cavity nest right there in the trunk of the big Sugar Pine where the chase was taking place. Nobody seemed interested in getting far from that nest. Young Chickarees in the Sierras are born in June and July so it didn't seem a good time for courtship shenanigans. It really looked like those two critters were having a chase just for the fun of it.
You can see what a Chickaree looks like at http://www.blevinsphoto.com/dougsquirrel.htm