Sorry for the very belated report. I hope some of this information is still relevant.
I birded various sites on Mount Diablo over the weekend, but since I was not on a birding trip, most of my forays were brief and many of my observations were done casually. However, I was able to get to Mitchell Canyon on both the days, but had to leave before some of the best hours of the morning. Still, I had a fine trip with a few highlights:
Golden Eagle-5; including 2 flying shortly after dawn!
Common Poorwill-7; I've grown to expect this species in spring on Mount Diablo. Despite relatively limited owling efforts, I found them in the chaparral just south of Juniper campground, twice in the grassland north of Juniper campground, and four times in Mitchell Canyon (including two visuals). Had I arrived at Mitchell canyon a half hour earlier on 5/5, I likely could've heard at least a few more.
Great Horned Owl-5; remarkably, this was my only owl seen despite camping in suitable Screech-Owl and walking by a large amount of grasslands at night
Rufous Hummingbird-1; at Mitchell Canyon on 5/5; female: outer rectrices studied; givin how many there were here just two weeks ago, it is striking that there migration has all but stopped
Calliope Hummingbird-9; on 5/4, I found 2 on Deer Flat road, 1 in upper Mitchell canyon, and two in the more traditional spot of White Canyon; on 5/5 I found at least 2 males and a female at White Canyon and saw a displaying male a few hundred yards farther up the main path from the white canyon entrance. Curiously, when I had 10 at white canyon earlier this spring, almost all of them were displaying. On this trip, though, only one was
Band-tailed Pigeon-2; Deer flat road 5/5
Hairy Woodpecker-1; Mitchell Canyon on 5/4
Western Wood-Pewee-1; Donner Canyon on 5/5
Hammond's Flycatcher-2; both at Mitchell Canyon (one a little ways up White Canyon) on 5/5
Pacific-slope Flycatcher-7; Mitchell Canyon on both days
Western Kingbird-1; Mitchell Canyon on 5/4
Cassin's Vireo-1; Mitchell Canyon on 5/5
Nashville Warbler-1; singing at Mitchell Canyon on 5/4
Black-throated Gray Warbler-1; Donner Canyon on 5/5
Hermit Warbler-1; Mitchell Canyon on 5/5
All in all, a fine weekend to be out birding on Mount Diablo.