Not paying much attention I assumed these were milk-maids. But inspection shows they have five petals not four. They are also similar to woodland stars (congeners which live in SB but are rarer here) and California Saxifrage.
I first noticed them blooming early March, by which time they were in full bloom. Still a few blooms in mid April. Finished in May (except on Arroyo Burro trail where they were still blooming on 22 May).
Seen on north slopes, generally higher in elevation than Milk Maids. Seen on Tunnel (just before Rattlesnake connector), on Rattlesnake (just below Gibraltar), in the larkspur patch above Forbush, and the West Fork of Cold Spring.
The flower has 5 petals (and I think 5 stamens). The stamens and ovary are hidden inside the tube below the petals. The petals are fragile and easily knocked off.