The first birds that came to our feeders were a small flock of Lesser Goldfinches. There were probably a half dozen to start, and the group quickly grew to between 25-30. The number peaked one morning after an all-night rain storm. I awoke to the sound of 40-50 chattering finches in the tree outside our bedroom. They were so loud I thought the neighbors might complain, but within the next week or so, the entire flock pretty much cleared out, leaving fewer than a half dozen visiting on a regular basis.
I spoke to one of our local experts about why they might have left, and she said it could be anything from a predator such as a neighborhood cat or a Sharp-shinned Hawk, to the presence of a bird such as a Northern Flicker, to a change in feeding patterns due to an increase in natural food availability. We did have a hawk chase after a house finch one day, and a Flicker dropped by briefly another day, but I haven't seen any predators lurking around on a regular basis.
Any guesses as to why our large flock disappeared? Is the abundance of natural food sources this time of year the most likely culprit? I'm in Northern California.