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. Over time, they became quite acclimated to me, with many of the closeup shots in the gallery taken hand-held from only 10-15 feet away. Such fun!
The number of finches 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 I soon learned that I had nothing to worry about. Within the next week or so, the entire flock pretty much cleared out, leaving fewer than a half dozen visiting on a regular basis.
It's hard to know why they left. I spoke to one of our local experts and she said it could be anything from a predator such as a neighborhood cat or a sharp-shinned hawk, to a bully 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 northern flicker dropped by briefly another day, but I haven't seen any predators lurking around on a regular basis. My guess is that it's related to the abundance of natural food available right now, but there's no way to know.
It turns out birding is not too unlike fly fishing, an activity I spent many years pursuing. The river, the mayflies, and the trout always held some surprise in store (and often, some frustration too). One day might bring a blizzard hatch and a river lit up with rising fish, another might bring a river that appears utterly devoid of life. It seems Mother Nature operates on her own schedule and in her own mysterious ways, whether we're talking about goldfinches or trout.