We live close to the outskirts of a very large city so we get an abundance of birds although typically, not all that many species. Also, we're practically surrounded on three sides by large areas of woodland, pasture and bottom land interspersed with smaller park areas, many with considerable trees and varying amounts of undergrowth.

I've also noted that at certain times of year, mainly sharp seasonal changes, the bird population thins out considerably for a few weeks and then slowly returns although with the addition or subtraction of a few species. I've always chalked this up to sudden changes in temperature and to some degree, the amount of cover left for nesting after a big freeze as an example. This is speaking of my yard and feeders, not necessarily the whole area...

We also have a city park close by that includes a small lake (perhaps 4 or 5 acres) and I've noticed the same addition or subtraction of species at certain times, typically spring and fall. In summer, the park and lake area is typically full of your standard park ducks and geese and about a million pigeons along with a small contingent of house sparrows and grackles. Let fall arrive with a single cold front though (it's Texas so the word "cold" is relative compared to some of ya'll) and here come the cormorants, ring neck ducks, Scaups, Coots, Canada Geese, and a few others in numbers that ebb and flow until warm weather and spring arrives again whereupon those species all tend to disappear over the course of no more than two to three weeks.

It might also depend on this weird weather year with it's floods, fires, oddly timed blizzards, and so on.

I'm in Southeastern Michigan and I'm going to assume along with TXSkeeter that it's "this weird weather year with it's floods, fires, oddly timed blizzards, and so on," because it's driving me crazy. I have a few birds, especially Nuthatches and Cardinals, but really miss so many of the others this year. So sad.