As we look out over our domains, we see through our eyes, not the eyes of the birds.
Suppose you were an Anna's Hummer and commonly made use of a certain feeder. Then someone put up a new feeder, that you, the Anna's preferred because of its location and/or its particular mixture of nutrients. Then you wouldn't bother with the old feeder.
I am new to this activity, but recognize the difficulty of separating three elements of bird populations: normal year-to-year variation, trends and cycles.
I don't know what to expect as a normal year-to-year variation. I have noticed such population variations, but draw no conclusions.
I expect that cycles occur. For example, a certain species may have a successful breeding season and the population increases. Then its predators notice and their populations subsequently increase. The prey bird population then declines and subsequently the predator population declines as well. I imagine that such cycles occur. Probably much more complex than the above.
Trends, or permanent changes also likely occur. Again, my experience is too limited to determine if such are occurring for certain birds in the local area. And, if so, would such permanent changes be "good" or "bad"? Just like humans migrate when seeking better opportunities, I expect that birds do too. That is, if permanent changes are occurring, are the birds fleeing ("bad") or finding better opportunities elsewhere ("good")?
Here's an example from the memory banks. When I was a kid growing up in Vancouver, I and almost every friend had a BB gun. In those days, the local feed store, Buckerfields had a bounty on Crows. We received 5 cents for handing over a pair of Crow's feet. Not surprisingly, there were no Crows to be found in any park or neighbourhood at that time. When the bounty was discontinued, the Crows returned, almost instantaneously (and other bird populations declined).
On the whole, birds are very successful. They live on every continent. They function in the air, on land and on water. They can function underwater, on the water and over the water. I think Canada Geese have been observed flying at 20,000 feet elevation. Only humans via the use of tools can claim the same breadth of use of the environment.
The data are too sparse to make any conclusions locally.