Definitely. I'm living in Thailand right now, but I follow a bird Kentucky list serve group that reports local rare bird sightings. A few years ago they began reporting tropical birds in Kentucky, and these reports have become increasingly frequent. They're seeing birds I only used to see in Mexico or FL.

Absolutely!

We used to get Northern Grosbeaks and I haven't seen one in years.

Blue jays also decreased here.

However by being on this board I found out that the Goldfinches we get here in the spring were coming up through New Mexico over a month ago now. They've now arrived. (Awaiting their plumage change!),

It may have more to do with global weather changes, habitats that have decreased and predators. These changes in migratory patterns.

It's been proven that feeding birds is not detrimental because they always use multiple food sources.

As for Hummers they have to be careful about where they reside because if it gets too cold they will just freeze to death. They have such a high metabolism. If it's staying warm enough for them? Migratory patterns definitely could switch.
But if it's that warm they must also have access to real nectar?

I feed them sugar water (I have a recipe) and two of my neighbors do as well (at least two!) and they also use the flowers as a resource.

Hope this helps!