Are you trying to say that the availability of food along the way can supplant a bird's entire migratory instinct?

That's an interesting point.

In the (UK) context of this study/article, blue tits and great tits (and indeed most garden birds) are generally non-migratory in the UK, so I think that's an orthogonal issue.

To be fair bird movements are quite complicated: e.g. a Robin in the UK between September to March might be a member of a migratory or a non-migratory population. And Blackcaps are an example of a mostly migratory species in the UK but where less migratory individuals come to feeding stations in winter months.

Hum, I don't think so. Even with a few birds dying, as long as they are replenished routinely bird feeders are definitely good for winter survivorship in birds.

Birds that evolved in Central Europe don't die by winter if they can find a constant supply of food. Can choose to do a shorter migration and overwinter in northern areas and this is good for the survival of young birds. Urban areas are warmer also. Migrating south is hazardous and exhausting and there are some places in the Mediterranean or Africa that are notoriously dangerous for birds when they are hunted massively.

There is a lot of migratory birds that just can't survive winter, let alone harsh one, even when fed.

Some have evolved in much warmer climate (a lot of migratory birds from Europe evolved in Africa) and never evolved to survive winter.