We have this problem here in Poland
Then you find birds dead because they are not adapted to surviving the winter.
We have this problem here in Poland
Then you find birds dead because they are not adapted to surviving the winter.
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.
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.
Some have evolved in much warmer climate (a lot of migratory birds from Europe evolved in Africa) and never evolved to survive winter.