Trees that have delayed softening and provide bird food in late fall and into mid-winter include: Malus ‘Prairifire’, Malus ‘Mary Potter’, Malus x zumi ‘Calocarpa’, Malus ‘Profusion’, and Malus ‘Anne E’.
The cultivars whose fruit softens in late winter often provide a little pick-me-up for the birds. I guess you could call it a little extra toot for the snoot. The fruits will have a bit of alcohol in them. It’s fun to watch! Two of these cultivars are ‘Bob White’ and ‘Sentinel’.
Two cultivars that I know of have fruits that soften very unevenly. These are ‘Birdland’ and ‘Ormiston Roy’. They feed the birds from mid-autumn into spring. I would say they are the ultimate in crabapples for birds.
A common misconception in the trade that yours truly had fallen prey to earlier in his career is that all small-fruited crabapples are taken by the birds. This is not true. Malus ‘Donald Wyman’, Malus ‘Red Jewel’, Firebird® Crabapple, and ‘Adams’ Crabapple are cultivars that retain their fruit all through winter. The birds mostly leave them alone. The little crabapples drop from the trees before flowering in spring. These are still among the finest ornamental crabapple trees for fruit color persistence in the winter. It’s just that they are not good plants for attracting birds or for siting in areas where fruit drop is not acceptable (by walkways or patios).
The list I have compiled is by no means comprehensive but is a start in helping you plan ornamental crabapples for birds.