Is it permissible to feed birds with milk expressed from the mother's breast, instead of discarding it, if the infant is in the incubator and cannot be breastfed naturally?

There is no harm in benefiting from human milk for non-humans. The disagreement regarding human milk concerned the ruling on its sale, not its use for non-humans. The Malikis and Shafi'is, and this is the more correct view among the Hanbalis, hold that it is permissible to sell human milk if it is milked, because it is pure and beneficial, and because its consumption is permitted, its sale is therefore permitted by analogy with other livestock. It is also permissible to take compensation for it in the context of a wet-nurse's hire. However, it is not permissible to sell it according to the Hanafis and some Hanbalis, because it is not considered property.