Hi KK
I wonder are your 'crows' actually Jackdaws. They always nest in holes in trees, cliffs, chimney pots etc. Crows build an untidy nest of sticks in trees.
I hope the Robin lays eggs and hatches out chicks, the box is only about 6' from my window and very easy to watch and hopefully the adults will become tame especially if I feed them with mealworms. Its the using of the box with the small hole that really surprises me because generally they prefer an open site in a shed, underneath tree roots etc. or an open box a bit like yours.