The Blue Tits hatch after 12 to 14 days of incubation, and if they are building a nest they will be laying in the next few weeks. The parents feed them for 16 to 18 days in the box, at which time the young will leave the nest. The adults feed them for another few days and then begin to prepare the second batch.
I don't know if this true, but someone once told me that you shouldn't put bird seed out at this time of year because the young chicks can choke to death. Has anyone else heard that?