Blue Tits nesting in box question

I have had some blue tits nesting in a box I put up in autumn

It seems very successful as birds have been coming and going since the beginning of April.

How long do broods last?

Today I noticed that a parent seems to be going back and forth (possibly with food for its young). I understand Blue Tits only have one brood so activity since early April seems a long time? Any explanation would be welcome.

Thanks

Nick