Webnear the episode's beginning, zenitsu gets a surprise message from his kasugai crow, or rather, his kasugai sparrow.
Anime fans can't see what the message.
Webduring the hashira training arc, a bird brings zenitsu a letter explaining what happened, and shortly after we see him being all serious.
Though if i remember correctly it's not.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder.
Webafter dinner, tanjiro visits zenitsu to give him his share of fish but finds him different than usual.
He leaves zenitsu reluctantly and we briefly witness zenitsu being.