Blue tits needing water!

Hi - we have a feeding couple in our box, think they started feeding young a day or so ago however today I’ve noticed one of them scouring our garden wall - there’s a lot of moss on it at the top and then the guttering on the house and I’m concerned about what they’re looking for - could it be water/moisture?  We’ve had no rain here for some weeks now, fields are very close by and we have many oak trees but I believe the tit population here is probably good so is it likely that there is a lot of demand on caterpillars and they’re struggling for food - my rose bushes that grow near to the box are absolutely covered in green flies but I don’t know if they’d feed those? 
Thank you for any advice