They could be building a nest in the chimney pot if the people who live there don't have a fire burning yet.
Use whatever talent you possess: the woods would be very silent if no birds sang except those that sing the best.- Henry Van ***
Jackdaws are known to nest and roost colonially and are the only species within the crow family which readily take to nestboxes, hence they're very well known for using chimney pots as suitable homes (with added central heating!).
Even with flue's which are still in use they will using the heat and rising smoke to bathe and groom each other - getting rid of lice or mites..
I'd imagine the birds here will of been gathering grit or perhaps ants.. sounds like something relating to feeding.
:)