The breeding season for Jackdaw’s is April – July. They only have one brood so if they lay early in April incubation takes 20 days. The nestlings then fledge at 32 – 33 days, so the first weeks of June would be when you would see the Juveniles.
If you are unsure about whether breeding has commenced or not, then the best advice is to wait until the end of August – September, if that is feasible. There are numerous telltale signs to confirm nestings birds in chimneys. Debris falling down the chimney into your fireplace, sightings of adult birds bringing food and removing faecal sacs. Noise from the chicks, enter your loft and listen for calls, if there is a nest you will hear them.