The Wildlife & Countryside Act 1981 is one of the most important pieces of Wildlife legislation in this country. It states it is an offence to...
Intentionally kill, injure or take any wild bird.
Intentionally take, damage or destroy the nest of any wild bird while that nest is in use or being built.
Intentionally take or destroy an egg of any wild bird.
If you are unsure there is a nesting Jackdaw, then the best advice is to wait until the end of August - September. 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 they were late breeders in July then the nestlings would fledge late August early September.
They are numerous tell-tale signs to confirm nesting 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, so enter your loft and listen for calls, if there is a nest you will hear them. Guano (excrement) and white splash on your roof near the chimney are other things to look for.
The maximum penalty that can be imposed for an offence under the Wildlife and Countryside Act - in respect of a single bird, nest or egg - an unlimited fine, and/or six months' imprisonment.