The Wildlife and Countryside Act 1981 is the primary legislation which protects animals, plants and habitats in the UK.
Basic protection
All birds, their nests and eggs are protected by law and it is thus an offence, with certain exceptions (see Exceptions), to:
Intentionally kill, injure or take any wild bird.
Intentionally take, damage or destroy the nest of any wild bird while it is in use or being built.
Intentionally take or destroy the egg of any wild bird.
Have in one's possession or control any wild bird, dead or alive, or any part of a wild bird, which has been taken in contravention of the Act or the Protection of Birds Act 1954.
Have in one's possession or control any egg or part of an egg which has been taken in contravention of the Act or the Protection of Birds Act 1954.
Use traps or similar items to kill, injure or take wild birds.
Have in one's possession or control any bird of a species occurring on Schedule 4 of the Act unless registered, and in most cases ringed, in accordance with the Secretary of State's regulations (see Schedules).
Intentionally or recklessly disturb any wild bird listed on Schedule 1 while it is nest building, or at a nest containing eggs or young, or disturb the dependent young of such a bird.
Fines
Penalties that can be imposed for criminal offences in respect of a single bird, nest or egg contrary to the Wildlife and Countryside Act 1981 is an unlimited fine, up to six months imprisonment or both.
The most notable exceptions to the Wildlife and Countryside Act include:
An authorised person (eg a landowner or occupier) may kill or take, in certain situations and by certain methods, so called 'pest species' and destroy or take the nest or eggs of such a bird. This is permissible under the terms of General Licences issues by government departments (see Licences).
You'd need more information about the situation and method used, but you can report it to the RSPB. www.rspb.org.uk/birds-and-wildlife/advice/wildlife-and-the-law/wildlife-and-countryside-act/exceptions/.