Installing a bird guard is one of the most effective ways to prevent wild birds from nesting in your chimney flue. As well as protecting your flue from intruders, they can also make your fire burn more efficiently.
If you’ve got birds nesting in your chimney already, you need to leave them be and not light any new fires until they’ve been vacated. This is for your own safety, as burning a nest in your chimney carries a high risk of causing a chimney fire, and it’s illegal to intentionally remove, damage or destroy an active bird’s nest under the Wildlife and Countryside Act 1981.
Once any birds currently nesting inside your chimney have been safely removed, you should have your chimney flue swept before installing a chimney bird guard. This new bird guard should then prevent any birds from getting into your chimney space in future.