The short answer
Leave an active nest alone. Under the Wildlife and Countryside Act 1981, section 1, anyone who intentionally harms or removes a wild bird's nest while it holds eggs or chicks, or is still being built, commits an offence. When the nest is empty, the flue can be cleared and a cap fixed on the pot so the birds cannot return.
Twigs in a flue are a nuisance, but a nest with eggs or chicks in it changes what you can do, and when you can do it.
What the 1981 Act says about active nests
The Wildlife and Countryside Act 1981 protects wild birds and their nests. The offence in section 1 turns on one word: intentionally. Deliberately pulling out a nest you know is occupied is the act it targets. Clearing a flue that you had no idea was in use is a different situation, although checking first is the sensible course.
Can a licence cover building work?
GOV.UK's guidance on wild birds lists no licence for development or construction. A repair or re-roof therefore gives no cover for disturbing an occupied nest, and the job waits for the birds.
If you cannot tell whether the nest is in use, listen at the fireplace for chirping and watch the pot for adult birds flying in and out. Where we are unsure, we hold off and come back later rather than guess.
This is general information, not legal advice. The Act is published in full on legislation.gov.uk if you want to read the wording for yourself.
Bats also use chimneys now and then. They have separate protection, so stop and take advice if you find one before anything is blocked off.
Clearing the flue once the birds have left
Nest material is a blockage. Twigs, moss, feathers and droppings pack together, and the pile can stop smoke and fumes leaving the house.
Who clears it?
A sweep cleans the flue from the inside with brushes and rods. A roofer deals with what is on top: the pot, the crown and the stack. The two jobs often go together, and our answer on roofer or chimney sweep explains who handles what.
If a fire, stove or boiler uses the flue, leave it switched off until it has been cleared and checked. Debris can fall onto a hot appliance, and a blocked flue sends combustion gases back into the room.
Resist the urge to poke at a nest from below with a brush or a pole, unless you are certain it is old and empty. A shower of soot lands in the room, and the nest may still be in use after all.
Nests sometimes sit outside the flue too, in the gap between the lead and the brickwork. We treat those the same way: wait for the young to leave, then close the gap.
Keeping birds out afterwards
Once the flue is clear, the way to stop a repeat is to close the top of the pot against birds while keeping air moving through the flue. An open pot looks like a ledge to a jackdaw, and the same bird will often return to the same spot.
What is a bird guard?
A bird guard is a wire or mesh dome that fits over the pot. The gaps are small enough to keep out nest-builders and big enough to let smoke pass. A cowl, a fuller cap that also keeps out rain, does the same job and more; whether you need a chimney cowl compares the types.
How it is fixed matters. A guard held by loose clips blows away in the first gale, so it should be strapped on or set into the flaunching, the sloping mortar crown round the base of the pot. A cracked crown is worth mending at the same time, and chimney flaunching explains how.
A flue that has gone out of use can take a capped pot with a vent, which keeps the air moving and still stops anything climbing in.
What to do while you wait
If the nest is plainly occupied and you can see it from the garden or a window, there is nothing to do but wait. If the flue is in use and you are worried about smoke, switch the appliance off until it has been checked.
What does the roofer do?
We look at the stack and pot from a safe position, judge whether the nest is active and photograph the top, so you can see the pot, the crown and any damage. When the birds have gone, we clear the debris from the stack, repair the crown if it has cracked and fit the cap or guard with fixings that will stay put.
A chimney with other faults, such as a loose pot or open joints, is best done in a single visit, so the cap goes on a stack that is ready for it. Tell us what you have noticed, and when the birds were last about. We then book the survey for a time when the nest is empty, and the written price comes after it.
Everything we do on stacks is listed on the chimney repairs page.
More on nests in flues
Can I block the chimney while the birds are inside?
No. Closing a pot over an active nest could trap the birds, which is the kind of intentional act the 1981 Act is aimed at. Wait until the young have left, then clear the flue and fit a cap. If you are unsure whether the nest is in use, ask a roofer to look before you do anything.
Will a cap stop the noise and the mess for good?
A well-fitted cap or guard keeps birds out of the flue, which ends the scratching, the soot falls and the droppings. It will not cure noise from a loose pot or flashing rattling in the wind, so we check those parts when we fit the cap.
The full picture: Chimney repairs in Taunton ยท More short answers