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How birds nests can cause havoc in a chimney

Posted on 14 March 2017


Having bird guards fitted (cowls, cages or caps for redundant chimneys etc) is essential if you don't want unwanted birds to nest in your flue. There is one bird in particular which likes to nest in chimneys, active or disused, solid fuel or gas, the jackdaw.

This blog has been put together to help those with a chimney to detect, or prevent, birds from nesting. The nest is often built by dropping or pushing twigs down the chimney. If the chimney is small enough they will bend the twig in the middle as they push it down the flue. When they release it, it may wedge itself in place. If not it will catch further down the chimney or will fall to the bottom of the flue. Irrespective of if it sticks in place or falls another twig is added and so on. Quite often the twigs will fall to the bend in the chimney, which is common in most chimneys. Twigs are then continually added until it is high enough to finish off.

As the pile of twigs are constructed all sorts of materials are added to the twigs in load. Flattened cigarette packs, paper, plastic bags, take away trays and so on are all added. As the nest reaches the top of the flue other materials are added such as horse manure, moss, grass, hair, fur and other softer materials are put in place where the eggs will be laid. This upper layer forms a solid dung/twigs/grass ball which is incredibly hard to break. Once they start nesting they are unlikely to stop.

There are a few things you can look out for if you suspect that you have birds nesting in your chimney. If you start to find twigs and other debris in your fire grate there is a strong possibility nesting activity is taking place. Take a few minutes to keep watch on your chimney pot, birds will be flying back and forth dropping or pushing material into the chimney pot if there is nesting activity. If you normally hear pigeons cooing or traffic noise and then it stops and all is silent, there may be an obstruction in the flue.

What to do

Do not try to burn the nest out by lighting a fire. You run the risk of setting the chimney on fire or filling your home with smoke. If you use the fire when there is a nest in the chimney, there is nowhere for the Carbon Monoxide to escape, except to build up in the room, this could result in deadly carbon monoxide poisoning. If in the early stages get a chimney sweep out. If left till the middle of March it will be too late,  as it is an offence under the 'Wildlife and Countryside Act' to destroy or remove a nest while it is in use, and you will have to wait until the end of August to remove it. Get the chimney swept and a proper British Standard/EU compliant bird guard fitted for your appliance immediately. There are different types for gas, oil and solid fuel.

There are a few criteria for fitting a suitable bird guard:

  • It needs to prevent birds from getting in
  • If the flue is going to be used, the bird guard needs to be fitted to ensure that the chimney can be swept fully into the unit to maximise the chimney clean and ensure there are no further blockages.
  • In the event that the flue is not going to be used, it must still allow for air flow in the chimney to prevent damp.

The bird guard should be suitable for the property e.g. our customer might like something that matches the colour of the pot if there is one.

Nest Removal

This is a very physical and time consuming job. All loose material at the bottom of the flue is removed. The nest is then broken up using a hard, small brush or a metal nest removal tool. Eventually the size of the brush is increased until the nest is completely removed. Ask your chimney sweep to let you see the brush coming out of the pot, and always ensure that a smoke draw test is done at the end of the procedure.

Problems associated with nesting

Often as the nesting season progresses the twigs dry out and shrink. As they do the weight from above, the mud and the chicks can cause the nest to slip down the chimney. It only has to fall a short way before the birds cannot get enough lift to fly out. They die in the chimney. As summer progresses nature and flies do their buisness and quite often there is an over powering smell,  and or a plague of flies in the room at the bottom of the chimney.


Nesting season is mid March until August or when the chicks have fledged. Signs of nesting are twigs at the bottom of the chimney or in the grate, a lot of activity around the pot, the chimney becomes quiet. If you suspect nesting do not try to burn it out but contact a chimney sweep to remove it. Then immediately fit a proper bird guard. A fly infestation is often a sign of a collapsed nest or a dead bird in the chimney.

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