Birds nesting in Chimney

I have one of those metal hat things on top of my chimney to stop birds nesting. However last year I was sure I could hear birds tweeting away and they seemed to be coming from the fireplace.

Now this year again, same twittering. I can hear it in the sitting room coming from the fireplace. Can the birds get in under one of those metal things - I want to light my gas fire but am afraid of damaging the nest, if there is one.

Bless em their little hearts......yeah a few yrs ago I thought they were coming down the chimney into the heartplace (almost) got the chimney swept and no evidence of a nest.

Still hear em tweeting away, its just the sound is amplified down the chimney stack. Even if there is a nest a gas fire ain't going to bother em and in fact might even give them a little comfort

A gas fire outputs carbon monoxide, if you believe there is any chance of
the chimney/flue being in anyway blocked or obstructed by a birds nest
have it throughly checked out before lighting your fire by a registered gas installer (rgii.ie)

Flue obstruction can easily cause flue spillage form the fire back into your living room and is a serious threat to life when they do. Do not take the risk.

As for the birds not minding the flue gas, if they were actually in the flue, maybe 30 mins to live before being overcome by CO

I just find it hard to believe that they could get in under the metal thing that is on top of the chimney. It is there over 2 years. Like Pudds, it sounds like they are nearly in the fireplace itself.

Is the metal guard seated properly? Is it intact? Sounds obvious but ours was knocked sideways in the wind lately. Use binoculars or look closely.