Neighbors flat is vacant and birds are breeding

My neighbors flat is vacant and birds are breeding inside the flat. Bad smell is coming and is increasing day by day. We informed the flat owner and he says he is not responsible for such act. My wife is pregnant and this smell is causing asthma related issues in our family. Can someone suggest what should my next course of action. The flat is locked from outside for last 4 years but windows are opened because of which birds have placed nest in their flat. Not cleaning the flat since last 4 years.