Birds in my Flue, help?

[wiperhunter2]

Yesterday I was under my house, working on a project, when I heard this scratching sound, I looked around a find that it was coming from the flue that goes to my hot water heater. Then I remembered my neighbor telling me about a bird that got stuck in his flue, so I figured that was what it was. Lucky for me I have a clean out for it at the bottom, so I took it out and left my door open going outside and the bird flew away when I opened it up. Lucky for that bird I was under my house and heard that noise but not so lucky for 6 other birds that died in there that I never knew about. My question to you, have any of you ever had this happen and if so what did you do about it? I'm hoping there is some type of netting I can put over the cap of my flue that can prevent this from happening again, have any of you even bought a net like I'm talking about that covers a flue cap and prevents birds from getting in?