[HiTech RedNeck]

This sounds like natural methane emissions concentrated to the point of asphyxiation. Birds would be quite sensitive to that in particular.

Nobody thought of tossing a burning object inside? (Hold my ancient ale and watch this!)

[Fire_on_High]

Some brilliant sorts decided to try that and burn off gas around a mining pit a while back...this was the result

[CodeJockey]

This sounds like natural methane emissions

Actually it was co2. I did not know it could be poisonous.

http://en.wikipedia.org/wiki/Carbon_dioxide

Check out the toxicity section.