5 dead chooks in one night

[dowey]

Hi ive just discovered 5 of my chooks de. 1 chook still alive and healthy looking and a guinea fowl still alive and healthy looking. None of the dead chooks are missing heads and they were out all day happy came running to there food as always had new water and were locked up does anyone have any ideas on what happened to them? This is how i found them

 

[Free Feather]

Check their necks for a small bite, like a weasel.
Could anything poisonous, like gas, get in the coop? Is there anything where they were roaming that they could eat or drink?