My four month old pullet, who had been sneezing only occasionally in the last month, changed her "chirp" this morning. She was eating but not hanging with the flock of six; at dusk she was outside the run but by herself. The others all headed for the run and coop. I went to get her, she tried to walk toward me (she was roosting under the porch) and fell on her face. Threw her wings out but couldn't lift her head or keep her legs straight. I picked her up and took her to the coop where the light was better. Not a sign on her of trauma, but she could not keep her head up. She'd convulse trying to get up, but couldn't. I'd put her in a nesting box, but the convulsions got so bad she was hurting herself, so I put her in the open straw-covered area. Another few convulsions and she died.
What could have caused this? I've read of two fairly common lung problems from fungus. I get organic feed but am trying deep litter this winter. I'm leaning toward cleaning it out tomorrow and starting fresh, maybe storing the food inside the house rather than the coop, where it isn't heated. We've had lots of rain lately, maybe humidity is affecting the feed? Could deep litter be the problem? I want to protect the remaining 5, who have all been pretty healthy since birth.