I've lost three birds from presumably the same thing in the past year, but they have been VERY spread out in time (2 last summer, one today). The birds didn't come from the same place and were all 1-3 years old and previously healthy. They have all exhibited the same symptoms and trajectory, starting with depression and lower body paralysis (flopping over, can't walk, but head upright), and ending with convulsions and death a few hours later. They wouldn't take water or food, did not appear egg or crop-bound, and were acting normal the day before. I must emphasize, in the time between these deaths all other chickens in the flock have been fine. I.e., I don't think it's disease but some kind of toxicity/poisoning.
I live on large acreage and they have a coop where they are locked up safe at night but free range during the day. It's the mountains and there is lupine in the yard, which I know to be toxic. All deaths have occurred during seasons when lupine is present, but I have never seen a chicken eating the plant. Otherwise the only thing I can think of is something in our grey water - our sink drains into the yard (I know, gross, but it's temporary) and they love to pick at that area (but again, they don't all get sick and die!). The only dangerous things I can think of coming out of the sink are a small amount of coffee grinds and soap suds (we use eco-friendly, biodegradable dish detergent).
Has anyone out there seen lupine or any other kind of poisoning? Is this how it presents? I hate losing birds and not knowing what is causing it or when it will happen again makes it even worse. :( Thanks in advance for any insight.