My African goose hatched 4 beautiful goslings on May 1st. Slowly, one by one, they have all mysteriously died. And I have found them all laying in the same position: This was my last one that I lost tonight. Every time it appears they just laid down and twisted their neck around and died. No respiratory issues (breathing is clear, no discharge, no sneezing or coughing, etc.). This is my first time having goslings, so I didn't know what to expect, but I wouldn't call them "energetic", so maybe they were lethargic, but I don't know why. Their poop all looked fine, nice green color. They are free range, so lots of grass, and I also feed them bagged feed from Tractor Supply. No skin lesions and they all had clear eyes. NO signs of apparent illness. WHAT COULD IT BE?