Can't say I've ever had a bird pass out no matter how long it was upside-down. Were they CX?
You can’t loose sleep thinking a headless chicken is thinking??
Fast and humane , off with the head while in the cone , the part you missed is that we hold them upside down til they pass out first .... they are not panicked and scared since they are unconscious.....
It’s never easy , but it must be done when you have animals ,
Good luck on your research