I have raised these birds for 3 seasons, 20 or so per batch, 3 batches per season. Today, I lost my first bird due to heart attack. A 30% loss happens, 15% is probably typical, 5% is good. I am just over a half percent (0.5%) loss rate now that this one went early.
There is another that came from the hatchery with a twisted leg, but that was how I received him and has nothing to do with my system of raising these birds to 10 pounds in 10 weeks.
I did experiment with a higher feeding rate early on with this batch. There was a limp in this group, so I scaled back to my normal feeding rate and the limpy one healed up and blended in with the rest of the flock. Perhaps the larger feed rate early on set the stage for heart failure today. Perhaps it was being tormented by the alpha jerk in the flock.
I could have harvested this flock this past weekend at 8 weeks and still had a 0% loss rate, but I will be harvesting the other 12 chickens on Saturday (day after tomorrow). New chicks are coming on June 17, Wednesday in 6 days and I need to get ready for the last batch this year!