I believe stress to be a significant cause of premature death with our birds.
My flock lives a cushy stress free life. Quality food, water, grit and oyster shell, 24/7.
Sufficient spacing, 5-6 square feet per bird indoor, an additional 16 -17 square feet per bird outdoor run available 24/7. 30% under cover for those rainy or snowy days.
Coop and run has never been breached by any predator since I built it 14 years ago. Many have tried, including fox, large domestic dogs, and the relentless raccoons...
The birds apparently know this. I have watched all sorts unsuccessfully try to grab a quick meal, while the meal watched with curiosity, until they got bored and went about their chicken business. Often feet, sometimes just inches, away. They have successfully pecked the whiskers off a kit fox through the 1/2" hardware cloth separation. They have lost their fear, and associated life stress. Although not all, many seem to live above average life expediencies, and continue to lay eggs throughout.