Where I live, it is common practice to feed only a corn product such as cracked corn or scratch, and table scraps, and the chickens are free range, often are not even locked in a coop at night. When some of the old folks here say "chicken feed" they mean cracked corn. I believe my parents did the same thing, and they were Yankees, and this is the deep south. My parents got some eggs from them, but all were eaten or in the freezer by the onset of winter.
I have read of studies that showed that chickens will survive on free range and corn, but may not grow as large, and may not "thrive" or be really healthy. I would say the roos you got were on little if any real feed. Maybe they got a small ration of feed, then had to find their own.
I currently free range and offer feed, with dogs for protection. I mean really free range; I am in the country and they could go miles if they wished; there are high weeds and woods easily accessible. They eat less feed than when they are locked up, as expected. The other day I mistakenly let their feeders go empty for maybe half a day. They really packed in the food when I filled them.
BTW, somewhere on here is/was someone who worked out that it takes 87 sq ft per chicken to free range them and still have grass that they won't strip the land of.