'Tis the feed that's the difference.they (about dozen "white" chickens - leghorns?) were penned in a maybe 5000 sq.ft. apple orchard overran by stinging nettles. They were fed old bread and cheap grains (millet I think) in small quantities. They were digging a lot of dirt searching for worms. The eggs were amazingly tasty, especially raw drunk from the shell - there was no internet at that time to tell us it could be dangerous. That was ~35 y ago.
I'd bet money on it(especially because there's no way to prove it-haha!!).
Maybe even the birds themselves.
A lot has changed in 35 years as far as hybridization.
Do you know if they ate the nettles?
Did the eggs taste them same all year around?
Were there other livestock around that they may have gleaned feed from?