I only just made the connection. It is Richard Mabey. I have the original edition of Food for Free. My mother was a big proponent and would send me out gathering stuff.I've been working on gathering the data for such a thing for quite a while now. There is really very little pertinent work done as yet, that I can find at any rate.
However, I did recently discover that fresh young beech leaves - which my chickens are very fond of, and will be appearing all over the hedge shortly - are “sweet as a mild cabbage though much softer in texture” (Mabey Food for free 2022: 29 [originally published in the 1970s]). He suggests adding them to salads; no nutritional data though. Google throws up lots about beech nuts, nothing much about leaves.
I've also discovered that even the best websites are not necessarily reliable here. I have seen my chickens eating creeping buttercup leaves, which the RHS says is “poisonous to livestock” – but since my chickens are free to forage on whatever they like, and are not eating it through lack of choice, I trust their instincts; maybe protoanemonin is not toxic to chickens, or maybe there is very little sap in the bits they choose to consume.
We all forget that cultivated foods, as well as uncultivated foods, carry risks and can have unexpected consequences; for example, a whole nutmeg can bring on days of hallucinations according to Mabey (who appears to be writing from experience!2022: 10). Most of what the chickens graze they just nibble, and the norm is a little bit of this and a little bit of that as they perambulate round the place.