I tried a new supplement today; the peel and pith of a small easy peeler type orange, after reading
https://www.arccjournals.com/journal/asian-journal-of-dairy-and-food-research/DRF-307 and in particular reading that "Citrus co-products obtained from juice production are composed of peel and pulp. Several bioactive substances, such as dietary fiber (pectin, cellulose and hemicellulose), minerals (potassium, calcium and magnesium), organic acids (citric, oxalic and malic), vitamins (vitamin C), phenolic compounds and flavonoids (hesperidin, narirutin) (Azizi et al., 2018) are abundant in these fractions."
I cut it into little pieces, and chanced to see a couple of birds try a piece and drop it, but there was none left when I went back to collect the bowls - and there was some grain, so some of them evidently thought it was OK, and didn't eat it just because there were no alternatives left.