The short answer for the cabbage patch
Yes — cabbage is a safe and genuinely useful treat for chickens. Hens will strip a leaf down to the rib, and the slow work of tearing pieces off keeps a confined flock occupied in a way scattered grain never does. Treat it as part of the greens rotation rather than a vegetable the flock lives on, and it earns its place in the run.
What a cabbage actually gives a hen
Cabbage is mostly water and fibre, with useful vitamin C and vitamin K and a little calcium riding along. That profile makes it a hydrating, gut-friendly bite on a warm day, and the fibre suits a bird built to process coarse plant material from dawn to dusk. What cabbage does not provide is protein or serious energy, so it can pad out a treat plate but never carry a laying hen the way her formulated feed does. The thick midrib and stalk are edible too — chop them finer, or leave the whole head intact and let strong beaks do the demolition work.
The one real caveat: cabbage is a brassica
Cabbage belongs to the brassica family, alongside kale, broccoli and Brussels sprouts, and brassicas carry compounds called goitrogens, which eaten in large amounts day after day can interfere with how the thyroid uses iodine. For a backyard hen this is a portion question, not a poison warning: a share of one head between several hens, a couple of times a week, sits comfortably inside sensible territory. The owners who get cabbage wrong are the ones dumping a whole surplus head in the run every morning because the garden produced a glut. Large sudden amounts can also sit heavily in the gut and leave a hen with loose droppings — a sign to cut back, not a crisis.
Serving it so the flock gets the good of it
Wash a shop-bought head and peel off any slimy outer leaves. The most enriching way to serve cabbage is suspended: skewer a halved head on a hanging holder or thread it on a rope at beak height, so hens reach and tear rather than trampling slices into the bedding. On the ground, coarse shreds work better than big sheets, which get dragged through droppings and wasted. Take away whatever is left at the end of the day; wilted cabbage left overnight draws flies and turns slimy, and hens should never be picking over spoiled greens the next morning.
When to keep cabbage off the menu
Skip cabbage that is mouldy, rotting or freshly sprayed, and be cautious while any hen has loose droppings from another cause, when extra watery greens only add to the mess. If a hen who has gorged goes quiet, stops eating her proper feed or is still producing watery droppings the next day, withdraw the treats, make sure her crop is emptying overnight, and talk to a vet if she is not back to normal within a day.
A treat rotation only stays a rotation when somebody owns it, so log cabbage days in KinsLe's Care Teams, where each helper can see the duty done rather than kindly repeating it.