Mine like the suet blocks with birdseeds/BOSS in them, the kind sold for suet cages they're only like $1 and give the birds something to chase around because I hang it from the ceiling in their run (chicken tractor, only 26" tall). They especially like the peanutbutter flavored ones.
I read on here to avoid them during the summer because the birds don't need the extra fat when it's hot, and of course feed them sparingly because they are just TREATS. I give mine one every payday (2 weeks) but not when it's sweltering. Edited to say: it takes 5 chickens less than 2 afternoons to eat the little square up, I immagine that it wouldn't last long if you had very many chickens --- but hey, it's cheap.