Eh? There is nothing at all wrong with hemlock wood. (If you are thinking of poison hemlock, a la Socrates, that is an utterly different plant, related to wild carrot or queen anne's lace)
Woods from the walnut family (walnut, heartnut, butternut) should be avoided for all hoofstock because of toxic compounds that can cause severe or fatal problems; I have no idea whether any such effects are known in poultry but personally I would not try it. Cherry and closely-related plants can have problematic concentrations of cyanide in wilted leaves so I would not use chippings of those species. And anything very strongly aromatic has at least the *potential* to cause respiratory problems (a la cedar) in an insufficiently ventilated space.
Otherwise, though, basically it boils down to: softwoods are less apt to mold than hardwoods so are more usually used, but most hardwoods can be used if kept a close eye on for problems.
Good luck, have fun,
Pat