Being a Pacific Northwest gardener, I think I can weigh in on this issue. The beauty bark that is common here is made from Douglas Fir, in almost all cases. It is dyed with a natural vegetable based dye so I would not worry about toxins.
But it can be a bit itchy on your skin- not really slivers but somewhat irritating. If the bark has been down for a long time and has softened as it composts, then I would not worry about using it as a base- the chickens will stir it into the sub-soil and create a wonderful mix! If it is relatively fresh and very deep then I would rake a section off into a pile so that they can scratch around in the soil. Add a section of sand for them to dust bathe in, too, and they should be happy.
My chickens scratch through areas of wood-chip mulch and composted bark and never complain. I would just watch sections that get quite dry in the summer and think about wetting it down if it seems to get too dusty (think about inhalation irritation). If you buy mulch in the future, start to buy a more composted type, which will still work the same but have less potential for irritation.
