question: do unused attributes go "rusty" like skills - or is there only plain attribute damage?
Oops, sorry, all is
there in the manual *(MENT_ATT_RATES).
Let's look at the parameter contradictions here:
poultry needs to be confined to go nuts, but
dwarfling doesn't. Lil' vunz are trained by spending some time with birds, but being locked in a small cell without any other activity causes mental deterioration (see memory and creativity decreasing on screenshots by monk12).
Quick experiment: a pair of geese didn't try to headbutt (like cats do) a "closed tight" hatch on short dead-end stairs, even after their pasture was reasigned back to the up side. So, the right question is: will confined birds try to do it
after going crazy merely to get into the next small airlock?
If not, the general idea has much greater potential than attempts to produce mad supersoldiers: breaks between "chicken gladiator" sessions could allow less intense, but still significant training while keeping possible crippling factors minimized. For that matter, do they train just as fast while sleepless and tired as when rested?..
Suppose a trainee eats and sleeps elsewhere, but regularly passes through the "birdcage" room and get locked there for a few hours by a water timer? Since the resistance to stress and fatigue differs, the timer can be made individually adjustable by changing its cistern volume via floodgates. Combined with the choice of oponents, this allows much better control of the process and keeping sanity without excessive costs and FPS loss on personal waterfalls.
Bed-to-food and back seem to be the most regular path. Since this requires merely setting up a chickenbowl and assigning a dorf the bedroom behind it, the training becomes useable not only in "sealed off children cave(s)" configurations, but at will. You deem an immigrant "badger-fodder weakling"? Prescribe him two fowl dances per day until he gets better!
And in the time not spent dodging an annoyed turkey's beak he
still can work, which not only keeps him at least somewhat useful, but right jobs can prevents skills and attributes getting "rusty". Plus, of course, the fowl also does something useful while remaining an emergency reserve of unrotten meat or living decoys.