Another important feature corsets/bodices/stays did, was to spread the weight of heavy dresses and skirts to the whole body from shoulders/waists. A lot of women that reconstruct the fashions of 17-19 centuries would tell you that those costumes are tiring without the corset or with a badly fitted one, but quite comfortable, when done properly.
Now, boning - inserting rigid elements of bone or metal - was not always a necessary part of the corset, just several layers of heavy non-elastic fabric were used.
Your avian humanoids would not need boning in their corsets, since their torso is rigid already, but if they have a fashion of heavy multi-layered dresses and skirts, they would certainly benefit from the weight distribution of a well-fitted boneless bodice.