I think I have read about this too and from recollection, all animals studied so far have been found to play and not just to learn to fight or dominate but apparently just because. I doubt chickens were in the group studied for all the reasons we know, but I do remember other non-mammalian species having been studied (reptiles and maybe even insects).I believe that I have read article on them playing. Maybe I can find 1 or two. I believe the answer is yes.
If you have seen video of the Panda in the Smithsonian doing summersaults in the snow you would be hard pushed not to conclude that adults play too!