Chick Jr. and Duck Jr. are a new chick and duckling that Joey bought Chandler as a housewarming gift in the final episode. (Joey named them "Chick Jr." and "Duck Jr." respectively).
In the end, Chandler tells Joey to keep them himself and, again after a few bumps (this time they got stuck in the Foosball table), he even considered getting a goose, but Chandler quickly discouraged him, telling him he was "set with the poultry". The birds did not make an appearance on the Friends spinoff Joey, and their final fate is unknown (it is possible that they did in fact move in with Chandler and Monica).