Collects New Mutants (1983) #55-70, New Mutants Annual (1984) #4.
Tragedy strikes the New Mutants! Meet Bird-Brain, a bizarre creature
who's about to become the team's feathered friend. The New Mutants face
off against their rivals, the Hellions, to track Bird-Brain down - but
before the quest is done, one of them will switch sides! And there's
worse in store for the team as they trace Bird-Brain's origins back to
the freak-filled island of the Ani-Mator. Between Ani-Mator's weird
science and the armored killers of the Right, one of the New Mutants
will not return home! Shattered by grief, guilt and loss, how can the
team possibly move forward? The New Mutants clash with Freedom Force,
and the alluring alien Gosamyr draws the team into an outer space
adventure. But as Illyana's rage grows, there's trouble brewing in
Limbo.