Complex sentence: Blu worries about Jewel and his kids as he attempts to adapt to the new environment, while Blu and Jewel's old nemesis, Nigel the Cockatoo, has returned for revenge.
Split into simpler sentences: Blu worries about Jewel and his kids as he attempts to adapt to the new environment. Meanwhile, the two love birds' old nemesis, Nigel the Cockatoo, has returned for revenge.