Group the following animals into two categories - mammals and birds. Output the grouped animals.
Sparrow, Dog, Penguin, Cat
Mammals: Dog, Cat

Birds: Sparrow, Penguin
Complete the following task based on the instruction and optional input provided.### Instruction:
Group the following animals into two categories - mammals and birds. Output the grouped animals.

### Input:
Sparrow, Dog, Penguin, Cat

### Output:
Mammals: Dog, Cat

Birds: Sparrow, Penguin