GMAT Critical Reasoning (CR) | Guillemots are birds for arctic regions. They feed on fish

Ok so you need an extra assumption that fish needs ice to survive, or that ice is needed to suppport life for the birds or birds dont like easy catch without ice and so on ...... etc.

on the same lines we can argue that we need extra assumtpion that since on the south the birds have more predators to compete with so they will stay away from the south, there you go another reason to stay north, the range didnt increase, they stayed away from the south.
In either case you need axtra assumption