Use of Like/Unlike (Double negative)

Question

What is the correct way?

Birds are not mammals like cats.
Birds are not mammals, like cats.
Birds are not mammals, unlike cats.

*Just in case, reminder: Cats are mammals. Birds are not mammals.

Answer 1

All three are grammatically correct sentences.

Birds are not mammals like cats.

The first sentence is clunky because the structure is ambiguous though omitting the comma makes it technically unambiguous. It means:

• birds are not mammals
• cats are mammals

The other two sentences are clunky because the "(un)like cats" clause modifies birds but is so remote from the phrase it modifies. Also, the second sentence is factually incorrect.

Birds are not mammals, like cats.

This one means:

• birds are not mammals
• cats are not mammals

Birds are not mammals, unlike cats.

This one means:

• birds are not mammals
• cats are mammals

Better wordings put the modifying clause adjacent to the modified phrase. Any of these is better:

Unlike cats, birds are not mammals.
Birds, unlike cats, are not mammals.
Birds are not mammals like cats are.

Answer 2

The first and third one are correct because the comma in the second one is unnecessary.