Do fish give birth to live young or lay eggs?

Birds are the only (known) animal that exclusively lay eggs rather than give live birth. Though the reason is not completely understood why some animals lay eggs and others do not. For birds, it could be because it would be difficult to fly while pregnant, though bats fly when pregnant, but birds have hollow bones, which would make them carrying that extra weight more difficult than bats.

Also, it's not true that fish and reptiles only lay eggs, although many do. There are some fish and reptiles that give birth to live young; such as guppies, some sharks, some snakes, also some mammals do too (the platypus, and spiny anteater). It's been theorized that animals that lay eggs have a better chance for their lineage to survive (laying many eggs), even if they themselves are killed - because the eggs have a change to hatch and live without the parent, as others sometimes take over the care of the eggs.

While those that carry their young, if killed, their potential offspring will be too. Not all insects lay eggs either, the fly gives birth to live young - maggots.