Let's look at the sentence you wrote: "Seabirds eat fish, so most of their lives they fly over in the sea following fish."
There are a few small mistakes to fix so the sentence sounds better and is grammatically correct:
- Instead of "most of their lives", it should be "most of their life" or better, "most of their lives" meaning the years they live.
- After "fly over", we don’t usually say "in the sea" when talking about flying over the water. We can say "over the sea" or "over the ocean".
- Repeating "fish" twice can sound a bit awkward. You can say it once and make the sentence clear.
A corrected and clearer version can be: "Seabirds eat fish, so they spend most of their lives flying over the sea, following their food."