Your original sentence is very clear, but we can make a small change to improve its grammar and flow:
Original: Seabirds eat fish, so they follow fish in the sea. They also spend time on land where they come together in groups to find food and keep their little ones safe.
Corrected Version: Seabirds eat fish, so they follow the fish in the sea. They also spend time on land, where they come together in groups to find food and keep their little ones safe.
Explanation: Adding the article 'the' before 'fish' is better here because it refers to specific fish in the sea. Also, adding a comma after 'land' helps separate the clauses clearly.