"Beautiful, white birds fly across the sea."
Translation:Szép, fehér madarak repülnek át a tengeren.
Emphasis. When the verb is not split, it is emphasized. When it is split, the word in front of it is emphasized.
For this sentence, imagine the question: "What flies across the sea?" - Asking about the subject in front of the verb.
For the "fekete hajók", imagine this question: "What do the black ships do?" - Asking about the verb, the activity that the ships perform.