Significance of Seagull in Navy flag

Question

In one-piece is there an in-universe reason for the seagull on the navy flag or is there another reason that they chose a seagull for the navy flag?

Answer 1

Symbolic meanings of Seagulls

• Freedom : Marines believe in absolute justice. They believe they provide true freedom the populace by catching people that put a stop to them. Thus they provide freedom to the general population by protecting them.
  Freedom is the most important symbol we have discussed. Luffy says he wants to be Pirate King because Pirate King is the most free person in the world. Throughout One Piece the importance of Freedom is emphasized. Blackbeard's sniper also shot down a Gull on Jaya. Did Oda foreshadow BB pirates will kill the "freedom"?

• Adaptation: Seagulls are present on Land, Sea and in Air. They can live in harsh climates on meagre sustenance. They can scavenge off anything. Sounds like the marines we know.

• Communication : How many types of Den-Den Mushi do marines have? I've lost count :p

• Opportunistic : Marines are highly opportunistic. They rushed ahead with Ace's execution to trap WhiteBeard. They'll use any and all means in their hand to carry out their own sense of justice.
• Resourcefulness : Marines are highly resourceful. Smoker, though just a Captain, was able to assist and capture a Shichibukai.

Going through the list, we can easily see the qualities represented by Seagulls and how any Marine would strive them to be. Sengoku must really believe in Freedom to wear that hat though :P

Also additionally... Its Oda's favorite bird!

This is an interview with One Piece author Eiichiro Oda that was published in the One Piece: Blue Grand Data File volume.
"Q.25 (Tashigi, Kuina, Nojiko and so on are characters with bird names but) What kind of bird do you like best?
Seagull"