I haven't got the opportunity to try it yet myself, but my best guess for the seagulls are:
1. When you start seeing birds flying in the sky while you are completely lost in the ocean, its a signal telling you that there are islands or lands nearby you. ( I learnt this from watching the series " Vikings" from Netflix)
2. The second guess is the seagull might have some kind of relationship with the glass bottle. When we own the bottle we usually use it right away since the first impression that the bottle gave us is to write SOS on the paper asking for help, hoping someone could find it and come to save us ASAP. At the same time it does not do anything at night time, with a 1 cost of energy to sent it out (same cost as fishing junk) all together create an illusion that it is for an instant use, the quicker you sent it out the quicker for people to find it. What if the purpose of the seagull (the game call it as a friend of ours) is a messenger, capable to grab the paper and fly to the closest island that have civilization?
These are my best guesses after 6 play through. From now on I would keep the bottle with me all the time until I met a seagull, wait till I see a boat or airplane then I will test my hypothesis and click it away haha. The placement of the seagull sitting at the front of the craft also gives me some extra thought, would it scared away the fish and affect the chance of catching a fish? I am still figuring it out!!! Sorry for the yapping and bad English (it is my third language) you can see how much I enjoy this game xD
Lastly thank you Dev for creating such an amazing and addictive game! <3