I believe the Warbler which lands on Santiago's skiff before flying off to meet the Hawks could be considered a minor character. I believe it serves as a symbol or something of the small comforts of life which are fine and enjoyable, but often leave us without warning or reason.
I think you are right, but I think you can also look at the warbler as almost a metaphor for Santiago.When the warbler lands on his line he is very tired and has just completed a long and hard journey and is resting for a bit before going off to meet the predatory hawks. This is much like Santiago's state just after he has caught the Marlin. He is very tired and worn yet has precious little time to rest before he must go and face the predatory sharks.