The Birds, a good thought, however the McKean birds might have been returning to their roost and McKean was closer to Howland and not too far from 337/157. That might have caused AE/FN to deviate to McKean. Considering that they were flying between 0900 Howland time and noon, the birds would probably not be returning to land at that time.
The early Polynesians navigated in outrigger canoes to widely separated islands using visual sightings of rising/setting stars and memorized information passed down from previous generations to get them close to their destination and then looked for seabirds returning to their roosting island to guide them in to the island. No sextants, no RDF. Just detailed info about ocean currents, winds, clouds, etc. that was passed down from generation to generation to those showing the talent -and the inclination- to become "navigators". I read somewhere (I forget where) that the angle of suspension and frequency of oscillation of their testicles under their loin cloths played a part in their navigation along a path. I'll have to try to find some documentartion of that.