Red-tailed Tropicbird

I enjoyed some time "sea watching" yesterday on my free day in Hawaii. By chance a small group of Red-tailed Tropicbirds came close enough to give me some good looks, and this one took a good look at me as it cruised along the cliffs of eastern Oahu. That long tail seems to be useful as a rudder as the bird maneuvers in the strong, gusty wind, but does anyone really know what purpose the red color serves?