In birds this sort of seemingly bad adaptation is usually related to mating. Like male cardinals being bright red or a peacock's tail. An fragile and energy-expensive appendage like that would be very indicative of the health of the individual, especially because it would get damaged pretty easily.
Or hell, maybe they used it to shade the water to see fish better. Or they used it to absorb heat from the sun to stay active in cool water. Or it helped them see other members of their species from a distance. I love mysteries like this!