Evolution wise, my understanding is that Ivory Gulls split off the main gull branch not long after the jaegers/skua group diverged from the gulls. I thought the bird in flight was reminiscent of a jaeger or maybe a peregrine as it would usually have fairly shallow but stiff wingbeats that allowed it to accelerate a lot quicker than gulls. Whether that's what one would expect based on the evolutionary lineage I'm not sure.
Here's a couple more distant montages that I think gives a person some sense of the wingbeat cadence
This next one is larger than most of the jpgs but I think an Ivory gull deserves it.