The degree-toting ornithologists call her pied-billed grebe but in Louisiana she answers to “di-dipper,” or “hell-diver” on a bad day. For her the water’s surface is only an interim point in space and time. That she spends precious few moments there is an aggravation for birdwatchers and boys with BB guns, not to mention the serious predators be they finned, feathered or scaled. Departing the planner ecotone dividing atmosphere and liquid is more graceful if she chooses the denser of the two mediums. The downward dive is faster than the eye can appreciate the choreography.