No, the title does not contain a spelling mistake. I really do mean plane, not plain.
This was an observation that would warm the cockles of the heart of any bird behaviourist: two male Common Mergansers were competing in their harassment of a female — and doing so while all were planing.
About the harassment: This is the breeding season and the males chased the female back and forth across the water before mating. The mating took place behind a dock, but subsequent behaviour made it clear what had happened. I will let the pictures speak for themselves on the issue of harassment, and concentrate on the feat of planing.
When moving across a water surface, most birds, mammals, and boats are in displacement mode: they are supported primarily by buoyancy. Their speed is effectively limited by (what is known as) their hull speed. Some birds have the power to temporarily plane, just as can some recreational boats. (Among adult birds, those with the power to plane seem to be mainly divers.)
When planing, a bird or boat is supported primarily by the rush of the water against its tipped-up body. For a boat to plane, there also needs to be a sharp transition between the bottom of the hull and the transom to force a separation of the water flowing underneath. A bird has a rounded butt, which normally would prevent planing. However, when a bird wants to plane, it changes its shape by forcing its tail down into the water to create the necessary sharp transition.
Male mergansers harass a female while all plane. Bodies are tipped up and tails are pressed down.
Planing is hard work. Sometimes when a bird tires, wings are used for supplemental power.