If you think dating is hard, be glad you're not a grebe.

These North American waterbirds have high standards when it comes to attracting and keeping a mate: If either a male or female grebe can't "walk" on water, they're out of luck.

Western and Clark's grebes engage in a maneuver called rushing during the spring mating season, in which they sprint up to 66 feet (20 meters) across the water in coordinated groups of two or more in about seven seconds. They're the largest vertebrates with the ability to walk on water, but no one had really looked at how they did it—until now.