When feeding, aquatic birds known as phalaropes often spin rapidly on the water's surface, pecking for food during each revolution. To execute these spins, phalaropes kick one leg harder than the other. This action creates upwelling currents. Because plankton on which phalaropes feed typically occurs in greater quantities well below the surface, it is hypothesized that by spinning phalaropes gain access to food that would otherwise be beyond their reach.
Which of the following, if true, most strongly supports the hypothesis?