Chickadees and nuthatches are frequently seen coming to feeders, selecting a seed and flying off to hammer it open.
According to a study by members of the department of biology at University of Vermont published in The Auk vol. 114 1997, Black-capped Chickadees and Red-breasted Nuthatches take oil sunflower seeds selectively and discriminate between seeds based on heft. Throughout the study, the chickadees and nuthatches choose the heavier and bigger seeds over the lighter seeds. This is probably because chickadees and nuthatches use energy to fly away with the seed and thus, choose the seed that gives them the most for their effort.