The response of different species of legitimate pollinators also varies. Some species, like the bumble bees "Bombus appositus" or "B. occidentalis" and many species of nectar-feeding birds can distinguish between robbed and unrobbed plants and minimize their energy cost of foraging by avoiding heavily robbed flowers. Pollinating birds may be better at this than insects, because of their higher sensory capability. The ways that bees distinguish between robbed and unrobbed flowers have not been studied, but they have been thought to be related to the damage on petal tissue after robbery or changes in nectar quality. "Xylocopa sonorina" steals nectar through a slit they make in the base of the petals. If nectar robbing severely reduces the success of legitimate pollinators they may be able to switch to other nectar sources.