Mimicry: Please help me understand how

Mimicry, as a form of camouflage, is quite common among various species of animals. Take, for instance, the numerous species of spiders, beetles, and butterfly/moth caterpillars etc., that mimic bird droppings as an avoidance strategy to predation. Any species that employs this strategy must have had some ancestral form that did not resemble bird droppings in any way. Given that starting point, how did bird dropping mimicry evolve?, considering that a random mutation that led to individuals appearing EVER SO SLIGHTLY like bird droppings would not have been significant enough (in my mind) to result in differential survivability leading to differential reproductive success. How can appearing EVER SO SLIGHTLY like bird droppings be advantageous? And then to assume that this happens over and over again to result in a form that is virtually indistinguishable from a bird dropping, seems to be a bit of a stretch. Your thoughts?Thanks