Robin behavior

Robins nested in a trellis off my porch, and I watched them daily until they disappeared. Two babies. But there was no sign of shells, not on the ground beneath the nest, nor (well after they were gone) in the nest itself. I have come across Robin's eggshells over the years--why not these? What do Robins do with the shells once the birds are born? The nest is pretty crowded--do they just nudge them overboard? Pick them up and carry them off and drop them from the air? Do other creatures take them? Why are these shells not easier to find than they are?