There's an old farmer's rhyme about planting corn seed: "One for the blackbird, one for the crow, one for the soil and one to grow."
Although it's not necessary to plant four times the amount of seed that you actually want to grow, a little extra doesn't hurt, if only to ensure good germination. As for the blackbird and the crow, they may be tough to beat in large cornfields, but there are a few ways to outwit them in the home garden.