The nature category is strictly photos of the natural world untouched by the hand of man. There should be no signs of humanity's influence in the photo. Some frequent 'gotchas' that are off limits in nature photos: contrails, domesticated flowers, zoo animals, small bits of litter, cut grass, sawed wood. Exceptions include bird bands or other wildlife tags or monitoring devices.
Photos must not be altered beyond basic exposure and color correction and cropping. Removing or adding anything in the photo (aside from items on the edge removed via cropping) is not allowed.
See updated Nature rules for a full list of what is and isn't allowed.