"Platform nests" are built mostly by raptors (birds that eat prey, like eagles and hawks). They are usually huge and very, very high up - on the sides of cliffs, maybe, or high up in a big tree. It takes months and months making these nests, adding branch by branch. They often return to the same nest year after year and continue to build it. It is because of this that platform nests get "so big": a Bald Eagle's nest, for example, can weigh as much as a ton (about the same weight as a small car). It is so heavy it can sometimes hurt the tree it is built in.