I wrote this a few years ago in a "fake fact" contest for a kids' magazine ("Muse").
It has been recently discovered by ornithologists that certain bird species have bellybuttons. The bellybutton is formed when the chick is still inside the egg. You can see this for yourself on any store-bought egg. The white stringy thing (the technical name for this thing is the avicord) is the bird equivalent of an umbilical cord, which forms the bellybutton. This information is evident in this photograph, which shows a Carolina Wren inspecting his bellybutton to see if he has an "innie" or an "outie."
— Hope Batcheller