Two opposing scenarios, the "arboreal" hypothesis and the "cursorial" hypothesis, have traditionally been put forward concerning the origins of bird flight.

Sure enough, these amphibious species show up on the triangular graph approximately half way between the "wet cluster" of sea turtles and the "dry cluster" of land tortoises.

The report notes that habitats and species in the habitats directive were chosen because they were under threat, and so were harder to conserve.