Photo: Sebastià Torrens. Question marks: Aina Bonner

Challenge 4

Today will be a bit of a trick question.

As you know, birds have two locomotive modules. One they use to fly (the wings) and another to walk (the hindlegs), and on the ground they usually move with one or with the other. In the sea there are birds that use one of the modules, others, the other one, and others, both. The question is: outside the sea (and lakes and rivers; i.e., on a solid substrat) what species occasionally use simultaneously both modules to get around?