To save energy? Well, yes, there's that. But the energy-conservation hypothesis went largely untested for many years. Now, results from a rather creatively designed experiment have confirmed what we've long supposed about flying formations – but not without raising some downright baffling questions in the process.
White ibises have experienced a huge drop in their reproductive rate, and mercury contamination was a prime suspect. But nobody expected pollution to alter the birds' sexual preference, as the male ibises were actually nesting with each other.