The dark, smudgy streaks on Xavi Bou's photographs suggest the jerky ink tracks created by a malfunctioning printer, but they actually record the various patterns birds trace while flying in flocks.

Flight is truly remarkable, no matter which way you look at it. And while it seems easy enough for the tiniest insects to take to the air, nature has also created its fair share of large, winged ...

Discover 10 birds that can sleep while flying, including swifts and frigatebirds, and learn how unihemispheric sleep helps ...