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.

Research looks at the evolutionary pathways to differences in bird plumage patterns between males and females -- and concludes that birds are able to adapt their appearance with remarkable ease.

Imagine zooming out on a giant family tree that includes every bird you have ever seen. Ostriches sprint across open plains, hummingbirds hover at flowers, penguins slice through cold seas, and eagles ...