Several years ago, a young girl started feeding crows near her home. After a while, the crows started "paying" for the food by bringing the girl small gifts --- mostly, small shiny objects.
Over the years, the crows have brought her hundreds of trinkets.
In addition to displaying a sense of social reciprocity, crows also are know for using tools (many animals use tools, but only humans and crows make hooks to drag objects from inaccessible places), and for engaging in play, just for fun.
Tool use: https://www.youtube.com/watch?v=41Z6Mvjd9w0 ; more crows using tools.
Crow improvises a snowboard: https://www.youtube.com/watch?v=2k1aoLAMRMc; more crow play.