"Feeding the birds"  is a hand gesture meant to replace the traditional high five or fist bump.
Actor Nathan Fillion considered the fist bump to be symbolic aggression, and invented the gesture as a more peaceful and nurturing alternative.
It is meant to represent a mother bird feeding one of her chicks. Fillion has promoted the sign in some of his works, including Castle, usually with Esposito. In his Twitter feed, he sometimes abbreviates it FTB.
The person to whom props are due should have their arm extended, fingers and thumb bunched together and pointing upward. The person who is giving props uses the same gesture, pointing downward, and "pecks" the other's upraised hand three times.
- When Lanie decides that the killer in "Fool Me Once…" is too tall to be a first grader, Ryan points out that one might have stood on another's shoulders. Esposito suggests that the pair might have hidden in a trenchcoat. Castle responds, "Diabolical! I like it." and feeds the birds for Esposito.
- When Ryan tries to explain his former romance with a girl in the vampire scene, Castle feeds the birds to Esposito to give him props for a well-timed joke about it. ("Vampire Weekend")
- In "The Mistress Always Spanks Twice", when Beckett secures a court order overriding an obstructive dominatrix lawyer, Castle says, "Ohh! Someone's about to get punished." Esposito laughs and feeds the birds.
- In "Punked", Castle suggests that the victim was found naked because his time traveling killer needed the clothes, having gone through the time ripple naked... like in Terminator. Beckett is defeated—she can't think of a better theory—and Esposito feeds the birds to Castle.
- When faced with the prospect of having to track down a non-human killer in "Close Encounters of the Murderous Kind", Esposito appeals to Castle to prove it wasn't E.T., and shows his gratitude for the reassurance he receives in this manner.
- As Molly Quinn describes Nathan Fillion as being like a second father to her in L.A., with her own parents living far away in Texas, she breaks into tears. Fillion holds her hand for a moment and then feeds the birds to her in gratitude. (Paley Arts Center 2010, An Evening With Castle)
- In the bloopers reel for Season 2, at the mark 2:25, Nathan exclaims "Somebody feed these birds!" and holds out his two hands to his side. Both Stana Katic and Jon Huertas feed the birds to Nathan.