TREE SWALLOWS AND TIT-FOR-TAT - RESPONSE

Koenig's (Ethol. and Sociobiol. 9:73-84, 1988) review of Lombardo's (Science 227:1363-1365, 1985) experiment testing the TIT FOR TAT model of reciprocity correctly concluded that the experiment did not demonstrate the existence of reciprocal altruism between parent and conspecific nest intruder tree swallows. However, his review contained errors that preclude a fair evaluation of the experiment and interpretation of its results. Here I respond to these criticisms, reexamine tree swallow behavioral ecology, and suggest that the lack of aggression between parents and intruders can best be modeled as by-product mutualism. I reiterate that the balance between parental aggression and nonaggression toward intruders can be maintained by TIT FOR TAT.