Well, technically.. this is an easy answer, it'd be the egg.
Comparing the two, it is a specific evolution of an avian compared to a method of protecting young while they mature. The egg came far earlier than the chicken, and can be seen being produced by various other animals out there today 
Lets face it, dino's laid eggs too... and while I could be wrong, I don't believe they had chickens to munch on ;D