Do you all have no understanding of the most basic concepts of evolution? Mutation does not occur after an animal has been born. The Egg was the first to be mutated to become a chicken egg. Thereby hatching a chicken. The parents did not need to be 100% chickens (though they did need to be pretty damn close).
A Chicken or Egg ,A Chicken or Egg, A Chicken or Egg ...MMMMMMMM really hard to tell .. I will bet on Chicken.
I agree with Ichigo91. Scientists proved that the egg came first in 2006 or maybe 2007, one of the two.
People this might be a very old question and I just like to give a shoot. For me who really comes first is a CHICKEN with an EGG and with a NEST.
The answer is obvious if you believe in God. God created mankind, then later animals and plants, so obviously it is chicken first.