You really ought to do a check to see if it's named "chick", or possibly check against the renamable var of the chick (which will be false if it's been renamed).  Otherwise, grown chickens will all be named "chick".

there is a byond builtin proc called initial(var) which is used to get the compile-time value of the var, for example
if(name != initial(name))
which is useful in circumstances like this as well.