First of all, sorry for the inaccuracies of my question or terminology, since I'm not Muslim myself and I ask it out of curiosity.
I became aware that in Europe it is allowed to give some animals met-and-bone meals (in a dried powdered form) as addition to the normal meal. The amount of such meal originated from the same species as the receiver is limited by law to very low amounts, therefore it is common to use other animals as source (in any case, the percentage of this meat-and-bone meal on the final meal remains very small).
In other words, we have pigs eating "powdered" cows and chickens, and chickens eating pigs, and so on.
Does this make the resulting chickens haram/forbidden?
Before simply answering "the chickens are not slaughtered properly anyway", I would like to point out that the question is limited to the food, not to the rest of the procedure. Moreover, I'm not sure, but I expect many Islamic butchers to buy live chickens grown industrially and to slaughter them themselves (to be able to have a reliable and controlled source), the question may still be relevant for them.
Back to the question: I would expect the chickens to be allowed, since their bodies completely processed the food they receive and probably no trace of pigs is left, but that is my opinion.
See also: Gelatin capsules from pig ash \ bones