The poultry you eat and some concerns.

Poultry are fed a rich variety of feed to enhance and speed their growth. They also receive injections for diseases & the prevention of them. They are also given growth hormones.

The feed may contain meat, dairy or other non kosher ingredients. The method of preparition may be a question of "bossur-bi'cholov" (meat n milk) issues. 

The injections are given either in the egg prior to hatching or in the chick after hatching. When given in the chick, it's usually done in the neck area which may make it a treifah. Some hatcheries in Eretz Yisroel have a full time Mashgiach to verify that it does not make it a treifah. 

Others give the injections in the egg. It's done in the side of the egg that's more round and there is an air pocket between the membrane and the shell of the egg. They must be careful not to inject past the 17th-18th day as it may make the chick a treifah.

We were able to verify that the Mehadrin Poultry (now includes the Vineland, Skver brands among others) only feed their chickens vegetarian feed and the injections are done in the egg not later than 18 and a half days. We don't know what the others are doing at this time.