So terribly sorry for the loss! I would contact the local extension service and see if they have any advice on how to go about the pursuit for compensation. Also, if you haven't already, document the events around the death of the birds carefully and meticulously including receipts for the feed that caused the tragedy.
I believe the manufacturer is liable for the safety of the product they sell; however, they might try to blame someone else such as either the feed store or yourself for poor handling of the product. In any case, act on it quickly. If you run into problems with the manufacturer, Im sure an attorney can give you advice or even help. Of course that route might become costly in no time. Whatever you decide, I hope that you will find closure in one form or another.
Would you mind sharing with us what type and brand of feed caused all this?