Were there visible injuries on these chicks? It’s unusual for a HOWR to leave them in the box. Most of the time they toss them out. If their bellies were bloated you can’t go by that as an indicator of poison. In death, they will bloat. Quite often cause of death remains a mystery.
Good idea to empty the boxes and leave them open. That might help with the HOWR but since they have established a territory they’ll probably stick around for another battle with you. So sorry! I deal with them all the time.