Respiratory signs cuts a cross several diseases, however depending on the brooding system you are using, your chicks must be getting chilled (cold) to death. Can you improve on warmth and have chicks separate from the mature birds. Having birds of several ahes in one coop too makes the younger flock whose immune system is still weak vulnarable to infections. Have all chicks on multivitamin(chick formular). Follow vaccination schedule acordingly and proper management practices.