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Environmental issues possibly relating to head shaking/gaping: Ammonia fumes from soiled bedding, mold spores inside the coop, improper ventilation, pollen, pesticide use, inhalation of feed granule(s) or feed dust, dust from dust bathing.

Since the gaping and head shaking has been going on for months, it's not gapeworm. Olive wouldve been dead long ago. You could be dealing with the possible onset of a respiratory disease since it's spreading through your flock, although an environmental issue could be the problem.