This recent paper is about birds of prey spreading fires. Some of the co-authors are firefighters which basically tell their personal experience as eyewitnesses to the behavior of these birds but do not present any hard evidence.
How can this be considered enough for co-authorship? Is this an accepted practice in social sciences?
I think the true answer depends on what the other co-authors think. In my field and particularly for application-oriented papers and case studies, we often add industry representatives or clinical staffs as co-authors, even when they don't contribute much to the writing of the paper.
We do this because it fosters good relationships, demonstrates their facilitation of the analytical work, and (in our belief) is good a "people-practice."
For others, we simply list them in an 'acknowledgements' section with some comments.
It all depends.
