You need to define what you mean by "behaviorist." It's an unregulated term. There's no nationally recognized credential like a CPDT, so idiots and charlatans abound.
When I use the term "behaviorist," I think of a board-certified veterinarian behaviorist
. They can bring medical, neurological, hormonal, and other physical concerns into the evaluation of behavior. In addition to the DVM, they've done a residency in applied behavior and satisfied the demanding qualifications for board cert. There are only a few of them in the country. You can find them through the American College of Veterinary Behaviorists:
For standard misbehavior, you don't need one. For OCD and related issues, I would want to work with one -- research just came out showing canine autism exists (!!!). The OCD stuff we sometimes discuss here (tail chasing, etc.) has some documented links to explosive rage in the study, and there's no longer a doubt it's neurological, not just "learned behavior." The researchers found what was going on in the brains of dogs' suffering from some forms of OCD closely paralleled autism in human children's brains -- it's a revolutionary finding. Those dogs need meds, though they're barely at the beginning of figuring out what helps. This is a new frontier in veterinary medicine -- the field is in its infancy.
One of the vet researchers was on NPR discussing his new book -- the interview was fascinating, and I literally sat in a parking lot of the grocery store listening to it instead of running my errands because I couldn't tear myself away: