I think the crux our pursuit in defining what it is to be a birder is that we’re looking to define criteria for membership of our tribe, which would be the group of people that “speaks the same language as us.” We recognize each other by our habits and field gear, and location. We can assume similar interests and motivations. There are many factors that may vary among birders, but the general idea remains, or at least insofar as I (and others) have taken a stab at defining it.
There are probably narrower definitions and thus “tribal affiliations” within this umbrella of birders. I think this is harder to define, because it varies among many factors, and is even perhaps evidenced in the small group birding friendships that are formed in a general area. For instance, how about the people I can find at a good birding spot, and they immediately know what I’m there for? Those are people with whom I feel the strongest birding connections, because it indicates a lot about how much we’re on the same page and interested in the same things. In that case, to me, it’s birder-specific, so then I find others who feel the same way to hang out with!
This isn’t meant to be an exclusionary statement or attitude, nor do I think I’m by any means limited to interacting with people that are like me in the birding world. I’m just trying to get at that shared sense of purpose and kinship, or “fellow traveler” feeling, that we get when we encounter others on the same path we’re on. I think that’s what many are trying to get at when we define what it is to be a birder, in part by looking in a mirror.