In re-reading the OP, I think we have all missed the point.
the OP is asking "IE. let's say I find myself constantly x feet too far to get a good shot of, let's say, easily startled birds. " and "How do I know if 105mm will be able to get me close enough, or 125, or 75, etc. ? Simply put, how does the Xmm translate into Xmeters/feet ? " in the case of wild life and easily startled or small birds, it does not matter what he has, it will be too short
but actually, aside from that realism, regardless of how people approach the issue it all comes down to the math I put a few posts ago.
I keep coming back to this because sometimes it is useful to know if you know how far away you will be, and what the subject is, you can calculate what you need and cur what you carry.