Look it's just not the right way to do it and, as you note, ebird doesn't even use the approach consistently internally.A reasonable distance from the edge of the town and the town itself, which--admittedly--varies between individuals,
The right way to do it is to collect observations as point occurrences---exactly where you made each sighting. Then provide 2 types of user-defined aggregations:
- You can combine sets of these point occurrences as a "trip" (or "checklist" or transect)
- Entirely separately, on the map, you can define hotspots. I suggest these can be of 2 kinds:
- An area (polygon) which you draw and which encompasses your local park (or whatever).
- A point which reports all occurrences within x of that location (where x is documented). You can already do this: if you zoom out from London until you see the big yellow/orange squares and then do a prolonged click it'll report the number of species (and hotspots)
Some other systems (don't ask me which) operate on these principles.
I really object to ebird throwing away the basic important underlying information--- which is exactly where you saw each thing. Observado (observation.org) doesn't do this afaik