How to identify pairings and repeats in population data

Eeek, let me think about that.

I don't really understand that. The formula give so far does nothing with the year. It looks for pairs of bird IDs and gives them an ID.

Also, once that pair exists in the DB the first time, they get an ID, period. So further down the list will detect that ID and reuse it, so I don't really see what new thing you're adding here.