Thanks very much for your help. This assigns the a unique pair_ID perfectly.

Is there any way for the formula to also identify the pair further down the spreadsheet to save me having to add everything twice when a line contains the identical ID numbers for both birds and the same year?

For example, imagine columns C and D have no contents and I enter 'EF70113' into cell C2. I would like the formula to search all of column A for 'EF70113' and assuming column B contains '1977' as per line 2, it enters 'ED81312' into column C and also copies the pair_ID of '1' into column D for that corresponding line.

Essentially, as I complete line 2 in the example, line 5 will be completed also.