The tail of the European curlew
The tail pattern of all subspecies of European Curlew versus Slender-billed Curlew
As can be seen from pictures and descriptions the tail pattern of Slender-billed Curlew is unique and therefore can be of help to tell Slender-billed Curlew in all plumages apart from all subspecies of Curlew.
see:
https://radioactiverobins.com/archive/snipes-waders/european%20curlew%20numenius%20a.arquata.htm
https://radioactiverobins.com/archive/snipes-waders/steppe%20curlews%20num%20arq%20sschkini.htm
https://radioactiverobins.com/archive/snipes-waders
/eastern%20or%20russian%20curlew%20numenius%20a.orientalis.htm
https://www.radioactiverobins.com/slender-billed-curlew/
https://www.radioactiverobins.com/the-tail-of-the-european-curlew/