Common Tern

Dark-blue ring with a white three numeric code.

note 1 : used codes: 400 - 499.

note 2 : birds ringed in Cumbria and North Lancs, UK.

note 3 : 

Dark-blue ring with a white code :
a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 2 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 3 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 4 :

Dark-green ring with a white code :
a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 2 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 3 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 4 :

Black ring with a white code :
a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 2 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 3 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 4 :

Red ring with a white code : a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 2 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 3 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 4 :

Yellow ring with a black code :
a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 2 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 3 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 4 :

White ring with a black code :
a triangle before a two alpha-numeric code (letter/number)
OR
a plus symbol before OR in the middle OR at the end of
a two alpha code OR a two alpha-numeric code OR a two numeric code.

note 1 : used codes are : ▲A1 to ▲Z9

note 2 : used codes are : +AA to +ZZ ; A+A to Z+Z ; AA+ to ZZ+

note 3 : used codes are : +A1 to +Z9, +1A to +9Z,  A+1 to Z+9, 1+A to 9+Z, A1+ to Z9+, 1A+ to 9Z+

note 4 : used codes are :  +00 to +99,  0+0 to 9+9, 00+ to 99+

note 5 :

Black ring with a white two numeric code (incl. a datalogger) (on left leg) and metal ring (on right leg).

note 1 :

White ring with a black 3-alpha code.

note 1 : most birds are ringed as pulli and have the c-ring on their left tarsus and a metal ring on the right.

note 2 : used combinations are : PAA-PZZ ;

note 3 : used letters are : A, C, E, F, H, J, K, L, N, P, R, S, T, U, V, X, Y, Z.

note 4 : 

Yellow flag with a black three alpha code (on left tarsus) and metal ring (on right tarsus).

note 1 : First letter R and flags are coded from  RAA to RZZ using 18-letter alphabet (A, C, F, H, J, K, L, M, N, P, R, S, T, U, V, W, X, Z).

note 2 : birds ringed in Romania starting with 2021.

note 3 :