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Black-headed gull (Chroicocephalus ridibundus)
The specific ridibundus is Latin for "abundant".

The black-headed gull (Chroicocephalus ridibundus) is a small gull breeding in much of the Palearctic, Europe and Asia.
Most is migratory, also many remain in the milder areas of northwestern Europe.

Charadriidae, Charadrius: Late Latin word for a yellowish bird mentioned in the fourth-century Vulgate; from Ancient Greek kharadrios a bird found in ravines and river valleys (kharadra, "ravine").

The genus name Chroicocephalus is from Ancient Greek khroizo, "to colour", and kephale, "head".
The specific name ridibundus is Latin for "laughing".

Brown-headed gull 
(Mòng bể đầu nâu, Chroicocephalus brunnicephalus)

The brown-headed gull (Chroicocephalus brunnicephalus) is a small gull.
The bird breeds in the high plateaus of central Asia from Tajikistan to Ordos in Inner Mongolia. 
This species is migratory, wintering on the coasts and large inland lakes of the Indian subcontinent. 

Charadriidae, Charadrius: Late Latin word for a yellowish bird mentioned in the fourth-century Vulgate; from Ancient Greek kharadrios a bird found in ravines and river valleys (kharadra, "ravine").

The genus name Chroicocephalus is from Ancient Greek khroizo, "to colour", and kephale, "head".

brunnicephalus: 
 Brunni- (Latin): from brunneus, meaning brown.
 -cephalus (Greek): from kephalos (κεφαλός) or kephalē (κεφαλή), meaning "headed" or "head". 

Chroicocephalus is a genus of medium to relatively small gulls which were included in the genus Larus earlier.
The genus name Chroicocephalus is from Ancient Greek khroizo, "to colour", and kephale, "head".

1/ Black-headed gull (Chroicocephalus ridibundus)
The specific ridibundus is Latin for "abundant".

2/ Brown-headed gull (Mòng bể đầu nâu, Chroicocephalus brunnicephalus)