The bird at left side is showing its white rump (in reality uppertail-coverts) that explains their name in English ans Spanish (in French also = Croupion blanc), and is a distinctive mark (please, see enlarged photo).
The bird at left side is showing its white rump (in reality uppertail-coverts) that explains their name in English ans Spanish (in French also = Croupion blanc), and is a distinctive mark (please, see enlarged photo).