A yellow wagtail of the subspecies Motacilla flava flava, which has a bluish-grey head with a white eyestripe, and is the usual form in most of Europe.
Origin
Mid 19th century.
Definition of blue-headed wagtail in US English:
blue-headed wagtail
noun
A yellow wagtail of the subspecies Motacilla flava flava, which has a bluish-grey head with a white eyestripe, and is the usual form in most of Europe.