Page "Wagtail" Paragraph 4

At first glance, the wagtails appear to be divided into a yellow-bellied group and a white-bellied one, or one where the upper head is black and another where it is usually gray, but may be olive, yellow, or other colours.

However, these are not evolutionary lineages ; change of belly colour and increase of melanin have occurred independently several times in the wagtails, and the colour patterns which actually indicate relationships are more subtle.