I am due to give a talk about blue tits in Norway later this month (October), so I thought I would rehearse some of it here.
DNA studies have revealed that the ancestors of blue tits originated in China about 7 million years ago. They dispersed across central Asia to Europe and eventually North Africa. Since then, a series of ice ages and climate changes caused these ancestral blue tits to go extinct, leaving the North African population isolated. Here their plumage probably attained its colourful blue, yellow, green, black and white tones. That is, all the colours that can be seen from space when looking at our planet. African birds tend to be colourful, because the brightness of plumage is an honest signal of health and freedom from parasites. Parasite abundance increases with proximity to the equator. About 3 million years ago, these African blue tits colonised the Canary Islands, forming new isolated populations on 7 of the islands, there are now 4 recognised species of blue tit on the archipelago, the Tenerife blue tit, the Fuerteventura blue tit, the Palma blue tit and the Hierro blue tit.
Meanwhile, about 5.33 million years ago, the Mediterranean Sea formed when a sun-parched salty crater separating Europe from Africa filled with water from the Atlantic Ocean via the Strait of Gibraltar. This historic event is called the Messinian salinity crisis and the Zanclean flood. After this, it became possible for the isolated blue tits of North Africa to recolonise Europe. They spread north, dispersing through Spain, France, central and eastern Europe, eventually reaching Britain where they again became semi-isolated forming our own sub-species of blue tit (Cyanistes caeruleus obscurus). Blue tits are still colonising Finland right now, and are being found further north each year. A few are now even breeding north of the Arctic Circle.
As blue tits moved north and particularly east, they became paler, eventually losing much of the blue and yellow of their western cousins. Another species was formed; this is called the azure tit (Cyanistes cyanus). Then as azure tits spread back into the warmer regions of Central Asia, they began to re-acquire their yellow hues on their chest, and yet another species was formed north of India and parts of China. This species is known as the yellow-breasted tit (Cyanistes flavipectus).