The great tit is a distinctive bird with a black head and neck, prominent white cheeks, olive upperparts and yellow underparts, with some variation amongst the numerous subspecies.

It is predominantly insectivorous in the summer, but will consume a wider range of food items in the winter months, including small hibernating bats.

The great tit has adapted well to human changes in the environment and is a common and familiar bird in urban parks and gardens.

[5] The great tit was formerly treated as ranging from Britain to Japan and south to the islands of Indonesia, with 36 described subspecies ascribed to four main species groups.

This form was once thought to form a ring species around the Tibetan Plateau, with gene flow throughout the subspecies, but this theory was abandoned when sequences of mitochondrial DNA were examined, finding that the four groups were distinct (monophyletic) and that the hybridisation zones between the groups were the result of secondary contact after a temporary period of isolation.

[11] The nominate subspecies of the great tit is the most widespread, its range stretching from the Iberian Peninsula to the Amur Valley and from Scandinavia to the Middle East.

The plumage of the female is similar to that of the male except that the colours are overall duller; the bib is less intensely black,[10] as is the line running down the belly, which is also narrower and sometimes broken.

[15] Young birds are like the female, except that they have dull olive-brown napes and necks, greyish rumps, and greyer tails, with less defined white tips.

P. m. excelsus is similar to the nominate race but has much brighter green upperparts, bright yellow underparts and no (or very little) white on the tail.

Higher levels of carotenoid increase the intensity of the yellow of the breast its colour, and also enable the sperm to better withstand the onslaught of free radicals.

Populations may become irruptive in poor or harsh winters, meaning that groups of up to a thousand birds may unpredictably move from northern Europe to the Baltic and also to Netherlands, Britain, even as far as the southern Balkans.

[22] The great tit was unsuccessfully introduced into the United States; birds were set free near Cincinnati, Ohio between 1872 and 1874 but failed to become established.

Suggestions that they were an excellent control measure for codling moths nearly led to their introduction to some new areas particularly in the United States of America, however this plan was not implemented.

[23] A small population is present in the upper Midwest, believed to be the descendants of birds liberated in Chicago in 2002 along with European goldfinches, Eurasian jays, common chaffinches, European greenfinches, saffron finches, blue tits and Eurasian linnets, although sightings of some of these species pre-date the supposed introduction date.

[26] Their larger invertebrate prey include cockroaches, grasshoppers and crickets, lacewings, earwigs, bugs (Hemiptera), ants, flies (Diptera), caddisflies, beetles, scorpionflies, harvestmen, bees and wasps, snails and woodlice.

[27] A study published in 2007 found that great tits helped to reduce caterpillar damage in apple orchards by as much as 50%.

[10] In England, great tits learned to break the foil caps of milk bottles delivered at the doorstep of homes to obtain the cream at the top.

One study in Germany found that 40% of nests contained some offspring fathered by parents other than the breeding male and that 8.5% of all chicks were the result of cuckoldry.

[37] Great tits are cavity nesters, breeding in a hole that is usually inside a tree, although occasionally in a wall or rock face, and they will readily take to nest boxes.

The nest inside the cavity is built by the female, and is made of plant fibres, grasses, moss, hair, wool and feathers.

Once feathers begin to erupt, the nestlings are unusual for altricial birds in having plumage coloured with carotenoids similar to their parents (in most species it is dun-coloured to avoid predation).

[10] Both parents provision the chicks with food and aid in nest sanitation by removing faecal packets, with no difference in the feeding effort between the sexes.

[43] Inbreeding depression occurs when the offspring produced as a result of a mating between close relatives show reduced fitness.

In natural populations of P. major, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.

It was originally a specialist tit flea, but the dry, crowded conditions of chicken runs enabled it to flourish with its new host.

Although these beetles often remain in deserted nests, they can only breed in the elevated temperatures produced by brooding birds, tits being the preferred hosts.

[52] In the face of winter food shortages, the great tit has also shown a type of peripheral vasoconstriction (constriction of blood vessels) to reduce heat loss and cold injury.

[53] Reduced cold injury and heat loss is mediated by the great tits' counter-current vascular arrangements, and peripheral vasoconstriction in major vessels in and around the birds' bill and legs.

[53] Vasoconstriction of blood vessels in the bill not only serves as an energy saving mechanism, but also reduces the amount of heat transferred from core body tissues to the skin (via cutaneous vasodilation), which, in turn, reduces heat loss rate by lowering skin temperature relative to the environment.

While there have been some localised declines in population in areas with poorer quality habitats, its large range and high numbers mean that the great tit is not considered to be threatened, and it is classed as least concern on the IUCN Red List.