Red junglefowl in the context of Galliformes

Flightless birds are birds that cannot fly, as they have, through evolution, lost the ability. There are over 60 extant species, including the well-known ratites (ostriches (Struthio), emus (Dromaius), cassowaries (Casuarius), rheas, and kiwis (Apteryx)) and penguins (Sphenisciformes). The smallest flightless bird is the Inaccessible Island rail (length 12.5 cm, weight 34.7 g). The largest (both heaviest and tallest) flightless bird, which is also the largest living bird in general, is the common ostrich (2.7 m, 156 kg).

Some domesticated birds, such as the domestic chicken, have lost the ability to fly for extended periods, although their ancestral species, the red junglefowl and others, respectively, are capable of extended flight. A few particularly bred birds, such as the Broad Breasted White turkey, have become totally flightless as a result of selective breeding; the birds were bred to grow massive breast meat that weighs too much for the bird's wings to support in flight.

The chicken (Gallus gallus domesticus) is a domesticated subspecies of the red junglefowl (Gallus gallus), originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is one of the most common and widespread domesticated animals in the world. Chickens are primarily kept for their meat and eggs, though they are also kept as pets.

As of 2023, the global chicken population exceeds 26.5 billion, with more than 50 billion birds produced annually for consumption. Specialized breeds such as broilers and laying hens have been developed for meat and egg production, respectively. A hen bred for laying can produce over 300 eggs per year. Chickens are social animals with complex vocalizations and behaviors, and feature in folklore, religion, and literature across many societies. Their economic importance makes them a central component of global animal husbandry.

Plumage (from Latin pluma 'feather') is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with sex and age classes. Within a few species, there can be different colour morphs. The placement of feathers on a bird is not haphazard but rather emerges in organised, overlapping rows and groups, and these feather tracts are known by standard names.

Most birds moult twice a year, resulting in a breeding plumage and a non-breeding plumage; one of the moults, usually the one just after breeding, is a complete moult replacing all the feathers; the other, usually the one just before breeding, is often only a partial moult, with new small body feathers but not replacing the larger flight feathers in the wings and tail. Some very large birds, like eagles, replace their flight feathers slowly but continuously throughout the year, to minimise loss of flight efficiency. Many ducks and some other species such as the red junglefowl have males wearing a bright plumage while breeding and a drab eclipse plumage for some months afterward. Many passerine species have only one moult per year, with changes in plumage resulting from the wear of differently-coloured feather tips. Young birds have a juvenile plumage, which is replaced in the months after fledging by the first-winter plumage; in long-lived birds with slow maturation like gulls, this is followed by a succession of second, third, and sometimes fourth year immature plumages.

Junglefowl are the only four living species of bird from the genus Gallus in the bird order Galliformes, and occur in parts of South and Southeast Asia. One of the species in this genus, the red junglefowl, is of historical importance as the direct ancestor of the domestic chicken, although the grey junglefowl, Sri Lankan junglefowl and green junglefowl are likely to have also been involved. The Sri Lankan junglefowl is the national bird of Sri Lanka. They diverged from their common ancestor about 4–6 million years ago. Although originating in Asia, remains of junglefowl bones have also been found in regions of Chile, which date back to 1321–1407 CE, providing evidence of possible Polynesian migration through the Pacific Ocean.

The junglefowl are omnivorous, eating a variety of leaves, plant matter, invertebrates such as slugs and insects, and occasionally small mice and frogs. These are large birds, with colourful plumage in males, but are nevertheless difficult to see in the dense vegetation they inhabit.

The gray junglefowl (Gallus sonneratii), also known as Sonnerat's junglefowl, is one of the wild ancestors of the domestic chicken together with the red junglefowl and other junglefowls.

The species epithet commemorates the French explorer Pierre Sonnerat. Local names include Komri in Rajasthan, Geera kur or Parda komri in Gondi, Jangli Murghi in Hindi, Raan kombdi in Marathi, Kattu Kozhi in Tamil and Malayalam, Kaadu koli in Kannada and Tella adavi kodi in Telugu.

The Sri Lankan junglefowl (Gallus lafayettii sometimes spelled Gallus lafayetii), also known as the Ceylon junglefowl or Lafayette's junglefowl, is a member of the Galliformes bird order. It is a common endemic bird in Sri Lanka, where it is the national bird. It is closely related to the red junglefowl (G. gallus), the wild junglefowl from which the chicken was domesticated. However, it is even more closely related to the grey junglefowl. Sri Lankan junglefowl and red junglefowl diverged about 2.8 million years ago, whereas time of divergence between the Sri Lankan junglefowl and grey junglefowl was 1.8 million years ago.

Evidence of introgressive hybridization from Sri Lanka junglefowl has also been established in domestic chicken. The specific name of the Sri Lankan junglefowl commemorates the French aristocrat Gilbert du Motier, marquis de La Fayette (1757–1834).