Generally, Old World sparrows are small, plump, brown and grey birds with short tails and stubby, powerful beaks.

Sparrows are physically similar to other seed-eating birds, such as finches, but have a vestigial dorsal outer primary wing feather and an extra bone in the tongue.

[4] Some classifications also include the sparrow-weavers (Plocepasser) and several other African genera (otherwise classified among the weavers, Ploceidae)[4] which are morphologically similar to Passer.

[7] According to a study of molecular and skeletal evidence by Jon Fjeldså and colleagues, the cinnamon ibon of the Philippines, previously considered to be a white-eye, is a sister taxon to the sparrows as defined by the HBW.

[7] Many early classifications of the Old World sparrows placed them as close relatives of the weavers among the various families of small seed-eating birds, based on the similarity of their breeding behaviour, bill structure, and moult, among other characters.

Like sparrows, the estrildid finches are small, gregarious and often colonial seed-eaters with short, thick, but pointed bills.

The 2008 Christidis and Boles taxonomic scheme lists the estrildid finches as the separate family Estrildidae, leaving just the true sparrows[clarification needed] in Passeridae.

[8] Despite some resemblance such as the seed-eater's bill and frequently well-marked heads, New World sparrows are members of a different family, Passerellidae, with 29 genera recognised.

In the Americas, Australia, and other parts of the world, settlers imported some species which quickly naturalised, particularly in urban and degraded areas.

The great sparrow is an exception, breeding in solitary pairs and remaining only in small family groups in the non-breeding season.

They form large roosting aggregations in the non-breeding seasons that contain only a single species (in contrast to multi-species flocks that might gather for foraging).

Attempts at large-scale control have failed to affect populations significantly, or have been accompanied by major increases in insect attacks probably resulting from a reduction of numbers, as in the Great Sparrow Campaign in 1950s China.

[17] Birds usually described later as Old World sparrows are referred to in many works of ancient literature and religious texts in Europe and western Asia.

[citation needed] They are also difficult to keep, as pet sparrows must be raised by hand and a considerable amount of insects are required to feed them.