The wing of Penguins is very little differentiated. It is covered at the margin by overlapping scales which gradually merge into scale-like feathers at the proximal end. The wing of the Penguin has nothing comparable to the remiges of other birds.
In some birds, such as Herons (_Ardea_), there occur in places plumulae of a peculiar kind, which grow persistently and whose summits break off into fine powder as fast as they are formed. These feathers are known as _powder-down_ feathers. They occur also in some Parrots and are then scattered indiscriminately all over the body.
Other exoskeletal structures besides feathers are commonly well developed. Thus the extremities of the jaws are sheathed in horny =beaks= whose form varies enormously according to the special mode of life.
In ducks and geese the beak with the exception of the anterior end is soft, and its edges are raised into lamellae, while in the Mergansers these lamellae become pointed processes supported by bony outgrowths. These lamellae act as strainers. In Parrots and Hawks, on the other hand, nearly the whole of the beak is hard.
The toes and tarso-metatarsus are usually featherless and are covered either with granular structures or with well-formed scales. The toes are nearly always provided with =claws=, and these vary in correlation with the character of the beak. Claws[105] also sometimes occur on the manus. Thus _Archaeopteryx_ and some Ostriches and Rheas have claws on all three digits. Most Ostriches and Rheas, and many Anseres and birds of prey, have them on the first two digits, while the Secretary bird (_Gypogeranus_) and many fowls, ducks, and birds of prey, especially kestrels, have a claw only on the pollex. In the Cassowary, Emeu, Apteryx and some Ostriches and Rheas only the second digit is clawed.
Claws should not be confounded with =spurs=, which are conical horny structures developed on bony outgrowths of the radial side of the carpus, metacarpus, or metatarsus. They occur in a number of birds, but are most commonly developed in gallinaceous birds, by which they are used for fighting. A single spur occurs on the metacarpus in _Megapodius_, in _Palamedea_, in _Parra jacana_ and in _Hoplopterus spinosus_, the Spur-winged plover. The Derbian Screamer, _Chauna derbiana_, has two metacarpal spurs, borne on the first and second metacarpals. The Spur-winged goose, _Plectropterus gambensis_, has a carpal spur borne on the radial carpal. Metatarsal spurs are quite common.