Amongst the families, the three Nestoridae species are all large parrots, and the cockatoos tend to be large birds as well. The Psittacidae parrots are far more variable, ranging the full spectrum of sizes shown by the family. The most obvious physical characteristic that characterises parrots is their strong, curved, broad bill.
The upper mandible is prominent, curves downward, and comes to a point. It is not fused to the skull, which allows it to move independently, and contributes to the tremendous biting pressure these birds are able to exert. The lower mandible is shorter, with a sharp, upward facing cutting edge, which moves against the flat portion of the upper mandible in an anvil-like fashion.
Seed eating parrots have a strong tongue which helps to manipulate seeds or position nuts in the bill so that the mandibles can apply an appropriate cracking force. The head is large, with eyes positioned sideways, which limits binocular vision, but greatly enhances peripheral vision. Cockatoo species have a mobile crest of feathers on the top of their heads which can be raised for display, and retracted.
No other parrots can do so, but the Pacific lorikeets in the genera Vini and Phigys are able to ruffle the feathers of the crown and nape. The predominant colour of plumage in parrots is green, though most species have some red or another colour in small quantities.
Cockatoos are the main exception to this, having lost the green and blue plumage colours in their evolutionary history they are now predominately black or white with some red, pink or yellow. Strong sexual dimorphism in plumage is not typical amongst the parrots, with some notable exceptions, the most striking being the Eclectus Parrot.
