As far as I know chaps...the Thickknees family including the Stone Curlews was so-called after the Thick-Kneed Bustard, and it refers only to the likeness of a thick knee, it is infact the extension of the area of the foot from the toes to the heel and ankle as you said. The true knees on birds are hidden by the feathers under the belly area. Even the Waders dont have knees where you'd expect a knee to be, as you say. As far as my knowledge extends, all birds have knees, they are high up on the leg, and are in proportion with the rest of the leg so as to provide the best possible locomotion to suit habits and tendencies for that species or family of bird, whether it be on land or in water eg.Divers (ungainly on land but masters underwater). Those birds that rarely use land or water as a medium TEND to have short legs, still with knees though, but they have adaptations elsewhere...eg.Hirundines (extra strong toes for clinging). In all cases knees are present but are used or underused as best suits the bird. Hope this is of help for you both, and I know there are more highly experienced birders on here that know a vast deal more about it than I do. Cheers Rob
I,m not sure how or why but i,ve got an issue of Birding World from 1993 (it,s not a woodcut) vol6 issue 3 which clearly indicates that waders at least have knees where you,d expect a knee to be maybe things have moved on since.
Forgive me for being pedantic as I'm not an expert, but I couldn't help being amused by a reference in another post to a wader's knees.
It's very rare to see a bird's "knees" as they are hidden by its plumage. The joint visible on a wader's leg is the "tibio-tarsal articulation" which apart from being a bit of a mouthful is actually the equivalent of the human ankle rather than knee .
maybe things have moved on since. 
