The three middle metatarsals become fused together into a cannon bone; the upper part of the third middle metatarsal projects behind and forms the so-called hypotarsus, which in various ways, characteristic of the different groups of birds (with one or more sulci, grooved or perforated), acts as guiding pulley to the tendons of the flexor muscles of the toes.
Ardeae.-Piscivorous, nidicolous, waders; with complicated hypotarsus and with long cervical apteria.
Ciconiae.-Zoophagous, nidicolous, waders; with simple hypotarsus and without cervical apteria.
Nidifugous, waders; with simple hypotarsus and without cervical apteria.
From the Mid-Miocene to the Oligocene of France are known several species of Palaelodus, Elornis and Agnopterus, which have relatively shorter legs, longer toes and a complicated hypotarsus, and represent an earlier family, less specialized although not directly ancestral to the flamingos.