Figure 1. Cranial kinesis. Left: Early evolution of cranial kinesis. Letters refer to similar bones, illustrating subsequent changes in their shape and presence during the processes of cranial fenestration and palate detachment. Grey areas are windows. Blue area is the quadrate (q) articulating at last with skull and lower mandible. Windows are the orbit (o), upper and lower temporal fenestrae (utf, ltf) and nares (na). Upper right: Prokinesis and coupled kinesis (crow). The right upper blue dot indicates the frontonasal hinge around which the upper mandible is elevated if the quadrate is swung forward by the protractor muscle (1) around the left upper dot. This action slackens the dashed red ligament that connects skull and lower mandible, so that the lower mandible is deblocked and can be depressed by the depressor muscle (2). Middle right: Palaeognath rhynchokinesis (ostrich). Lower right: Neognath rhynchokinesis (plover). Red bars refer to flexible areas. Dotted areas refer to upper mandible and jaw apparatus. aof, anterior orbital fenestra; f, frontal; j, jugal; l, lacrimal; ltf, lower temporal fenestra; m, maxilla; n, nasal; na, naris; o, orbit; p, parietal; pf, prefrontal; pm, premaxilla; po, postorbital; pof, postfrontal; q, quadrate; qj, quadratojugal; sq, squamosal; utf, upper temporal fenestra.

Figure 2. Ingestion by the pecking mechanism. (a) Behavioural patterning. (Redrawn after Zeigler et al., 1980, Journal of Comparative Physiology and Psychology 94: 783–794.) (b) Process of ingestion. Seed 1 is tested by touch and taste, and repositioned for proper oral transport. Seed 2 is stored and tested by taste at the lingual base prior to swallowing. A bill tip touch organ lies underneath a hard horny rhamphotheca. The dashed lines indicate areas of taste organs and mucus orifices. (c) Phases of ingestion.