The beak or bill of the bird is made up of two jaws, the upper mandible and the lower mandible.
The upper jaw of birds, unlike that of humans, is capable of movement. This is technically known as cranial kinesis. In prokinesis, the mobility of the upper mandible is at the junction with the skull. When the mobility is further out, as in the Common Snipe (Gallinago gallinago) where the tip of the upper mandible can be flexed upwards, it is known as rhynchokinesis LINK. This is usually seen among birds feeding in water LINK.
Cranial kinesis allows birds to use their bills like a pair of pincers to pick up food. In some birds the cranial kinesis is poorly developed such that the bill cannot open wide. This is related to their feeding habits. Birds that need to open their bills wide so as to seize a frog for example, have considerable kinesis.
In Javan Myna, prokinesis is reasonably well developed to allow the mandibles to part the soil, exposing hidden insects and other arthropods like earthworms (see image at top and video above).
YC Wee
Singapore
15th April 2018
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