Before the beak
THE beak of a bird is special as it is one of the unique features which distinguishes the whole class of birds from other animal classes. Birders like beaks because they help to identify, age and even sex species, but what most birders don’t realise is just how flexible beaks are. The beak is part bone and part keratin, the latter forming an outer shell which forms the most visible part of the beak. It is this combination which makes a beak both light and strong. There are two main parts to the beak: the upper part is the maxilla or upper mandible, and the lower is the mandible or lower mandible. The maxilla is attached to the skull by a central bone joined to the forehead, and two side bones which attach to the skull by a series of flat palate bones. The mandible is formed from two bones which are joined at the tip, each of them being fixed to the side of the skull via the jaw muscles. The lower jaw can be moved up and down while fixed against the skull. Meanwhile, the upper mandible can be flexed up and down using the mobile palatine bones and the frontal hinge.
Such flexibility is the key to how woodpeckers extract their beaks from wood. Studies of Black Woodpeckers using highspeed film which can then be slowed down, showed that when hammering, the upper mandible can be slid slightly backwards out of the hole and then by tipping the head, the fixed lower mandible slides under the upper and the beak can be freed. The whole process takes a mere 70 microseconds, allowing three pecks per second. Some beaks are flexible in other ways. Nightjars have an intramandibular joint which enables them to open their beak wide both vertically and horizontally to produce an extremely wide gape when feeding. Similarly, hummingbirds can bow their jaws when opening to help them to catch insects, which form an important part of their diet. Many waders have a flexible tip to the upper mandible which can be bent upwards, enabling a food item to be secured – a process known as distal rhynchokinesis.
The classification of birds is based on many features and in the 1860s it was first suggested that the structure of the maxilla and associated palate could be used to distinguish between two groups, the paleognaths (ratites like Ostrich or Emu, as well as tinamous) and the neognaths (all other bird species). The ratites and tinamous have a maxilla which is rigidly fixed to the skull and cannot be moved, while other birds have a flexible joint which enables them to move it. This means the beaks of neognaths can be flexible in lots ways, which may be useful for nest building, preening, feeding and many other activities. It was always presumed that the ratites possessed an original, ancient form of beak, while the others had more recently evolved from this. Now this has been shown to be wrong, thanks to a fossil found in Belgium. The fossil species has been named Janavis finalidens and a recent study shows that it had a mobile upper jaw, like modern neognaths. While its toothed beak fits with it being an ancient species, the modern jaw should not have existed then, which means that the ‘modern’ beak structure is actually much more ancient and may even predate the original paleognath.