Flying is a highly successful way of life, but not without drawbacks. Among birds, a major difficulty is the inability to manipulate small objects with hands, these now being part of the flight mechanism. So what's a bird to do? Among various solutions that evolution has come up with is the use of the bill as a specialized tool.
One neat example of this, well represented among Chihuahuan Desert birds, is a bill adapted to removing the hard coat from seeds. Finches generally have grooves on the under edge of the upper bill and sharp edges on the lower. A seed picked up by the beak is manipulated into the groove; then the lower jaw moves back and forth, the edge acting like a knife to cut through the coat as the seed is rolled along the groove. Side to side movements of the jaw then eject the pieces of coat and send the eatable kernel into the mouth. Not exactly the slice and dice of modern kitchen appliances, but it's worked well enough for millions of years.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.
Lower jaw and bill of Black-headed Grosbeak. The lower jaw (on left) has the biting surface facing to the right. The arrow points to the structure that initiates the cutting of a seed coat. The upper jaw is shown from below (and slightly obliquely). The two arrows are at the ends of a groove formed by the edge of the bill and a ridge that rises in parallel but nearer the center; this is the structure that holds the seed while the lower bill works on it.
Welty, J. C. 1982. The life of birds. Saunders College Publishing, 3rd ed., Philadelphia, 754 pp. [See p. 123 for diagram]