A Feeding Adaptation of the Jaw Articulation in New World Jays (Corvidae)

Abstract

Abstract The jaw articulation of most endemic New World jays (Corvidae) has a condyle of the quadrate and an opposing cotyla of the lower jaw not found in other birds. They also have well-developed meatic articular facets of the quadrate and cranium. The tip of the rhamphotheca of the lower mandible is chisel shaped. These and other features constitute a functional unit, the buttress complex, that braces the partially opened lower jaw and enhances its use as a chisel. The buttress complex stabilizes the lower jaw by anchoring the jaw on the quadrate and reducing torque on the quadrate during pounding. A hypothesis of pounding with the lower mandible was confirmed by field observations of Aphelocoma coerulescens coerulescens, which stabs acorns with the lower mandible and then tears off the shell using both mandibles. This may be an unusually effective method of peeling acorns, and it differs from the techniques used by Garrulus and Pica. The origin of the complex may not be associated with acorn eating. A slight modification of the jaw articulation in Cyanolyca probably represents the evolutionary precursor of the buttress complex. The distribution of the complex in the Corvidae suggests that Cyanolyca is the sister group of other endemic New World jays. Gymnorhinus is related to the New World jays, not to Nucifraga. An example of convergent evolution is provided by Hemignathus wilsoni (Drepanidinae).