A feeding adaptation of the jaw articulation in new world jays (Corvidae)

--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 i related to the New World jays, not to Nucifraga. An example of convergent evolution is provided by Hemignathus wilsoni (Drepanidinae). Received 15 August 1986, accepted 22 April 1987. MANY members of the Corvidae pound hard food items with the tip of the bill while holding the food against a firm substrate with one or both feet. Most birds that pound with their bills on a hard substrate (e.g. woodpeckers) use both mandibles or primarily the upper for the transmission of force (Spring 1965). Because of its unstable suspension from the cranium, the lower jaw alone appears not to be an effective implement for strong pounding. For this reason my curiosity was aroused by an unusual configuration in the lower jaw and quadrate of a New World jay that implied use of only the lower mandible in pounding. I studied modifications of the lower jaw and its articulation as a functionally integrated character complex. The variation and taxonomic distribution of this apomorphous complex have some phylogenetic implications within the Corvidae (sensu Blake and Vaurie 1962). Although acorns and other nuts are important food items for many species of Corvidae, the only well-defined modification of the bill for cracking nuts is the rhamphothecal bulge in the lower jaw of the nutcrackers (Nucifraga; references in Turcek and Kelso 1968). Most corvids have stout bills capable of a variety of manipulations. The modifications I describe are 665 found only in certain New World jays and are clearly associated in some species with a special technique for opening nuts, especially acorns. Within the Corvidae, however, the occurrence of this character complex does not correlate well with the occurrence of acorn eating; it appears to represent an evolutionary novelty of broader ecological significance within the family. MATERIALS AND METHODS I examined skulls of 76 species from all genera of the Corvidae except Temnurus and Zavattariornis. The jaw articulations of 2 additional species were observed in skin specimens by softening and reflecting a flap of skin. I also examined 13 species from 4 families allied to the Corvidae. To determine the action of the jaw articulation and its ligaments, I manipulated a freshly killed Blue Jay (Cyanocitta cristata) and American Crow (Corvus brachyrhynchos). Information about ontogenetic stages of the features under study was obtained by clearing and staining (Taylor 1967) two nestlings of the Florida Scrub Jay (Aphelocoma coerulescens coerulescens) and by dissection of a hatchling Fish Crow (Corvus ossifragus). Jaw muscles of an adult Eurasian Jay (Garrulus glandarius) and one adult Blue Jay were dissected. I drew the original illustrations in pencil, using a dissecting microscope and drawing tube for all anatomical figures; final figures were inked by an illustrator. Field observations, phoThe Auk 104: 665-680. October 1987 666 RICHARD L. ZUSI [Auk, Vol. 104