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A comparison of mandibular structure in ruddy and masked ducks (Oxyura jamaicensis and Nomonyx dominicus).

Article Excerpt
Abstract.-In order to test inferred differences in foraging behavior of both the masked duck (Nomonyx dominicus) and the ruddy duck (Oxyura jamaicensis), this study examined nine characters and three ratios in the mandibles of 25 male and 25 female museum specimens of both species. Statistical analysis indicated that O. jamaicensis had longer lamellae and a slightly broader bill than N. dominicus (data for both sexes combined), which may adapt O. jamaicensis for efficiently sifting through bottom silt. Specimens of N. dominicus exhibited a stouter bill (greater height/length ratio of the maxilla), supporting the premise that this species is principally adapted for grazing on tough plants. Most bill characters were greater in length or width in O. jamaicensis as opposed to the masked duck and in ruddy duck males as opposed to ruddy duck females. Also, maxilla height (absolute as well as relative to dorsal maxilla length) was greater in male than in female masked ducks, suggesting a possible signal function in bill shape. Based on bill morphology, this study supports the premise that the genus Nomonyx is less advanced than the Oxyura/Biziura group.

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Ducks of the tribe Oxyurini are distinguished from all other waterfowl by their elongated and pointed tail feathers and stiffened shafts (Johnsguard & Carbonell 1996). Within the tribe Livezy (1995) proposed the generic addition of Nomonyx (originally proposed by Salvadori 1895) differentiating the masked duck (Nomonyx dominicus) from other Oxyura based on a variety of characters including morphology and behavioral traits. Considering the premise that morphology represents aspects of the relationships between the organism and its environment (Bock 1977), this study examined mandibular traits of masked and ruddy ducks as a reflection of their foraging ecology.

Methods and Materials

The following measurements were made on 50 museum...

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