SEXUAL SIZE DIMORPHISM IN RED-NECKED PHALAROPES AND FUNCTIONAL-SIGNIFICANCE OF NONSEXUAL BILL STRUCTURE VARIATION FOR FEEDING PERFORMANCE

Sexual size dimorphism is widespread in shorebirds, yet no tests of th e assumption that such size dimorphism extends to functionally signifi cant dimensions of the bill exist. This report presents tests of: (1) the assumption that sexual size dimorphism extends to the feeding stru ctures in a sexually size dimorphic bird, and (2) the hypothesis that bill-size variation influences feeding performance in Phalaropus lobat us, the red-necked phalarope. Discriminant function analysis revealed that the sexes of this species can be distinguished on the basis of fi ve body size/bill length variables, but with low accuracy in sexing of females because of misclassification of small females as males. In th e shorebird literature, the assumption is generally made that in the a bsence of selection to the contrary, bill size scales to body size and hence sexual size dimorphism extends to bill size. However, discrimin ant function analysis of measures from red-necked phalaropes failed to separate the sexes on the basis of either external or internal bill d imensions other than length. Nonetheless, internal dimensions of the u pper jaw combined with exposed culmen length explained 86% of the vari ance in feeding performance of phalaropes; high feeding performance de pends on a wide, shallow, complex internal bill structure. This study provides evidence that internal bill dimensions determine feeding perf ormance in a manner consistent with the mechanics of surface tension t ransport of prey. These results suggest that some dimensions of bill s ize may be constrained by performance demands and demonstrate that var iation in bill morphology has functional consequences. (C) 1996 Wiley- Liss, Inc.