Ma. Rubega, SEXUAL SIZE DIMORPHISM IN RED-NECKED PHALAROPES AND FUNCTIONAL-SIGNIFICANCE OF NONSEXUAL BILL STRUCTURE VARIATION FOR FEEDING PERFORMANCE, Journal of morphology, 228(1), 1996, pp. 45-60
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.