Latitudinal dines in sex ratio during the nonbreeding season occur in some
shorebirds of the Scolopacidae. We compared populations of nonbreeding Dunl
in (Calidris alpina pacifica) from two latitudes along the Pacific flyway:
the Fraser River Delta, British Columbia, and Bolinas Lagoon, California, t
o determine whether, and to what degree, they exhibited sex ratios consiste
nt with a latitudinal cline. Dunlin are plumage monomorphic, so we used a m
aximum likelihood model to estimate overall and monthly sex ratios for each
population based on culmen length distributions. Sex ratios in the Fraser
River Delta were corrected for sex differences in habitat use. Monthly sex
ratios were similar at the two sites but varied throughout the winter, like
ly reflecting differences in seasonal movement patterns between the sexes.
Both populations showed an overall bias toward males (Fraser = 61% males, B
olinas = 65% males). Since there is no evidence to support the possibility
of a skew toward males in C. a. pacifica as a whole, our data are consisten
t with some form of latitudinal dine in the sex ratio of C. a. pacifica. Ho
wever, additional data from the Oregon coast, southern California, and Mexi
co are required to resolve this question. We also tested the hypothesis tha
t mean body size within each sex is larger at the higher-latitude site (Fra
ser River Delta), but this did nor appear to be the case.