White-throated Sparrows (Zonotrichia albicollis) display a plumage dimorphism (white-striped and tan-striped) with attendant behavioral differences, including greater aggression levels in white-striped birds and negative assortative mating, in which tan-striped birds pair with white-striped birds. To determine whether morph influences migration timing, which could influence patterns of assortative mating, we evaluated the phenology of northbound migration among White-throated Sparrows from a long-term banding dataset collected at a southern Ontario banding station. White-throated Sparrows are sexed by wing-chord length, but there is an intermediate size for which sex cannot be assigned. When all birds were considered together (both known and unknown sexes, n = 6,243), the white-striped birds migrated earlier by slightly more than 2 days. The sexing criteria, however, appeared to yield a sample that was not representative of the whole population: when we included only birds for which sex was assigned (n = 2,794, 45% of all birds), white-striped birds apparently migrated earlier by more than 4 days, but separate analyses of males (n = 1,511) and females (n = 1,283) revealed no differences in migration timing between morphs. By measuring wing-chord lengths of internally sexed specimens (from the Royal Ontario Museum) collected during April to June (n = 273), we found that in both sexes the wings of white-striped birds were about 2% longer than those of tan-striped birds. When we used these specimen data to recalibrate the sexing criteria, (a) it was possible to assign sex to 1.47 times as many birds (n = 4,121; 66% of all birds), (b) sex ratios of the banded birds more closely approached what appears to be the natural sex ratio (approximately 1:1), and (c) within-sex analyses indicated that white-striped females migrate earlier than tan-striped females by about 1.3 days, whereas there was no statistical difference between male morphs in migration timing.
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Vol. 118 • No. 3