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1 September 2006 COMPARATIVE SPRING MIGRATION ARRIVAL DATES IN THE TWO MORPHS OF WHITE-THROATED SPARROW
SARAH S. A. CALDWELL, ALEXANDER M. MILLS
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Abstract

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.

SARAH S. A. CALDWELL and ALEXANDER M. MILLS "COMPARATIVE SPRING MIGRATION ARRIVAL DATES IN THE TWO MORPHS OF WHITE-THROATED SPARROW," The Wilson Journal of Ornithology 118(3), 326-332, (1 September 2006). https://doi.org/10.1676/05-047.1
Received: 25 April 2005; Accepted: 1 February 2006; Published: 1 September 2006
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