eWestern Birds

The Quarterly Journal of Western Field Ornithologists

Vol. 49, No. 3
August 2018
Western Field Ornithologists


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Sexually Dimorphic Plumage Characteristics in the Northern Black Swift
Carolyn Gunn, Kevin J. Aagaard, Kim M. Potter, and Jason P. Beason

ABSTRACT: Most published accounts of the Northern Black Swift (Cypseloides niger borealis) contain disagreements and inaccuracies concerning the species’ sexual dimorphism in plumage. These reports represent small sample sizes and are not based on birds recaptured and followed over time. In our study, to gain a better understanding of dimorphic plumage characteristics, we captured swifts at four breeding colonies in the Rocky Mountains from 2004 through 2017. We banded breeding adults, recorded eight metrics (wing chord, flat wing, length of the outermost [r5] and innermost [r1] rectrices, the difference in these lengths, length of white tipping on the abdomen and undertail coverts, and weight), and photographed the birds’ underparts. We followed these characteristics in birds recaptured for up to 13 years. The difference in the mean depth of the tail fork (r5 – r1) was the most definitive plumage characteristic distinguishing the sexes of adults, being accurate in 95% of the birds examined. With few exceptions, males had only slight white tipping of the abdominal feathers and undertail coverts, whereas females had significantly longer white tipping on those feathers. Individuals that were recaptured did not change appreciably over time in wing chord, depth of tail fork, or degree of white tipping on the abdomen and undertail coverts. This is the largest and longest study of sexual dimorphism in the Northern Black Swift and clarifies previous misinterpretations of these characteristics.

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