Overlapping Life-History Stages in Migrating Songbirds: Variation in Circulating Testosterone and Testosterone Production Capacity

Document Type

Article

Publication Date

1-2017

Department

Biological Sciences

School

Biological, Environmental, and Earth Sciences

Abstract

Understanding the extent of overlap between life-history stages is fundamental to understanding full-life cycle biology, especially for migratory species. Testosterone levels vary throughout the annual cycle in seasonally reproducing vertebrates. In migratory songbirds, testosterone increases associated with breeding preparation may overlap with the vernal migratory period; however, this overlap remains largely unexplored. We test the hypothesis that both circulating testosterone and the capacity to elevate testosterone increases throughout vernal migration in long-distance songbird migrants. Here we relate testosterone in songbirds sampled en route with the stable hydrogen isotope ratios in their feathers as a metric of breeding ground proximity. We determined the capacity to elevate testosterone using field gonadotropin-releasing hormone bioassays and related this to feather hydrogen ratios as well. Males that were closer to their breeding grounds had higher circulating testosterone, whereas there was no relationship between testosterone and breeding ground proximity in females. Similarly, while capacity to elevate testosterone was not related to breeding ground proximity in female migrants, this capacity was greater in males closer to their breeding grounds than those further away. These results reveal that male migrants prepare for breeding during their vernal migration, whereas the schedule for breeding preparation among females is less clear and may be more complex.

Publication Title

Journal of Ornithology

Volume

158

Issue

1

First Page

203

Last Page

212

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