Patterns of testosterone in three Nearctic–Neotropical migratory songbirds during spring passage

• Testosterone levels were evaluated during vernal migration in 3 songbird species.
• Testosterone is higher when males are closer to breeding grounds.
• Testosterone does not change during vernal migration in female songbirds.
• Increases in testosterone are related to date.

Preparation for breeding may overlap extensively with vernal migration in long-distance migratory songbirds. Testosterone plays a central role in mediating this transition into breeding condition by facilitating changes to physiology and behavior. While changes in testosterone levels are well studied in captive migrants, these changes are less well known in free-living birds. We examined testosterone levels in free-living Nearctic–Neotropical migrants of three species during their vernal migration. Testosterone levels increased during the migratory period in males of all three species but significantly so in only two. Testosterone levels in females remained the same throughout their migration. Our results support the extensive overlap between vernal migration and breeding preparation in male songbirds. The pattern of testosterone changes during vernal migration is far from clear in females.