Transition Between Phases of the Annual Cycle: Spring Migration to Breeding in Nearctic-Neotropical Songbirds

Appropriate timing of each life-history stage is crucial for seasonally migratory species. The temporal constraints faced by migratory songbirds require that they overlap preparation for breeding with spring migration. However, previous work has focused primarily on male birds and has produced inconsistent results regarding the degree of overlap between these two life-history stages. I study the degree to which migrating male and female songbirds prepare for breeding throughout spring migration as they move towards their breeding grounds. Overall, male migrants show a significant degree of breeding preparation during spring migration as determined by circulating testosterone levels and their ability to elevate testosterone. Female migrants, on the other hand, did not vary in their degree of breeding preparation throughout the migratory period. That said, in both male and female migrants, some degree of breeding preparation had occurred previous to their passage through my migratory study areas. It is possible that while male migrants continue to prepare for breeding throughout spring migration, female migrants delay the final stages of breeding preparation until they arrive on the breeding grounds. Since testosterone increases in some birds during the migratory periods and mediates a wide range of effects during the breeding season, I also investigated whether testosterone was related to migratory stopover biology. To do so I looked for correlates between testosterone and the following measures: likelihood of stopover, stopover duration, level of competition for resources, foraging movement rate, and prey attack rate. However, I was unable to detect any effects of testosterone on the measures of migratory stopover biology that I used in this study. It is possible that since levels are relatively low during migration, testosterone does not directly influence the expression of migratory traits