The transition from migration to breeding and demography of yellow warbler (Setophaga petechia)

Migration and reproduction are often considered discrete stages of birds' life cycle with unique physiological adaptations. The speed of the transition from a migratory to a reproductive physiology is likely to have significant fitness consequences. I examined the transition from migration to breeding in yellow warbler (Setophaga petechia), a long-distance neotropical migrant, and explored the factors and mechanisms through which migration influences annual survival, breeding phenology and productivity. I showed that females arrived on the breeding grounds with elevated plasma triglyceride levels compared to males. Some females had plasma triglyceride levels consistent with an advanced stage of yolk precursor production suggesting that they can initiate these changes while on migration. Next, I confirmed that the timing of breeding in yellow warblers is linked to weather conditions on migration and investigated potential pathways in which this carryover effect may arise. Crosswind speed experienced during the 14-day period before arriving on the breeding grounds influenced the delay between female arrival and egg laying, but not arrival timing, residual mass or their reproductive state (triglyceride levels) on arrival. However, females that arrived with higher plasma triglyceride levels took less time to initiate their first clutch. Wind speed on migration and reproductive state on arrival independently influenced the delay between arrival and egg laying, which in turn influenced productivity because initiating reproduction early increased both their chance to raise at least 1 nestling and the number of nestlings fledged. Finally, I used spatial Cormack-Jolly-Seber (CJS) models to show that sex-differences in dispersal can lead to erroneous conclusions about survival, and that incorporating dispersal can both reduce the bias and increase the accuracy of survival estimates. The spatial CJS model describing dispersal using a t-distribution that varied with sex estimated annual survival of yellow warblers to be 0.08-0.16 higher (18-42% higher than apparent survival). The longer dispersal distances of females largely explained the sex differences in apparent survival estimates obtained using conventional CJS models. Together, these findings highlight the importance of considering events that occur in previous stages of the annual cycle when analyzing demographic data.