Connecting the dots: Understanding migration in the context of other periods of the annual cycle
Migration is a fundamental characteristic of the life history of many organisms. Large-scale seasonal movements expose migratory species to an array of differing environmental conditions such that the ecology, behavior, and life history strategies of migratory species must balance selection pressures associated with each phase of the annual cycle. Therefore, scientists increasingly realize that to understand the ecology of a migratory species in any one phase of the annual cycle requires an understanding of how other phases may interact with and influence the period of interest. That said, following individuals throughout the year poses a serious challenge. The focus of my dissertation research was to examine factors important to migration in the context of other periods of the annual cycle. From a broad-scale perspective, I examined how global climatic cycles such as El Niño Southern Oscillation in geographically different regions of the world carry-over to impact the migratory success of numerous intercontinental migratory bird species. I directly associated climatic variability experienced at over-winter areas with factors important to the success of migration, namely timing and condition, providing strong evidence that not only are migratory birds during spring migration influenced by events occurring during the previous phase of their annual cycle, but where they over-winter determines how vulnerable they are to global climatic cycles. From an individual-scale perspective, I examined (1) how conditions prior to the onset of migration carry-over to affect migration the success of black-and-white warblers ( Mniotilta varia ), and (2) how a warbler's migration stopover strategy interacts with other periods of the annual cycle. I found that the quality of a bird's over-winter habitat strongly influenced the timing of migration with cascading impacts on a warbler's migration strategy during stopover. However, the distance remaining to a bird's final breeding area destination also played a strong role in the strategy a bird utilized at a stopover site. Last, I integrated information from stable isotopes and genetic markers to geographically link individual Wilson's warblers ( Cardellina pusilla ) captured at a stopover site in the southwestern U.S. during spring migration with their ultimate breeding destination in North America. These studies demonstrate that we can begin to disentangle the complexity of migration when we examine factors known to be important to the success of migration in the context of other periods of the annual cycle. This is a critical step towards understanding population dynamics of migratory species, given that migration is the phase of the annual cycle most often thought to be limiting migratory birds.