Migratory birds complete different stages of their life cycle in locations that can be thousands of miles apart. Conditions during one stage can impose carry-over effects on subsequent stages. It is critical that researchers examine how environmental factors act as carry-over effects across different annual stages in order to fully understand how migratory bird populations are regulated. In the American redstart (Setophaga ruticilla), males wintering in superior quality habitats arrive earlier at the breeding grounds and have greater breeding success than those wintering in poor habitats. While these seasonal interactions are well documented, the underlying mechanisms are not well understood. Energetic demands of spring migration can be compounded by the additional demands of transitioning from the non-breeding to breeding state. This study focuses on how ecological and endogenous factors interact during this transition in Jamaica, West Indies, and if hormones usually associated with breeding can also help migrants reach the breeding grounds earlier. Relationships among environmental conditions, energy stores, breeding preparation, and migration will be examined. Testosterone will be manipulated (enhanced or inhibited, compared to controls) in male redstarts during the period leading up to spring departure. Stable isotope signatures in feathers and/or claws will link wintering ground events with migration distance to the breeding grounds. Breeding readiness and migratory condition will be collected in other Neotropical migrant species wintering in Jamaica. This study will significantly add to our understanding of how wintering and breeding events are linked, and the role of T in a migration context. It will help resource managers develop effective management plans for migratory populations. The study brings together U.S. academic institutions and resource management agencies in Jamaica, and provides many career development experiences (field observations, sample collection, hormone manipulation, laboratory techniques) for graduate and undergraduate students interested in behavioral ecology, eco-physiology, and conservation.
Evolution has intricately honed the behavioral and physiological processes that enable migratory birds to reach their destinations. However, in spring, migrants face an enormous challenge as they not only have to cope with the increased energy demand of flight and the higher risks of predation and unpredictable food availability along the route, they also have to prepare for the annual event of reproduction. Those that can arrive the earliest and are ready to breed stand the greatest chance of high breeding success. Once considered to be separate and distinctly discrete stages of the annual cycle, researchers are uncovering how much migration and preparation for breeding overlap and that life history stages are defined by events occurring within the individual not by their movements or locations. This study helped further our knowledge of how events occurring during these periods of the annual cycle can influence each other, as ‘seasonal interactions’. In this study, in which we used experimental manipulations of hormone levels in conjunction with surveys of naturally occurring hormone patterns in the American redstart (Setophaga ruticilla) and other songbird species, we found that birds have the ability to begin breeding preparation well before they leave the wintering areas, but how well they do so can vary. Our study illustrated the important role that testosterone, previously considered primarily in a breeding context, plays during the development of migration preparation. In this way, testosterone regulation, as well as the environmental conditions affecting it, can be seen as a major physiological mechanism underlying ‘seasonal interactions’ in migratory birds. Species can differ, however, in how environmental factors affect breeding preparation and this variation among species likely reflects the different ways evolution has acted to shape different life history strategies. The results of this study demonstrate that managing and conserving migratory populations requires a detailed understanding of how different stages of the life-cycle overlap and interact within the annual cycle. Conserving species is a challenge: assessing what environmental perturbations may have greatest impact on a population and when requires a detailed understanding of physiology and behavior throughout the annual cycle. While global climate change models predict increasing precipitation and temperature on redstart breeding grounds, the major predicted change on the wintering grounds is a dramatic decline in precipitation. In this study, we show that male redstarts are capable of beginning the transition to breeding while still on the wintering grounds, presumably impacting an important determinant of reproductive output, the timing of arrival at breeding sites. Thus, for this and other species, research on adaptation to climate change should focus as much on the impacts of drought on migration phenology and breeding onset on their tropical wintering grounds as on the impacts of warmer temperatures on breeding success on their temperate breeding grounds. This illustrates the critical need for research that focuses on the entire life-cycle of organisms. Our project provided many opportunities for training students, several of whom have gone on to further their education and training in environmental research through graduate study and post-doctoral fellowships. The study maintained an important link between the Smithsonian Institute, The Field Museum, the Jamaican National Environmental Protection Agency, and the University of Maine. The results from this study have been prepared for publication and presentation not only for the scientific audience but have also been featured in public outreach and education presentations and in 'citizen science' programs sponsored by the Smithsonian Institute and the University of Maine. In summary, our results point to substantial species specific and annual variation in the interactions between the breeding and non-breeding stages of the annual cycle. This highlights the need for more extensive study across species with variable life-history strategies, ideally spanning many years. Successful full life-cycle management will critically depend on such detailed information.
