This National Science Foundation-funded project will continue a 20-year study of long-distance migratory birds during the non-breeding season in Jamaica. Specifically, the new phase of research will measure how birds' physical condition and survival are influenced by population size and climate. Survival is expected to decrease as population size gets higher, a phenomenon known as regulation. However, survival will also respond to factors, such as weather, that act independently of population size, a process known as limitation. Understanding how population regulation and limitation interact to influence survival and abundance is essential in order to predict how future climate change will affect migratory birds. This research program will experimentally determine how overwintering migratory birds respond to future droughts predicted to occur throughout the Caribbean region over the next 50 years, thereby providing unprecedented insight into the consequences of climate change on the behavior, abundance, and ultimately the evolution of these species.
This project will facilitate the professional training of at least four graduate and over 20 undergraduate students in the United States, as well as the training of Jamaican students and scientists. The investigators will continue their existing collaborations with Jamaican wildlife agencies and local conservation groups, and will communicate their research results to scientists, policy makers, and citizens in the U.S. and the Caribbean.
Nearctic-Neotropical migrant birds comprise the majority of birds breeding throughout large areas of North America, and a large proportion of them spend the winter in the Greater Antilles Islands of the Caribbean Basin. These birds provide ecologically and economically valuable roles, including control of insect pests. However, their populations are threatened by habitat conversion, and increasingly by the intensification of droughts as a result of human-caused global warming. Our research has contributed greatly to understanding how populations of these birds are limited by drought and other ecological conditions encountered in winter, based on our field studies and analyses focused on the American redstart (Setophaga ruticilla), a long-distance migrant, where it winters in Jamaica. The research based on the five years of funding provided by this grant resulted in 41 publications, including two book chapters, three book reviews, and 36 peer-reviewed publications of original research. A few research highlights: Using Breeding Bird Survey data (publicly available through the US Geological Survey), we have shown that eastern, but not western, N. American redstart populations decline predictably in response to reduced winter rainfall levels in Cuba and nearby Caribbean islands, as measured using satellite images of winter vegetation greenness (Fig. 1). This is the first continent-wide study showing migratory bird populations limited by ecological conditions in winter. Intensive research at our two main Jamaican study sites provides an explanation for this continental scale impact of winter conditions, namely by documenting density-dependent effects on adult survival, i.e., poorer survival in winter in response to both higher population density and less food. Preliminary results support our main hypothesis motivating this study, namely that two mechanisms of population regulation both contribute to density-dependence, i.e., to controlling—and not just limiting—American redstart populations wintering in Jamaica. These mechanisms involve negative feedback on population growth: (1) local, territory-by-territory competitive interactions involving food exploitation and aggression that increase the denser the local population, and (2) a "site-dependent" mechanism in which individuals survive more poorly the larger the population size as a result of individuals in better habitats constraining other birds when population is large to occupy poorer habitat. A major discovery is that ecological conditions in winter can alter natal (after fledging the nest) dispersal, which in species like the American redstart is far more plastic than previously appreciated. Our results also indicate that migratory species like the redstart disperse better than resident birds through human-disturbed Jamaican habitats. Study of redstarts’ and other migrant species’ diet while sharing winter habitats in Jamaica—shaded coffee plantations analyzed to date—indicates feeding opportunism, consistent with their migratory life histories, as well as the potential for (1) food competition between species, suggested by species dietary differences (Fig. 2), an important finding in terms of helping understand wintering population dynamics, and (2) all five migratory bird species studied to date help control the coffee berry borer, the most economically important coffee pest globally. We have pioneered new methods to study birds wintering in multiple Caribbean habitats, including new models to quantify local abundance accurately using point censuses, and—for the first time in birds—documenting three-dimensional home ranges of individuals in relation to habitat, vegetation, food, and overlaps with neighboring individual birds (Fig. 3). Populations of birds like the American redstart survive poorly and decline following winters of relative drought, which are forecast by published climate models to intensify in the future in the Caribbean Basin. This has important implications for reduced pest-control services by these birds in coffee plantations, but also for reduced potential control of insect populations throughout these birds’ breeding and migratory ranges in North America. Understanding mechanisms of population limitation and regulation are critical to model future migratory bird populations, our ultimate goal, and to understand ecological consequences. Broader impacts of this research included collaborations with three senior level scientists and four junior level scientists or graduate students; and the training of two post-docs, two Ph.D. students, 28 undergraduate field assistants, and a technician. Collaborations are contributing to a variety of additional studies involving use of stable isotopes (e.g., collaboration with other U.S. Geological Survey scientists on the Smithsonian Institution-based "Migratory Connectivity Project"), physiological mechanisms of ecological processes in birds, ecological interactions involving other species, the role of birds in agriculture, and life-history traits of birds (e.g., sexual traits, annual feather molt patterns). Other activities resulting from the funding associated with this research include publishing book reviews (e.g., on Jamaican birds, and climate change impacts on birds) that reach large audiences, our mentoring of many current and former Jamaican students, international activities involving Jamaican school children, and Smithsonian Institution educational activities that reach large and diverse audiences globally.
