Neotropical-Nearctic migrants have one of the most complex annual cycles in vertebrate animals. Each year, over 350 species migrate thousands of kilometers between their temperate breeding grounds and tropical wintering quarters.
Between and within these periods, individuals are adapted to a remarkable diversity of ecological and social conditions. Because of this complexity, however, we still have a poor understanding of the factors that influence individual success and overall population dynamics.
Research on migrants during the breeding period suggests that population abundance is primarily influenced by reproduction. Factors that may limit reproductive success in migrants include:
- habitat fragmentation resulting from increased rates of nest predation and brood parasitism by Brown-headed cowbirds, Molothrus ater
- food abundance
- territorial exclusion mediated through behavioral dominance
- nest site availability; or the interactions between these factors
Migrants may also be limited on their tropical wintering grounds by factors affecting overwinter survival.
Alternatively, migrant populations can be viewed as being controlled by a dynamic equilibrium between demographic events on the breeding and wintering grounds. This idea implies that events in one season act as a buffer against detrimental effects (e.g. habitat loss) in the previous season.
Recent evidence now points to the importance of these seasonal interactions on the population dynamics of migrants. At the individual level, events in each period can be viewed as being tightly linked with each other, such that ecological success in one period affects success in the next period. Marra et al. (1998) formalizes this idea into the Seasonal Interaction Hypothesis.
The essential concept of this hypothesis is that events in one period produce residual effects that carry-over into the following season. Examining seasonal interactions and determining which events produce the strong carry-over affects will be the key to understanding the migrant population dynamics.
However, we currently lack a synthesis that integrates interactions within the year-round cycle of any one migratory species. Using stable-carbon isotopes as novel technique to link summer and winter events, I am testing multiple predictions of the Seasonal Interaction Hypothesis in the American redstart (Setophaga ruticilla). The goal of this project is to understand how events, such as habitat selection and reproduction, in different periods of the annual cycle integrate to determine the individual success of long-distance migrants.
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