This project focuses on the evolution of winter family structure in western bluebirds, which form kin groups through delayed dispersal of sons that remain with their parents and settle to breed nearby with neighboring females or with females that have immigrated into their winter group. Delayed dispersal, where offspring remain home for longer than is required to achieve nutritional (or financial) independence, is thought to be the starting point for the evolution of complex kin groups. Once families remain close together, there are many possibilities for family-centered social behavior. Observational studies have implicated wealth of resources and nepotism as two key routes to family stability. Where resources are plentiful, it is advantageous for parents to preferentially share resources with their offspring, promoting offspring survival and, ultimately, family inheritance and the formation of dynasties localized in space. While these principles are thought to guide the evolution of social behavior in everything from ants to humans, there are relatively few systems in which both resources and parental nepotism can be manipulated. The western bluebird`s heavy reliance on mistletoe, a clumped and highly visible winter food resource, and the ease with which they can be captured and held in captivity, make them traceable for experimental manipulation of both resources and parental nepotism. Prior work has manipulated mistletoe abundance on winter territories, showing a reduction in delayed dispersal of sons, even though their parents remained on territory for winter.
Future experiments will investigate the importance of parents to determine whether sons leave when deprived of parents on intact territories. We will also continue long-term monitoring to assess the impact of both resources and parents on the proliferation of genetic dynasties within a nest box population.

The work informs basic behavioral ecology, conservation strategies, and understanding of the evolution of families, because the experiments tease apart factors influencing two key life history traits, dispersal and survival. This information is critical for understanding what will happen to bluebird populations with changing resource abundance due to development and anthropogenically caused alteration of the social group structures.

Experiments and monitoring have begun, including quantifying the winter food resource on over 3,000 trees on the study area comprising all trees within 200 m of fall nests.

One key incidental outcome of this study is recognition that mistletoe, which grows on two declining species of oaks, is a keystone species for the western bluebird in California. Conversion of oak woodlands to agricultural lands and housing developments stands to affect all oak woodland birds, but we predict that western bluebirds will be affected primarily due to loss of winter food supplies—mistletoe berries provide a constant food supply for the bluebird in winter.

National Science Foundation