November 1, 2005

Male Bluebirds Reluctant To Leave the Parent Nest

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Sometimes, parents just can’t get their kids to leave. New research at Cornell has found that young male bluebirds will stick around in their parents’ nests through the winter months (when the female bluebirds usually depart), and not take off until their parents’ food source has run out.

The study, performed by Prof. Janis Dickinson, natural resources, while she was at the University of California-Berkeley, was reported online last month in the Proceedings of the Royal Society B.

“Both birds and humans show patterns of female-biased dispersal,” she said. “There [was] evidence linking resource wealth with a tendency for offspring to remain home and breed near their parents.”

“An experiment [was] needed to tease apart cause and effect. Staying together could cause families to accumulate wealth, or wealth could cause families to stay together,” Dickinson said.

To conduct the experiment, Dickinson manipulated the birds’ main food source or “resource wealth” – mistletoe berries that grow on California oaks. The research showed that while half of all male offspring normally stay in the nest during the winter, only eight percent remained when she removed half the mistletoe from trees near the parents’ territory. The birds, Dickinson said, seemed to leave the nest at their own volition, rather than being kicked out by their parents.

Dickinson’s research showed that male bluebirds left the nest once their parents’ food supply ran out, even though it meant giving up the benefits of living in their home nests.

“[Male bluebirds] leave even though their parents remain behind and their mothers, under ordinary circumstances, would be less aggressive to them than would an unrelated adult female in a new winter group they might join,” she said. “The parallel with other bird species and with humans here is that non-experimental evidence has suggested that resources are important in [understanding] why sons stay home.”

While some females did fly from the nest, this was rare and not affected by mistletoe removal, Dickinson said.

Dickinson’s research is crucial to preserving western bluebirds, as it suggests that these birds are dependent on mistletoe. However, development along California’s Central Coast to build vineyards has lead to the destruction of oaks that support the mistletoe, Dickinson said. Through her research, she hopes to collect data on bluebirds in vineyards and oak woodland landscapes.

“The importance of mistletoe to bluebirds suggests that clearing of old growth oak forests in California will have a dramatic impact on the distribution and viability of populations,” she said.

Dickinson said bluebirds were a good system to test the relationship between wealth and staying home, as mistletoe are measurable and removable. In addition, they do not produce berries until October, and, because offspring migrate in late summer, this factor ensured that she saw an effect of wealth on dispersal, and not an effect on survival from lack of food.

Beginning in 2001, Dickinson conducted her research with undergraduate interns at UC Berkeley’s Hastings Research and nearby ranches in Carmel Valley, C.A.

Archived article by Olivia Oran
Sun Staff Writer