Nov. 4, 2005 — Bird wealth is measured in food, not money, and researchers have discovered that berry-rich bluebird young with edible inheritances prefer to stay in the nest.

Since humans and many other animals also often stay close to home when the pickings are good, the discovery supports the theory that wealth can promote family stability and togetherness.

The study, which will be published later this year in The Proceedings of the Royal Society B, focused on Western bluebirds, Sialia mexicana, which live in California.

While bluebird daughters usually flew away from home at the expected time in August, bluebird sons stayed with their parents all the way through winter when desired mistletoe berries were plentiful.

When the sons finally decide to leave home to breed, they often do not go far and still may reap familial benefits, including meeting females that their parents might know.

"Males potentially gain a portion of the family territory, the ability to come back home easily if things do not work out, and perhaps even assistance with acquiring a nearby territory," said Janis Dickinson, lead author of the study and associate professor of natural resources at Cornell University.

She added, "It is also possible that familiarity with the area makes them more competitive for and aware of newly open vacancies nearby, so that they can find mates more easily. Sons also commonly mate with females that have been attracted into their winter groups, which may be an advantage over a vagabond lifestyle."

Dickinson explained to Discovery News that she and her colleague, Andrew McGowan, selectively removed mistletoe shrubs in oak tree bluebird territories. The birds usually eat bugs during the spring and summer months, but mistletoe berries are their primary food source from October until spring.

Only eight percent of sons overwintered with their parents in areas when the berries were removed, while half of all bluebird males continued to stay with their parents when resource wealth, in this case mistletoe berries, was left untouched by the researchers.

Male birds and humans tend to stay closer to the nest than females do, especially when good food is around.

"This pattern is very similar to what we see in humans, with sons tending to live closer to their parents than daughters," said Dickinson, adding, though, that even female bluebirds do not fly too far away when the parents' food supply is tempting.

She said, "There are families of a father and mother with three sons nesting nearby, a daughter a kilometer (.62 miles) away, and an uncle up the hill, so it's common for family members to live quite close together."

Stephen Emlen, professor of behavioral ecology at Cornell University, formulated several theories in 1995 concerning what may lead to family stability.

He told Discovery News that one of his predictions was, "Families that control high-quality resources will be more stable than those with lower-quality resources."

Emlen said, "This is exactly what was found (by Dickinson and McGowan)."

Vittorio Baglione, an expert on ecology and evolution at the University of Vallodolid in Spain, told Discovery News that wealth "makes the situation at home more 'pleasant,' that is, risk safe, for the offspring and may be an incentive to postpone dispersal."

Baglione said other data suggest that for humans living in rural societies, "the stability of the family is linked to its wealth."

Jan Ekman, professor of ecology and evolution at Uppsala University in Sweden, told Discovery News that ample wealth also tends to lessen parent/offspring conflicts over resources, but he wondered how parents could stand having their children around for so long.

"The logical continuation of this study would be to follow up on the tolerance of offspring by their parents," Ekman said. "Does parent behavior vary with resources so that they can be made more and less tolerant by manipulating 'wealth'?"