"Wealthy" Bluebirds Have Stay-at-Home Sons

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These behaviors gave researchers the opportunity to manipulate the quantity of mistletoe to see whether abundant food would keep sons at home.

In central California, mistletoe berries are not edible until October, after bluebird daughters have already made their late-summer dispersal.

Sons are left to estimate the "wealth" of their parents and decide whether to stay or leave well before winter arrives.

By removing mistletoe in late summer, the scientists could be certain they measured sons' decisions to stay, and not simply the impact of available food on survival.

Mistletoe Experiments

Three summers ago, researchers removed half the mistletoe by volume from 13 experimental territories, matching the sites with 13 control territories.

Compared to control areas, territories with reduced mistletoe retained a lower proportion of bluebird sons, on average, but recorded no difference in the retention of bluebird daughters.

Wealthy bluebird parents who lived in control areas thick with mistletoe had five times the proportion of stay-at-home sons than bluebird parents who were forced by the experiment into reduced circumstances.

While Dickinson acknowledged that it is difficult to extend these ideas to human economic systems, she nevertheless found room for speculation.

"Our experiment indicates that stability of families is tied to resources, and that reducing the availability of resources reduces the stability of families," she said.

"In humans," she said, "this translates into a pattern where sons of wealthy families are more likely to stay home and retain close family ties, including economic ties, than sons from less wealthy families."

"It is interesting to think about the potential impact of a taxation scheme that promotes stable, inherited wealth for a small segment of the population, while reducing resources available to the rest of the population," she added.

For bluebirds, reducing the amount of mistletoe didn't make the parents abandon their territory. They simply stayed where they were, able to survive on less food because their kids had left the nest.

Bluebirds don't depart because they are hungry. Rather, both parents and fledglings make a visual estimate of how much food will be available over the winter.

If it looks like the parents are rich, sons stay to enjoy the easy living. If not, they fly away.

Voluntary Departure?

Do the sons leave voluntarily, or do the parents evict them? Researchers couldn't say. They observed few instances of aggression toward offspring during the summer when dispersal occurred.

But the scientists acknowledge that a single unobserved act of aggression could drive a young bird away, an event that could be missed even with many hours of close study.

The researchers conclude that abundant winter resources among the bluebirds can lead to delayed dispersal, which sets the stage for helping their parents raise the next generation the following spring.

This, the researchers say, agrees with observational results in other bird species and in humans, where resources are linked to home-loving males and the growth of family dynasties.

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