This study, conducted by Renée Duckworth, Ph.D., suggests the birds may play more active roles in their own natural selection than traditional models of evolution would support.
"The traditional view of evolution is that organisms are passive creatures on which natural selection operates," said Duckworth, who just completed her doctoral training at Duke. But her research results, published online on Wednesday, April 12, 2006, in the journal Proceedings of the Royal Society B, suggest a different model, at least among these bluebirds.
"By selecting the environment in which they live, animals can actively affect the natural selection they experience," Duckworth said in an interview. "The main message of this study is that the ability of organisms to choose their environment needs to be made a more explicit part of evolutionary theory."
In her studies, funded by the National Science Foundation, Duckworth followed up on previous findings that adult western bluebirds aggressively defend large breeding territories and also use different foraging strategies in wooded and open habitats.
In gathering worms and insects to feed their young, birds living in wooded environments "mainly forage by perching on trees to scan the ground for prey," Duckworth said in her article. But in open areas with few trees, the birds must be agile in order to "hover or hop along the ground to search for prey."
Despite such advance knowledge about behavior, "the relative importance of behavior in driving or inhibiting evolutionary change remains largely unresolved," Duckworth said. At the same time, aggressive behavior "has a great potential to affect selection pressures since aggression is known to play a role in securing breeding territories," she added.
So Duckworth set out to investigate the evolutionary consequences of aggression, selecting western bluebirds as her study animal. Western bluebird males prefer to breed in territories with more than one nest available. And they readily accept human-made nest boxes as substitutes for the hollowed out cavities they occupy in the wilds.
On a ranch in Montana, Duckworth created a breeding ground by setting out nest boxes to attract the birds. In addition, she purposely manipulated the densities of the nest boxes, putting two rather than one in some territories in order to test whether more-aggressive males were better at acquiring territories with multiple nests.
Duckworth measured each male's aggressiveness by observing its response to a tree swallow that she placed near a nest box. Tree swallows are a rival species known to compete with bluebirds for nesting spaces in the wilds. In order to avoid injury to the birds, she enclosed the swallows in cages placed near individual nest boxes. She observed the bluebirds' reactions from a nearby blind.
She found that when she manipulated the densities of nest boxes prior to the birds' arrival at the breeding ground, more-aggressive males did, indeed, acquire territories with multiple boxes. But when she manipulated the densities after the birds already had settled on territories, the responses of more- and less-aggressive males did not differ.
"Taken together, these experiments show that, first, aggression plays a key role in determining the outcome of territorial interactions, and, second, male aggression is set before they ever get to their territories," Duckworth said. "These results support my previous findings that males are highly consistent in their aggressive behavior and can be categorized as either aggressive or nonaggressive."
Duckworth also evaluated western bluebird aggression in more realistic settings by installing nest boxes near natural breeding cavities to duplicate natural breeding territory densities.
She found that open areas had higher densities of nest cavities and that more aggressive male birds were preferentially attracted to these areas. Moreover, bigger males with longer legs and tails living in the open habitats produced more offspring.
Do the results of these tests mean that western bluebirds are diverging into two different species due to behavioral interactions that sort males into different environments -- one favoring open environments and the other tree cover?
"If the sorting of aggressive and nonaggressive males into different habitats is consistent over time, then it could set the stage for ecological divergence," Duckworth said.
But such a divergence seems unlikely, she said, because the frequency of highly aggressive western bluebird males can shift or even diminish over the years.
"This should allow more nonaggressive males to spread into open habitats," she said. "Another factor is that an environment itself is always changing," as fires, pests and weather remake western bluebirds' surroundings. Consequently, the link between behavior and settlement patterns is dynamic and ever-changing.
"This study does suggest that behavior can play a leading role in evolutionary processes," Duckworth said. She plans to follow up with a study investigating whether differences in aggression among male western bluebirds have underlying genetic differences that have the potential to evolve.