"These results support my previous findings that males are highly consistent in their aggressive behavior and can be categorized as either aggressive or nonaggressive."ĭuckworth also evaluated western bluebird aggression in more realistic settings by installing nest boxes near natural breeding cavities to duplicate natural breeding territory densities. "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. 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. 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. She observed the bluebirds' reactions from a nearby blind. In order to avoid injury to the birds, she enclosed the swallows in cages placed near individual nest boxes. Tree swallows are a rival species known to compete with bluebirds for nesting spaces in the wilds. 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.ĭuckworth measured each male's aggressiveness by observing its response to a tree swallow that she placed near a nest box. On a ranch in Montana, Duckworth created a breeding ground by setting out nest boxes to attract the birds. And they readily accept human-made nest boxes as substitutes for the hollowed out cavities they occupy in the wilds. Western bluebird males prefer to breed in territories with more than one nest available. So Duckworth set out to investigate the evolutionary consequences of aggression, selecting western bluebirds as her study animal.
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. 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."ĭespite such advance knowledge about behavior, "the relative importance of behavior in driving or inhibiting evolutionary change remains largely unresolved," Duckworth said. 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. 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. "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." "By selecting the environment in which they live, animals can actively affect the natural selection they experience," Duckworth said in an interview. 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. "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. This study, conducted by Renee Duckworth, suggests the birds may play more active roles in their own natural selection than traditional models of evolution would support.
In the process, she found that more-aggressive and milder mannered birds also tended to breed in different settings that favor different body types. In findings that may offer insight into how evolution operates, a Duke University evolutionary ecologist reported evidence that aggressive male western bluebirds out-compete less aggressive males for preferred breeding territories.
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