The Swedish study, reported in the August 16 issue of the journal Science, was designed to test a "public information hypothesis," which says that some animals assess the reproductive success of fellow species in a given habitat as part of efforts to scout out environments that are optimal for their own breeding.
Other studies have demonstrated that birds depend on various clues—public information—in making individual decisions about where to settle. But scientists don't know much about all the factors that influence that information-gathering process.
Clearly, reproductive success is an important indicator, and this is what Blandine Doligez and her colleagues focused on. The number (quantity) and fitness (quality) of existing offspring born and raised in a specific habitat served as the measures of reproductive success.
For the purposes of the experiments, the scientists changed the makeup of infant birds in the nests of collared flycatchers (Ficedule albicolis) to manipulate local reproductive success.
During two breeding seasons, from 1997 to 1999, they transferred seven-day-old nestlings among nests in different breeding plots, thereby altering the quantity and quality of the offspring. In broods that were increased in size, the nestlings were less healthy than those in other broods because the parents had more babies to feed.
In line with the hypothesis and the researchers' predictions, more birds prospecting for breeding sites flocked to the areas where scientists had artificially increased reproductive success. Also, fewer of the birds already at the sites with bigger broods and healthier offspring left those areas. The opposite was true when reproductive success decreased.
"These results could only be explained by the use of public information, and they imply that flycatchers, like many avian species, prospect to gather public information," the researchers concluded in their paper in Science.
The experiments were done by Blandine and her co-authors, Etienne Danchin and Jean Clobert, at Laboratoire d'Ecologie CNRS and Université Pierre et Marie Curie in Paris, France. Doligez is also at Uppsala University in Sweden and is now at the University of Bern in Switzerland.
Their results add to a rapidly growing body of evidence showing that the cognitive abilities of birds are much more complex than previously thought.
Upscale Attraction
The discovery that some birds are particularly attracted to wealthy neighborhoods emerged from the Central Arizona—Phoenix Long-Term Ecological Research project, a multi-year study to determine how development and related human activities affect the urban environment and the surrounding Sonoran Desert ecosystem.
Kinzig and her colleague Paige Warren found that the parks in upscale areas of Phoenix had the highest number of bird species, while the bird populations of parks in middle and lower income communities were progressively less diverse.
The researchers were surprised to find that the size of the parks and the nature of their vegetation—mainly trees—did not seem to affect how many species of birds flocked to the various sites, said Kinzig. "Instead, the characteristics of the neighborhood, including the income of the residents, seem to play a significant role in influencing the number of species that live in the park," she said.
Past studies have shown a strong correlation between the diversity of birds in a plot and the size and predominant vegetation of that plot, with birds most attracted to areas that have larger and more diverse trees and other vegetation. In the Phoenix study, however, the highest levels of tree diversity generally existed in the parks near lower income residents —perhaps because those parks tended to be older and their trees were planted earlier, Kinzig suggested.
The researchers said it's not clear what specific factors encouraged the birds to hang out in more affluent neighborhoods.
"Something that happens in the radius of 200 meters from the park boundaries is influencing the diversity of birds," said Kinzig. "There's a variety of things—it could be what people are planting, it could be socio-economic differences in how often you feed birds, maybe the rich people have more bird feeders."
"Whatever people are doing is having an influence, because we can't explain it with the park itself," she added.
More research must be done to sort out the factors that attract birds to affluent neighborhoods, the researchers said.
Among the possible influences, they suggested, are zoning decisions, the type of existing landscaping, proximity to factories, local cat populations, the presence of bird feeders—even things such as the number of dog dishes or other sources of water and food in the communities.
"We don't know, but it's something about the differences in people's lifestyle," said Kinzig.
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