Bird population changes may be natural

For the last several decades, bird populations across the globe have fluctuated greatly, sometimes threatening the survival of a particular species. Scientists have looked for various causes of the population decreases and have often attributed the declines to human encroachment. Now, two University of Missouri-Columbia biologists claim that nature may be as big a culprit in the population decreases as humans.

"By tracking their migration and counting their populations over the long term, we're trying to understand whether the differences in populations are due to natural events such as a draught or if they are a result of human encroachment and loss of habitat," said John Faaborg, professor of Biological Sciences. "We also need to have a better understanding of what happens during their migrations to warmer climates and if there are any problems once the birds arrive at their winter destinations in the tropics."

Currently, Ernesto Ruelas-Inzunza, a doctoral candidate at MU studying under Faaborg, is counting the number of Broad-wing Hawks, Swainson's Hawks, Turkey Vultures, and Mississippi Kites, among other raptors, as they migrate south. Due to the way these birds ride the wind currents to South America, they must pass through a mountain pass at a specific point in Veracruz, Mexico. This natural restriction funnels birds' flights through a narrow bottleneck and has allowed Ruelas-Inzunza to count and track an average of 4.6 million birds each year.

"This site is where you can find most species of migrant birds in the Western Hemisphere, which includes two-thirds of all species migrating between North and South America," Ruelas-Inzunza said. "In the last decade, we have seen an increase in the population of Swainson's Hawks and Mississippi Kites. After reviewing our long-term data, it's likely that the population increase is due to an improvement of habitat availability in North America. However, we still need to determine the reasons for the improvement of that habitat before we start pointing fingers."

Geographically, the Kites are expanding their breeding range, which could be a trend induced by humans. Yet, not much is known about other research to explain the increase. While scientists have studied bird habitat, range and breeding populations in the United States and other parts of North America, there has been less research in the winter zones and a continuing need to study birds' ecology.

"While the Kites are expanding their population, we're seeing a decrease in other bird species," Ruelas-Inzunza said. "However, we think the decrease in the other species could be due to forest land that is recovering. With different types of habitat, different bird species will do better and worse, depending on what they need for survival. Once we understand why the birds' populations move up and down, we can decide if we should do something about it."

Additional links:

John Faaborg
Division of Biological Sciences

<< back to news
<< back to archives