March 7, 2010

Cornell Researchers Addresses Climate Impact on Bird Migrations

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The pattern between winter weather and bird migration seems to be common knowledge: birds fly south for the winter. But how does climate really limit bird distributions and what changes have scientists seen as global climate patterns continue to fluctuate?

Prof. Benjamin Zuckerberg, ornithology, attempted to answer these questions. In his lecture March 2 in Fernow Hall, Zuckerberg discussed the link between climate and bird occupancy. A bird occupancy pattern describes the settlement of birds in different locations as they migrate throughout the year.

However, the theory that temperature affects bird occupancy suffers several flaws. For example, the climate is not constant, and species adapt to different environmental conditions over time. Arguably, food availability affects bird occupancy more significantly than climate patterns does.

“Obviously range boundaries are telling us something, but I think there’s been an overemphasis,” Zuckerberg said. “Although range boundaries are interesting, it’s not the whole story.”

In order to study bird occupancy patterns, researchers need to know the locations of birds. Although radio frequency identification tags can track birds, the Lab of Ornithology gets most of its data through volunteer service. This method of data collection, called “citizen science,” allows researchers to amass large amounts of data, engages the public in research and promotes science education.

Ontario’s Long Point Bird Observatory conceived “Project FeederWatch” in the mid-1970s, and it became a joint project with the Cornell Lab of Ornithology in 1987. Now, FeederWatch is a continent-wide network of over 10,000 volunteer birdwatchers, who contribute bird-sighting data for scientific analysis. FeederWatch participants count and identify the birds they observe at their feeders from the months of November through April.

Zuckerberg and his associates take climate data obtained from the Northeast Regional Climate Center, overlay the FeederWatch sites and look for associations between them. So far, they have discovered that the minimum temperature of an area is likely an important component in whether birds occupy that area during winter or not.

Researchers see a strong correlation when comparing the areas’ minimum temperatures to temperatures in areas of extinction — characterized by bird absence due to mortality or emigration — and areas of colonization, showing bird presence due to a shift in food resources or immigration. For the species studied by the FeederWatchers, the probability of occupancy drops dramatically when the minimum average temperature of an area is below negative 4 degrees Celsius.

A major problem in occupancy modeling is imperfect “detectability,” meaning that, just because birdwatchers do not see a bird, does not mean it is not there. Since bird feeders are only supplements to a bird’s normal diet, birds tend to visit feeders less often — and be counted less often — when temperatures are warm and food is plentiful. Birds can be misidentified by new and inexperienced birdwatchers. Also, birdwatchers typically spend only a few hours a week watching their feeders, missing many potential sightings.

The unreliability of climate data can create problems too. Different climate models place importance on different variables, and no model can perfectly reproduce such a complex system.

Zuckerberg stated that the future could bring multi-year and multi-scale studies of bird occupancy patterns. With enough data, future backtesting could test the accuracy of current predictions and hopefully stabilize understanding of the link between climate and bird occupancy. It could even answer the question of whether migrating birds might be sentinels of climate change.

Data from the FeederWatch project participants is instrumental to the completion of these goals, since, as Zuckerberg noted, “Winter bird behavior is a really understudied field.”

Original Author: Jacquelyn Heim