What kills cows, saves birds

Sick cows in Europe may help grassland birds in North America, a paper published today in the __Proceedings of the National Academy of Sciences__ suggests.

Image: Steven Mileham, flickr

Poring over four decades' worth of data, researchers at Trent University in Ontario stumbled upon a sort of __butterfly effect__ in which mad cow disease outbreaks that periodically strike Europe's cattle result in an increase in the population of grassland birds in the US and Canada a few years later."This is an intriguing study," said linkurl:Peter Vickery,;http://www.umass.edu/forwild/faculty/vickery.html an avian ecologist from the University of Massachusetts, Amherst, who was not involved in the study. "It's a great starting point for trying to get conservation biologists and others to look at bigger pictures and potential connections between international events that may not seem so obvious at first blush."The possibility of such an unlikely connection first occurred to linkurl:Joseph Nocera,;http://people.trentu.ca/joenocera/ a conservation biologist at Trent University and lead author of the study, back in 2004, when he was a PhD student studying grassland birds in a farm in Nova Scotia. "That year we noticed increased reproduction in grassland birds in that farm," Nocera said. Hay harvesting had occurred later than usual because the farm had sold most of its cattle due to an increase in demand brought on by a mad cow disease crisis in Great Britain. With a smaller herd, there was less need for hay. Because North American grassland birds make their nests in the ground, they are particularly vulnerable to destruction during harvest season. "Currently, we're in a situation where hay harvesting tends to overlap with nesting periods," Nocera said.

(From left to right) the Eastern Meadowlark, Grasshopper Sparrow, and Sedge Wren are among the birds most sensitive to changes in hay production. Images from Wikipedia.After examining data on bird populations, hay production, cattle exports, and European cattle outbreaks extending all the way back to the 1960s, Nocera noted a recurring three-year pattern that starts with an outbreak of mad cow disease in Europe and ends with a jump in North American grassland birds. The outbreak would result in increased cattle exports from the US and Canada the following year, which in turn reduced the standing herd sizes of cattle in North America, thereby reducing the demand for hay and saving the grassland birds' habitat."There was more grass that was left untouched or harvested later and so more birds were able to reproduce that year," Nocera said. Still, ecologist Peter Vickery remains skeptical. "I'm not convinced that reality falls into place as neatly as this manuscript seems to describe," said Vickery, noting that the study doesn't account for other factors that affect hay harvesting, such as precipitation, temperature, and fertilization practices. Yet he adds these sorts of studies are valuable. "I think it will have a lot of people scratching their heads and thinking more closely about it, and it will have served an important benefit for doing that, even if in the end, some of these linkages break down."J.J. Nocera and H.M. Koslowsky, "Population trends of grassland birds in North America are linked to the prevalence of an agricultural epizootic in Europe," PNAS, doi:10.1073/pnas.1018904108, 2011.
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[25th April 2008]