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Table 1 Environmental variables used to predict AIV in wild birds in the US.

From: Mapping the risk of avian influenza in wild birds in the US

Variable Source Manipulation
Binary variable: 1 if the county ever froze, 0 otherwise ftp://sidads.colorado.edu/pub/DATASETS/NOAA/G02156/metadata/ Calculated from freeze and thaw date
Freeze date ftp://sidads.colorado.edu/pub/DATASETS/NOAA/G02156/metadata/ Calculated as the first day of the year when more than half of the county was frozen
Harvested cropland (ha) http://www.nationalatlas.gov/atlasftp.html None
Human population density http://www.nationalatlas.gov/atlasftp.html The total human population was divided by the area of the county
Mean farm size (ha) http://www.nationalatlas.gov/atlasftp.html None
Minimum temperature (°C) http://www.prism.oregonstate.edu/products/matrix.phtml?year0=2000 Calculated as the average minimum temperature of all of the 800 m pixels in the county
Percent family owned farms http://www.nationalatlas.gov/atlasftp.html None
Percent of county in cropland http://www.nationalatlas.gov/atlasftp.html None
Poultry density http://www.fao.org/geonetwork/srv/en/resources.get?id=12720&fname=glbpo25cor.zip&access=private The data were aggregated to the county scale and the total number of poultry was divided by the area of the county
Road density http://www.nationalatlas.gov/atlasftp.html Calculated as the number roads in each county that connect major population centers
Thaw date (°C) ftp://sidads.colorado.edu/pub/DATASETS/NOAA/G02156/metadata/ Calculated as the first day of the year when more than half of the county was not frozen
Total annual precipitation (mm) http://www.prism.oregonstate.edu/products/matrix.phtml?year0=2000 Calculated as the average precipitation of all of the 800 m pixels in the county