Not sure if that rate is the same in the US. There are a lot of chicken farms in NL and they generally keep good track of outbreak statistics so this was easy to find.

I've wondered about this, too. The Dutch "knowledge platform for livestock and public health" (collab between Ag Department and Universities, etc.) estimates the risk of avian influenza infection for free range farms to be 6x the risk for indoor farms.
(www.kennisplatformveehouderij.nl/vogelgriep)