A population decline of about 50% from 1974 to 1981 for Turkey Vultures (Cathartes aura) was found at a traditional roosting site at Malheur National Wildlife Refuge in Oregon. This decline may have been due to a region-wide reduction in this species, to the possible but improbable formation of a new but undiscovered roost, or to a reduction in feeding opportunities caused by the decreased use of the refuge by cattle.
Taylor, Daniel M.
"Turkey Vultures decline at a traditional roosting site,"
Great Basin Naturalist: Vol. 46
, Article 22.
Available at: http://scholarsarchive.byu.edu/gbn/vol46/iss2/22