See "Bird Ownership Statistics in USA"
http://www.parrots.com/parrot-ownership.htm
"Using the AVMA [American Vet Medical Association] data, along with previous surveys in 1991 and 1996, the following conclusions have been drawn: Birds are the third most popular companion pet owned, behind dogs and cats. 4.6% of all households own at least one bird, with households in the Pacific region having the highest concentration, averaging 6.3% of households".
It goes on to show a map of how pet bird ownership varied across the US. An "eyeball" comparison between this and the Willis paper showing the distribution of PD suggests it shows roughly opposite patterns.
John