The Birds is the Duke of Portland's copy; Waldemar Fries suggests that the set was bound for the 5th Duke of Portland, William John Cavendish Cavendish-Scott-Bentick (after being acquired by either the 5th Duke himself or his father). The Christie's catalog description reads "Presumably purchased sometime after 1838 as a bound complete set" by the 4th Duke (with a qualifying note later in the description). Whichever duke first acquired it, the set has been in the family's library at Welbeck Abbey in North Nottinghamshire since the middle part of the 19th century.
Tom Lecky from Christie's appeared on a Bloomberg radio program to discuss the sale; you can listen here (mp3 file).
Just 13 copies of the Birds remain in private hands; when the hammer comes down on this sale, will this copy still be among them? Stay tuned.