On its migration south, each subspecies flies over winter areas already occupied by the subspecies that breeds south of it (Welty 1975:472).
(each subspecies; flies; over winter areas; T:On its migration south) (winter areas; occupied; by the subspecies; T:already) (the subspecies; breeds; south of it)
each subspecies winter areas the subspecies
flies occupied breeds
over winter areas T:On its migration south
by the subspecies T:already
south of it