Which of the following statements is false?
a.
Parrots, hummingbirds, and oscine songbirds have dramatically different song circuits.
b.
Parrots, hummingbirds, and oscine songbirds all share the distinction of being vocal song learners.
c.
Vocal learning was once a trait shared by virtually all groups of birds, but now is retained in only parrots, hummingbirds, and oscine songbirds.
d.
At present, most groups of birds are not vocal learners.