It would seem to me that the only way such an abundant species could
have been eliminated was through avian disease, such as avian malaria,
etc. This disease may have been introduced from introduction of foreign
birds, such as domestic pigeons, or by importation of alternate hosts,
such as rats, cats, dogs, and their accompanying vector vermin, such as
lice, fleas, etc. In Hawaii, most of the remaining indigenous birds are
apparently at relatively high altitudes where the avian malaria isn't present.

Tom Weir
weir at u.washington.edu
(206)747-8480