Workers in New Zealand saw numbers increasing constantly. Basically the two sides are at an impasse. How about the effects of distance? Diamond found in the New Hebrides that the furthest island have fewest birds, but Abbot and Grant in New Zealand and Australia found no such correlation. David Lack said that even the most remote island has a large number of vagrants, so distance does not really affect island populations (but dispersal is not the same as colonization). Hawaii has very few vagrants (2000 miles from mainland)
How about size? There are many examples that indicate that there is a fairly straight line correlation between size and number of birds, but it may not actually be size that's affecting bird diversity. Size is also correlated with elevation, diversity of base resources, and habitat types available.
David Lack says all this equilibrium stuff is bunk--islands behave just like mainland. But Willis, working on Barro Colorado island (2-3 sq mile island in the middle of the Panama Canal), found that the island looses 10+ species/decade. The only difference is that it is now an island (it wasn't before a dam was erected), when a new equilibrium was presumably created.