Diamond's hypothesis was that competition, not random immigration, was the main force structuring the species composition of islands.
The two birds are about the same size, and both use their curved bills to sip nectar; Diamond argued that competition affects their distribution.
[2] Ted Case tested the assembly rule that species occurring together on islands should have less niche overlap than random assemblages because they have undergone specialization.
Testing the assembly rules is a complex process that often uses computer simulations to compare experimental data with characteristics of random assemblages of species.
[4] Daniel Simberloff led the arguments against these rules claiming that theory as the one developed by Diamond "has generated predictions that are either practically untestable, by virtue of unmeasurable parameters or unrealizable assumptions, or trivially true".