There are bascially two types of islands: oceanic: start with rock and gives rise to a community never connected with mainland. And continental: broke off from continent and share communities with it.
NEW ISLANDS have only a few plants, so flora depauperate. Some mainland birds are therefore excluded--only a select few can disperse to an island and, of course, you need a male and female. These must nest successfully. Migrants have to quit flying (most migrants arrive in nonbreeding condition) --physiology has to change or perhaps immatures stay (Andean and Franklin's Gulls in the highlands of Peru may be an example of this).
Andean (above) and Franklin's gull