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Speciation and isolation mechanism

1. A species of wren arrives on a chain of volcanic islands off the coast of New Zealand that previously had no wrens. After several thousand years, wrens from these islands are able to interbreed with occasional migrants from the mainland, but the resulting offspring are poorly adapted to compete with the local wrens and produce no offspring. This is an example of what type of speciation and what type of isolation mechanism?
If wrens on each island were able to interbreed, and produce fit offspring, with wrens on adjacent islands, but crosses involving wrens from distant islands produced sterile and unfit offspring, this would be what type of species? explain

2. An series of isolated population of salamanders lives on a ring of mountaintops in the Ozarks. Individuals occasionally migrate to other populations, but this is rare. On the average, one migrant reaches a new mountaintop every five generations, and only half of these reproduce. Is the rate of migration sufficient to stop the affects of genetic drift? Over time, would separate species of salamanders develop? Why or why not?