Apart from sea turtles, the only wild animals I saw on Heron Island were birds. Lots and lots of birds. They made so much noise, I needed earplugs to sleep at night.
But I can't really complain; they were there first. In addition to birds and bats, many islands were colonized by plants and nonflying animals, often long before humans evolved.
How did they get there? When enough water was tied up in glaciers, sea levels were lower, so some of today's islands were connected to the mainland. Darwin showed that plant seeds can be carried in mud on the feet of birds. Coconuts can migrate by floating in ocean currents, although there are other hypotheses to explain their presence on particular islands. But what about animals that can't fly ? Even if they can swim short distances, what about sharks, like those below (seen from the beach at Heron Island)?
Small animals may have floated to even distant islands on driftwood "rafts." That only works if currents flow from the mainland to the island, however.
CurrentlyAt present, currents flow from Madagascar to the African mainland. In Mammalian biodiversity on Madagascar controlled by ocean currents, published in Nature, Jason Ali and Matthew Huber argue that Madagascar was previously reachable by currents from Africa. Since then, continental drift moved Madagascar into its current position, cutting off the supply of rafting animals from Africa.
Rafts can be made of ice, rather than trees. An earlier paper, Frequent Long-Distance Plant Colonization in the Changing Arctic used DNA analysis to show that plants reached Svalbard from multiple mainland sources. Surviving there seemed to be a bigger problem than getting there. (Thanks to Odd Arne Rognli for calling my attention to this paper.)