You need to specify which islands.

I can’t give you a useful answer without knowing exactly which islands you mean.

Spring and fall are great seasons for wildlife viewing on many islands, but “the islands” covers everything from the Galápagos to the San Juan Islands to the Maldives. Each has completely different endemic species and migration patterns. You wouldn’t look for Galápagos tortoises in Washington State.

If you mean a specific place (Channel Islands, Galápagos, Hawaii, etc.), drop the name and I’ll give you the real answer. Otherwise I’m just guessing, and you don’t need guessing.