After my research the answer is that they are forms of electromagnetic radiation as far as I know.
I believe it is behavioural isolation. When there are changes within a certain species, e.g. new birdcalls are formed in certain individuals in a species, "traditional" birds may not like such mating calls and only choose to mate with the more common birdcalls from majority of the birds. There are also some birds that may like the new birdcalls, and choose to mate with birds with those birdcalls. They become genetically isolated as they continue such behavior. Eventually, through genetic drift and other mutations, different species form from a single specie.
Hope this helps! :)
