All are possible, one population becoming dominant in a habitat is more likely if the other population can move away. In an isolated situation with less chance of a population moving out, niche separation is probably more likely with one or other adapting to utilise a different food supply. This can lead to the evolution of different beak shapes as this would not happen independently so enabling the bird to use the new food source. It happens because they use the new food source, the beak modification happens over generations resulting from those best adapted surviving to breed more successfully. The