In this question, there are no options given to choose from. So i would answer this question based on my knowledge. The most likely cause of the observed variation is adaptation and evolution. The birds of that island adapted to the food source and then they evolved as per the requirement of the place and food source.
The biological levels of organization of living things arranged from the simplest to most complex are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and biosphere.
SO CELLS, ORGANS, SYSTEM AND ORGANISMS I GUESS?
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Answer:
D. Predator-prey!
Hope This Helps! And that I wasn't too late!