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.
Answer:
Glacial striation is your answer
A massive landslide Mud flows in the Toutle river.
<h3>How changes have occurred in the North Fork Toutle River?</h3>
There had been many earthquakes in the area prior to eruption but this happen to be a massive land slide because a moving magma have over steepened the mountain slopes.
This geologic process uses gravity to cause rock, soil, artificial fill or a combination of the three to move down a slope. Slow weathering of rocks as well as soil eruption and volcanic activity also causes landslides
Thus, massive landslide Mud flows in the Toutle river.
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