The sentence which best concludes the paragraph is that birds have developed effective skills to survive.
The migration is a movement to another place, often of a large group of people or animals.
Migration can be internal or international. Internal migration does not change the size of the population, but influences the distribution of population within the nation. While the external migration change the size of population.
Thus, option D is true that birds have developed effective skills to survive because it best concludes the paragraph.
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