First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of migration in birds. It is a general statement that does not need other sentences for context. It does not start with a conjunction or use pronouns. It also does not use the definite article 'the' for a defined subject. Why other lines are not chosen? Paragraph 2 starts with 'This behavior', which uses a pronoun and needs context. Paragraph 3 starts with 'For example', which is a transitional phrase. Paragraph 4 starts with 'However', which is also a conjunction. All these reasons make them not independent.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it concludes the discussion on migration. It introduces a problem that affects the migratory patterns, wrapping up the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it explains the behavior of migration. It uses the Noun Pronoun Rule, where 'This behavior' refers back to the migration mentioned in Paragraph 1.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it gives an example of the behavior mentioned. It follows the Broader to Specific Strategy, where the general statement in Paragraph 2 is followed by a specific example in Paragraph 3.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it shows a contrast to the example given. It uses the Contrast rule, where 'However' indicates a different aspect of migration.