Meaning: 'Opportunities' refers to favorable circumstances or chances for something to happen, in this case, for feeding.

Wrong options:

• materials: 'Materials' refers to substances or components used for making things, which does not fit the context of feeding.
• contents: 'Contents' refers to what is contained within something, which is not relevant to the idea of feeding opportunities.
• regiments: 'Regiments' refers to military units, which is unrelated to the context of feeding.

Correct answer: cause

Explanation: The word 'cause' is appropriate as it indicates a reason or factor leading to the need for migration, fitting grammatically and contextually within the sentence structure.

Meaning: 'Cause' means the reason something happens, in this case, the reason for migration.

Wrong options:

• result: 'Result' implies an outcome rather than a reason, which does not fit the context of explaining why migration occurs.
• bearing: 'Bearing' refers to a direction or relevance, which does not convey the idea of a reason for migration.
• level: 'Level' refers to a position or standard, which is not applicable in the context of migration.

Correct answer: need

Explanation: The word 'need' fits grammatically as a noun that indicates a requirement or necessity, aligning perfectly with the context of finding food resources due to migration.

Meaning: 'Need' refers to something that is necessary or required, in this case, the necessity to find food.

Wrong options:

• gain: 'Gain' implies obtaining something, which does not convey the necessity aspect of finding food resources.
• store: 'Store' refers to a place where items are kept, which does not fit the context of a requirement for food.
• exist: 'Exist' means to be present or alive, which does not relate to the concept of needing food.

Correct answer: sustain

Explanation: The word 'sustain' is grammatically correct as a verb that means to maintain or support, fitting well in the context of how birds and animals manage to keep themselves fed in warmer regions.

Meaning: 'Sustain' means to provide what is needed for someone or something to continue, in this case, to keep themselves fed.

Wrong options:

• restore: 'Restore' implies bringing something back to a previous state, which does not fit the context of maintaining food supply.
• vary: 'Vary' means to change or differ, which does not apply to the idea of keeping oneself fed.
• conserve: 'Conserve' means to protect from loss or harm, which does not align with the idea of actively finding food.