What can be a main reason for the movement of migratory birds?

Correct answer: Season and food availability

The direct answer is B: season and food availability. Migration means the regular movement of animals from one place to another, often at a particular time of year. Birds may leave a cold, dry or food-poor region and travel to a place with a more suitable temperature, nesting condition or greater food supply. Step by step: seasons change; temperature and rainfall change; insects, seeds, fish or other food may become scarce; birds move to a safer place where food and conditions are better; they may return when conditions improve. Option A, farming, is an activity that people do and is not the main natural reason for bird migration. Option B is correct because seasonal weather and food are major causes. Option C, road construction, may disturb birds locally but does not explain normal migration. Option D, mining, is a human activity and may damage habitats rather than cause regular migration. Exam cue: migration is seasonal movement for survival, food, breeding or suitable climate.