Context: In the tropics there is little variation in the length of day throughout the year, and it is always warm enough for a food supply, but altitudinal migration occurs in some tropical birds. There is evidence that this enables the migrants to obtain more of their preferred foods such as fruits.

Question: Where is there little variation in the length of day throughout the year?
Answer: In the tropics

Question: Why does altitudinal migration occur in some tropical birds?
Answer: to obtain more of their preferred foods

Question: What are tropical birds preferred foods?
Answer: fruits

Question: Where is it always warm enough for food supply?
Answer: In the tropics