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: సంవత్సరం పొడవునా రోజులో కొద్దిపాటి వ్యత్యాసం ఎక్కడ ఉంది?
answers: In the tropics