Background: 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: Why does altitudinal migration occur in some tropical birds?
Answer:<extra_id_0>.
<extra_id_0> to obtain more of their preferred foods<extra_id_1>