According to MacArthur's observations, five closely related species of warblers living on the same tree were able to avoid competition and co-exist primarily due to:

Step-by-step solution

MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioural differences in their foraging activities. This is a classic example of resource partitioning, where species evolve to share resources by utilizing different parts of the habitat or employing different foraging strategies. Answer: Behavioural differences in their foraging activities