The article is paywalled but it's a pretty plausible result of group selection.
The self-interested birds will force out the cooperative birds at first, but they are naturally self-limiting by cutting off their own resources.
On the other hand the cooperative birds can ensure that their entire species grow quickly, eventually covering areas that selfish birds haven't reached.
In an alternate universe the cooperative birds might even evolve a "moral" behavior where they actively gang up on and punish cheaters who eat unripe fruits.
There are multiple ways evolution tries to solve this, but it's hard. Insects are particularly good at it.