Have you ever wondered why rare plant species still exist instead of going straight to extinction? It is actually because fruit eating birds maintain the rare plant species. These birds help structure the diversity of forests and increase plant diversity which is good for many ecosystem services like nutrient cycling. Birds eat the fruits and spread it around (i.e. regurgitate the seed) which helps the plants reproduce. Even if there are more common plant species around birds will still eat some of the rare plants and help with the reproduction of the rare plant species.
Tomás Carlo, a professor in biology at Penn State and his colleague documented how much seed rain (“seeds that fall to the ground because of birds activities”) are made up of rare species. They had 10 plots in Puerto Rico and they looked for the bird activities, seed rain and establishment of plants. 91.3% of seeds from 6 common fruiting plants were available for birds but of those only 18.4% was spread by the birds (seed rain). The rest (81.6%) of the seeds that were distributed by the birds came from 43 rare fruit plants. They also found that when the common plant species became rare due to the season the birds ate more of that causing faster regeneration of the forest and more diverse plant communities.
One of the reasons for more rare fruits then common fruits is because a varied diet provided more nutrition for the bird. Younger birds with no experience on what foods to eat will try any fruits. Birds know what fruits are there even if it is in low amounts. It is part of their survival skills to go find it.