Many trees and shrubs reproduce by seeds, which usually fall just beneath them, so it’s really hard for them to reach new places to grow in further than a few meters. But Evolution gave them the way to get a long distance free ride up to hundreds of meters, even kilometers from their born spot. Thus, they achive a greater dispersion and a higher genetic variability. They got fruits with seeds on the inside.
In the middle of the Summer, even in Autumn, fruits are an extremely tempting, fresh, juicy and full of sugar and proteins product. So, that’s what birds and mammals think when they approach to this fruit supermarkets. They find plenty of high quality food. In exchange, they hay to pay a low but really efective price.
It has to do with the dispersion of the seeds inside the fruit. Once they have fed on the fruits, they move to other places, sometimes closer, sometimes further away, depending on wether they are migrating or not. When they poo, they spread the seeds they previously ate, wherever they go.
This erratic sowing allows a huge genetic diversity. Birds are the main actors in this show and there are studies http://www.agenciasinc.es/Noticias/Las-aves-dispersan-semillas-de-arboles-y-arbustos-de-los-bosques-cantabricos showing how important they are for this process.
But it is in Autumn, when a lot of species are migrating, that the fruits and berries intake becomes a crucial power source, provinding them with energy to face the incoming Winter and their long journeys to warmer, distant lands. Eating these fruits bring seeds to further places, increasing the chances for reforestation.
Some of the bird species acting as seed transporters are:
Song thrush https://www.seo.org/ave/zorzal-comun/
Eurasian jay https://www.seo.org/ave/arrendajo/
We have an interesting example with the blackcap and the sticky mistletoe, which provides food in exchange of dispersion.