Birds and mammals disperse strawberry seeds by eating the fruit and defecating or spitting the seeds out. The red color of a ripe strawberry is a signal that the fruit is tender and sweet, encouraging animals to eat them, scattering the mature seeds after digestion.
Strawberries are just one fruit that scatters seeds through digestion and defecation, and the maturation process of the fruit includes ensuring that the seeds are ready to germinate. Strawberries that are not yet ripe are green and hard to the touch, with a sour flavor. This discourages birds and animals from eating them; instead, they wait until the fruits turn red before eating them.
Over time, the process of natural selection has ensured that those strawberry plants that produce the reddest and sweetest strawberries have genetic descendants. The more flavorful and the more brightly colored a strawberry is, the more birds come for the fruit, and the more widely the seed dispersal takes place. When birds made the mistake of eating a sour, green strawberry, or even a strawberry that had just turned red but was not quite ripe, they released seeds into the wild that were not yet prepared to germinate, bringing that plant's family tree to an early end.