These species get separated due to geographic isolation. These birds get separated due to a tsunami. Both of them live on different islands.
Isolating Mechanism
Now both islands have medium winged birds. One long winged bird comes to an island and mates with a medium winged bird. They have a long winged offspring.
Variation
Due to their long wings they can fly better and search for food.
Selection
The medium wing birds mate with the birds with large wings. They mate and pass their genes to their offspring. They also passed the long winged trait to their offspring.
The above couple gave birth of this bird. He got the long winged trait from his dad
Inheritence
Like this, many birds give birth with this trait.The offspring give birth with long wings. Over time the population increases. The long winged bird population overtook the medium winged population.
Time
Due to food scarcity the birds on the other island arrive at this island. During the process of searching, these long winged birds reach the first island. Mating is not happening because the medium winge birds became completely different from the long winged birds.