Birds migrate from their original habitat to new habitats to search for food and better climate. how is bird migration a kind of gene flow?

Gene Flow is an exchange of genes between two different populations. If birds migrate from their original habitat into another habitat that is inhabited by another population, they might breed and exchange genes. 

For example, a population of short birds migrates into the habitat of a population of tall birds. This might result in the hatching of medium height birds. 

This is what i did 45-5=4040÷4=10so there are 10 white marbles but am not sure is just a question that am not sure