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Mice became two species because of the geographic separation caused by river as they could not swim and move to both parts. This caused them to adapt and reproduce in the habitat of either side of the river whereas birds did not face any geographical isolation because they could fly over the river and live and reproduce like before.

Explanation:

Evolution of a species of a population into two different species is termed as speciation.

Speciation occurs due to the reproductive isolation of the organisms in the population due to change in habitat, reproductive cylcle, mating approach or sometimes change in genetic makeup of organs.

In the case of mice which inhabited the field but when the river was cut in between they could not go from one part of the field to other by crossing the river. They adapted to the part of fields they were in and mated to produce progeny with the phenotype required to adapt the habitat.

They became two species due to geographic separation and will be called as allopatric speciation.

While the birds nesting in the field were able to cross the river by flying over it their movement was not hindered by the river hence they mated and produced as before and did not evolve in two species.