when birds migrate in to or out of a defined population and mate with birds from another population the existing pool of genes may be altered migration can increase or decrease gene frequencies in population resulting in...

These genes will mix and the population will increase.

Explanation:

This is beside the point though, this will make both groups of birds join forces. resulting in overpopulation.

answer:

convection currents in the magma drive plate tectonics. heat generated from the radioactive decay of elements deep in the interior of the earth creates magma (molten rock) in the aesthenosphere. the aesthenosphere (70 ~ 250 km) is part of the mantle, the middle sphere of the earth that extends to 2900 km.

genetic variation will increase because of a new habitat and food source.

explanation: