Answer: The eastern population of mice will most likely to change their behavior and become diurnal ( perform most of their activity during day time).
As per the question, due to geographical isolation mice population becomes separated as eastern and western population. Eastern population is inhabited by owls that hunt at night.
Owls capture mice when they go out in search of food during night time. Over a period of time, mice most likely to change their behavior and become active during day time.
This prevents them from being captured by their predator during night time.