Why can’t a flock of birds be called a population?

A population is a collection of living things related to each other by a common territory and often by a common gene pool.
For example, all squirrels living in one grove are a population, while several squirrels living in a hollow are a family.
All rooks living in the forest are a population, and a group of rooks flying south is a flock.
The main feature that unites all individuals in the population is constancy. The flock is formed only for the duration of the flight, and the population exists for a long time.