Fact 1: Migration is a type of movement that changes population size.

Fact 2: Bird Migration and Isotopes Tracking animal migration is a difficult task.

Given the two facts above, answer the question "What undergoes a movement that changes population size?" with the following options: 
- Wasps
 - camels
 - sheep
 - insects
 - animals
 - sharks
 - birds
 - ducks


 

       birds  