Answer: 2. Population bottleneck
Explanation:
A Population bottleneck is a term used to refer to an event that causes the population of a species to reduce in very large numbers. Such an event can be environmental disasters like famine, the overhunting of a species or the destruction of its habitat.
In the case of the Cha-tham Island Robin, the bottleneck event must have been the introduction of cats and rats which then went on to hunt the birds to near extinction.