Why do European rabbit fleas reproduce when young rabbits are born?

The life cycle of the European rabbit flea (Spilopsyllus cuniculi) is perfectly synchronized with its host. The sexual development of these fleas is directly controlled by the rabbit’s hormones.

Specifically, female fleas can only mature their eggs while feeding on a pregnant doe. When the young rabbits are born, both male and female fleas mature and migrate from the doe to the nestlings and the nest environment. This timing ensures that the fleas copulate and lay their eggs in a location that provides a suitable habitat for the development of the flea larvae.1