Complex sentence: Females hibernate inside their nests in the winter after mating, and then from the end of May to the beginning of June they will begin to build new nests, in which they store foraged pollen and nectar.
Split into simpler sentences: Females hibernate inside their nests in the winter after mating. But from the end of May to the beginning of June, they will begin to build new nests, in which they store foraged pollen and nectar.