In the spring and in the fall, the beehive will engage in fervour for cleaning the beehive. This is because the bees in the hive, throughout the winter, move slowly to maintain energy and are incapable of breaking cluster to tidy up the hive, making them particularly susceptible to disease and stress. So, by cleaning out the hive, the bees are taking preventative measures to ensure their hive is clear of fungi and viruses that may take over in the sensitive winter months. This is an inherent behaviour that bees have in these seasons. There is a lot of news about hygienic bees, that bees can be bred to be specifically better cleaners then generic honeybees.
Honey bees will begin to cluster at temperatures below 18C in the fall. So this means that when you look in to your beehive at these times, your hive will look as though many of the bees have disappeared. This is because the bees in the hive have compressed in to a small ball.