Bumblebee feeding habits
Bumblebees are vegetarian at all stages of their life. The workers gather pollen and nectar for food. Pollen is a good source of protein. Most of the pollen is fed to the larva, and the workers and males eat very little - they live on nectar that has been turned to honey. The queen eats pollen to give her protein for egg formation, and larvae that will be raised as queens and new queens eat the rest.
A bumblebee worker is capable of carrying half her body weight in nectar carried back to the nest in her honeystomach or crop, and if she is not gathering pollen as well this load can be increased to as much as 90% of her body weight. This takes up most of her abdominal space and is why some bumblebees are predated on for this sweet load. Pollen is carried back to the nest in pollen baskets on the 2 hind legs. When both baskets are full they can weight about 50% of the worker's body weight.
So bumblebees get all their food from flowers. Most species of bumblebee show a preference for violet or blue flowers, but will forage from other colours once they have learned how to.
Metabolic rate. It is often thought that humming birds have the highest metabolic rate of all animals, however the metabolic rate of bumblebees is 75% higher than a humming bird's!
Above you can see a photograph of a Bombus hortorum male deep inside a flower. The photograph was taken in the morning before the sun had warmed up the garden and the bee could barely move.
Once males leave the nest they do not go back, so they have to find somewhere to spend the night. Hanging underneath the heads of flowers or even getting right into them is what they normally do. Below is another male hanging underneath the head of a teasel.
Their temperatures will drop and by morning they will have used up their stores of energy, so until they warm up by either drinking nectar or sitting in the sun or both, they will appear listless and sick.
I get a huge number of emails from people asking me why their bees are sick, when in fact they are just males who have spent the day chasing queens and drinking nectar and then stayed out all night. Sometimes it rains and they get soaking wet, but they will recover once they drink or get warmed up by the sun.
Sleeping inside a disk or bowl shaped flower is a good strategy for these bumblebees and other insects as research has shown that the temperature at the base of the bowl, near the source of nectar, can be as much as 10 °C higher than the surrounding air temperature (see the bumblebees inside the courgette flower at the top of the page).
Males patrol circuits
All bumblebee males patrol mating circuits laying down a pheromone to attract new queens. The pheromone is used to scent-mark prominent objects (tree trunks, rocks, posts, etc) on the circuit.
The circuit is marked in the morning, and after rain. The scent of some species can be detected by some humans. Usually they patrol at species specific heights. Bombus lapidarius, terrestris and lucorum males patrol at tree-top height. Bombus sylvestris and hortorum within 1 m of the ground. However this depends on the habitat.