Warming up the flight muscles
At rest a bumblebee's body temperature will fall to that of its surroundings. To raise the temperature of the flight muscles high enough to enable flight the bumblebee shivers, rather the same a we do when we are cold. This can easily be seen in a grounded bee as her abdomen will pump to ventilate the flight muscles. The rate of pumpling can give an indication of the temperature of the bee. Ranging from around 1 pump per second when she is at 10^oC, to 6 pumps per second when she reaces 35^oC. The time taken to raise the thorax temperature has been studied and is laid out in the table below.

When food is plentiful and outside temperatures fall below 10^oC bumblebees generally stay inside the nest and live off theri stores. At times when food is scarce or stores are low they will forage when the outside temperature is as low as 6^oC, and queens will forage at even lower temperatures. In severe conditions they have even been known to vary their flying height to and from the nest to take adavantage of any temperature differences.

Nest searching by queens
Generally you will find bumblebees on flowers, in the nest, and travelling between the two. However in spring you will find bumblebees in odd places. These are the queens and they are searching for a suitable place to nest, or just somewhere to have a rest or spend the night. She will investigate dark corners, mouse holes, garages and sheds. There have even been cases where a queen has gone into a pocket. She will hover and fly low over the ground oblivious to everything, and is quite easy to follow at this time.

Maintaining dominance
Bumblebee queens appear to maintain dominance purely by aggressive behaviour, though it is believed that a dominant queen secretes a pheromone that suppresses the glands in workers that would otherwise lead to their ovaries developing. In many species the queen is bigger than the workers, she uses her size to dominate workers by opening her mandibles and head-butting the most dominant worker from time to time. This is usually sufficient until unfertilised eggs are laid, or a worker's ovaries develop. Although bumblebees produce nectar, the quantity produced is not enough to make it worth while domesticating them as has been done with honey bees.

Feeling threatened
So how do you know if you are upsetting a bumblebee? Well it's quite simple really. If the bee is on a flower or other surface and is feeling threatened it will raise one of its middle legs. This is a sign that you are too close and should back off a bit. The Bombus impatiens in the photograph on the right is showing this defensive behaviour. B. impatiens is native to the eastern U. S., but is sold commercially in the western states as a pollinator for glasshouses etc. A visitor to bumblebee.org kindly allowed me to use this image.

In cold weather the bee may even fall to the ground to avoid you, as it hasn't built up enough heat to fly off. It is said that bumblebees don't like human breath, so if you want to observe one closely then don't breathe on it.

Angry buzz
While marking and measuring bumblebees I noticed that one would sometimes give out a high pitched buzz similar to that when they sonicate to dislodge pollen from tomatoes and other similarly constructed flowers. The bee was trapped at this time with no chance of flying away, so I have always believed that it was angry, perhaps I was holding it down too firmly, or perhaps it was just fed up being handled.

Feeding.
Bumblebees are vegetarian at all stages of their life. The workers gather pollen and nectar. 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. So bumblebees get all their food from flowers.

Male behaviour
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. 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.

On the right is a Bombus pratorum male who spent the night hanging on to a spike of flowers. Look at his hind leg, the tibia is convex as opposed to the concave tibia that forms the female pollen baskets.