Warmth.
Bees cannot take off for flight unless their flight muscles are at least 30℃, yet the ambient temperature is often low as 10℃. While flying the movement of the wings and the energy they use up while flying is enough to keep the flight muscles warm enough to continue flight, however when the bee is stationary its hemolymph is the same temperature as ambient temperature, resulting in d the bee not being able to take off and sustain flight. This is a problem because in colder weather bees need to be foraging for more food to provide the hive with, if they bee cannot activate its flight muscles to take off, it can't forage. To combat this bees will ‘shiver’, they will vibrate their flight muscles without actually moving their wings, this