This This time of year early springtime, when there are days that seem warm enough that the honeybees should be flying but they are not, and you know that the hive is living, the reason is that they are sitting on their brood and eggs inside of the hive to keep them warm the same way that a bird sits on it's eggs and chicks to keep them alive and well. If the bees have just enough of them selves to keep brood and eggs warm, but not enough to send worker bees out to fly, then there will be no flying during those times. That is why this early springtime, the bees must be carefully looked into to be sure that they have enough honey and pollen because they are feeding it all to brood as fast as they are able, but are not able to go out and gather nectar and pollen. If they do not have enough honey,syrup must be fed. The warmer the weather is, the more bees will be flying. 
       Here is a link to an article called: "How to autopsy a honey bee colony".