Interesting, University of Maine, Extension publication explains to Blueberry farmers how to determine a "crude" hive strength by counting bees at the entrance.
Colony Strength – Considerations
To ensure that a good pollination service is being received, the grower needs to know about differences in colony strength. Colony strength refers to the number of bees in the hive and the population structure of the colony (does the colony have a queen and is the colony rearing brood, necessitating workers foraging for pollen. A strong colony has a minimum of 15,000 bees in each deep section (hive body or story). When the hive is opened, bees should immediately appear to “boil over” and cover the tops of the frames Figure 4. Beekeepers utilize different types of equipment in migratory operations. The width of the hives generally varies from 8 to 10 frame supers or boxes. Also, some beekeepers transport colonies in one deep and one shallow super. A colony for pollinating blueberry should be housed in at least a two-story hive (preferably two deep hive bodies), containing at least 30,000 bees, and have 6 to 10 full frames of brood in all stages of development. Remember that the physical size of the hive (number of boxes) is not a good indicator of the strength of the colony. It should be stressed that an accurate assessment of the pollinating strength of the colonies cannot be made merely by counting boxes. A hive might consist of several hive bodies, but the bee cluster size inside may fill only a single hive body.Some quick indications of colony strength can be obtained by watching the flight activity of the bees at the entrance. On a bright, warm day (greater than 55º F and winds less than 15 mph), dozens of bees should be constantly coming and going at each entrance as shown in Figure 5.
On a bright, warm day (greater than 55º F and winds less than 15 mph), dozens of bees should be constantly coming and going at each entrance.
Fewer flying bees in front of some colonies may indicate that the colonies are not strong. Keep in mind that an examination of the colony inside the hive gives the best indication of its strength. To obtain a very crude “ball park” estimate of the foraging bee strength, count the number of bees RETURNING to the hive in 15 seconds. Then multiply this number by 0.06 (a factor that represents the proportion of an individual foraging bee’s makeup on a well covered comb in one minute). This product (number of returning bees in 15 seconds x 0.06) will provide a crude estimate of the number of full frames in the hive well covered with bees. So, for example, if you count 125 bees returning to the hive in 15 seconds then 125 x 0.06 = 7.5 or the estimated number of full frames well covered with bees is 7.5, a good strong colony. If the number of bees returning to the hive in 15 seconds is too high to count, then count the bees returning in 10 seconds but multiply the number of bees by 0.09 instead of 0.06.