The Behaviour of Honey Bees Preparing to Swarm

Swarming is a topic that pre-occupies beekeepers around the world, in particular identifying if and when their bees are going to swarm. In this article we review some examples of observed bee behaviour prior to and during swarming as reported in various research papers and then we look at which of those behaviours we can identify using remote hive monitoring.

Most swarming occurs during a period of only a few weeks, generally linked to plant phenology (the timing of the bloom). The bees will normally swarm during or towards the end of the first decent nectar flow. Tom Seeley the author of several amazing books on bee behaviour (Honeybee Democracy 2010 and The Wisdom of the Hive 1995) cites the first nectar flow as a key indicator of swarming. Hive weight data can not only track the nectar flow but it can show the weight of the swarm bees leaving the hive, as in the graph below.

Graph 1: Hive Weight

You can see a strong nectar flow in early May. The swarm can be identified by the sudden drop in weight of the all the swarming bees leaving the hive (in this case 3.5kg). This swarm was collected, but as most beekeepers will know, it is difficult to catch all the swarm, particularly if the bees have clustered in an awkward position high up a tree. However any bees left behind when a swarm is collected will often make their way back to the hive.

Examining this weight graph more closely you can see the weight of the bees returning to the hive the following day (about 1kg). It’s nice to know they found their way home!

Graph 2: Hive Weight

One of the most fascinating articles about swarming behaviour was written by Delia Allen way back in 1956 (British Journal of Animal Behaviour). This gives an amazingly detailed account of the events in a colony from the time the first queen cup was formed until after the final cast departed. One of the most interesting behaviours described is that the queen and the worker bees are ‘shaken’. A bee will rest its forelegs on the body of another worker or the queen and quiver vigorously for a few seconds before departing. This behaviour becomes more frequent and in the last 1-2 weeks before the swarm the queen is shaken almost continuously, with pauses of only 30 seconds in the succession of workers shaking the queen. This is thought to be a means of exciting the queen and the colony to greater activity in preparation for swarming, as if to say C’mon let’s go! We have captured some great video footage of this behaviour which you can watch in the video below (see the bee at the top centre 12 o’clock shake 3 bees in a row)

The Calm Before the Swarm

Other fascinating behaviour Delia Allen observed was that from about a week or so before the swarm left, an abnormal number of bees were completely stationary, with their heads pointed upwards and their wings at an angle of 45° to their bodies. It is recognised that swarm bees can become inactive during the period before the swarm departs. The swarm bees are also known to consume honey in preparation for the swarm. This behaviour of the swarm bees preparing to leave the hive can be identified through a combination of weight and acoustic data from Arnia hive monitors. There will be fewer bees foraging, identified by a reduction in flight noise. There will also be a corresponding slowdown in the weight gain of the hive. The noise of the actual swarm itself can also be identified by the microphone. The graph below of the same colony shows nicely this sequence of events.

Graph 3: Hive Weight and Foraging Activity

The swarm can be easily spotted on 15th May with the large peak in flight noise (blue line) and the corresponding drop in hive weight (green line) as the swarm departs the hive. In the week preceding the swarm it can be seen that the daily hive weight increase slows down from about 3kg-4kg per day to 1.5kg–2kg per day. Flight noise is also reduced during this period. This indicates reduced foraging activity and consumption of stores. The second large peak in flight noise on 16 May is the bees that were left behind after the swarm collection returning to the hive.

Here is a different colony exhibiting the same reduced foraging activity (pink line) and weight gain (blue line) leading up to a swarm. In this case weight becomes static the last couple of days before the swarm.

Graph 4: Hive Weight and Foraging Activity

Delia Allen also describes how the queen plays little part in deciding when a swarm will depart; indeed she will attempt to remain in the hive when the swarm leaves. The workers push the queen around the hive, attempting to force her downwards and out. The queen resists and tries to force her way back into the hive. Sometimes the queen successfully resists and does not leave with the swarm. If she remains in the hive, the swarm bees will return shortly after swarming and try again later.

Other behaviour witnessed by Delia Allen at the time of swarming was that the bees became restless and excitable, and there was increased fanning inside the hive and at the hive entrance. The bees fanning will also increase ventilation and cause humidity in the hive to drop. This fanning behaviour and fall in humidity can also be picked up by the monitors as shown in the graph below.

Graph 5: Hive Weight, Fanning Activity and Humidity

The increased fanning noise (red line) is clearly identifiable at the time of the swarm. Humidity (blue line) can also be seen to fall with the increased fanning.

In the pre swarming period Delia Allen also observed that the there was a progressive reduction in the number of bees feeding the queen. In the last fortnight before the swarm, hardly any bees fed her. The queen continued to lay eggs each day up to and including the day of swarming, although at a much reduced rate. While the queen is still laying and there is brood, the bees will continue to regulate the brood temperature, although on occasions we have seen that it is not as tightly regulated during this period as normal (it is usually tightly controlled around 34°C). Other researchers (S. Ferrari et al Computers and Electronics in Agriculture 64(1):72-77 · November 2008)have identified that brood temperature can increase above the normal 34°C just before swarming. This can be seen in the graph below. On this colony the brood temperature (blue line) remains stable at 34.5°C up until the swarm. However at the time of the swarm brood temperature rises to 36°C.

Graph 6: Hive Weight and Brood Temperature

The temperature peak can be explained by the fact that in order to take off bees need to reach a flight-ready threshold temperature of 35°C (Seeley and Tautz, 2001). With lots of swarm bees preparing to leave together this generates heat that causes the rise in temperature.

The behaviour of bees preparing to swarm is a fascinating area of investigation. Hive monitoring can bring a new dimension to this work. The ability to overlay data from different hive sensors such as scales, brood temperature, colony acoustics and humidity, enables beekeepers to build up a picture of what is happening in the hive and it can provide additional insight into the lives of these amazing insects.

If you have any observational or hive monitoring examples of bees before or during swarming, please get in touch, we’d love to hear from you....