The astonishing behavioural versatility of nest-site scouts
In my book Honeybee Democracy, I summarised the behaviour of nest-site scouts as they work to find a home for their swarm: “Once bivouacked [in a beardlike cluster], the swarm will field several hundred house hunters to explore some 70 square kilometers (30 square miles) of the surrounding landscape for potential homesites, locate a dozen or more possibilities, evaluate each one with respect to the multiple criteria that define a bee‘s dream home, and democratically select a favorite one for their new domicile” (page 6).
At the time I wrote these words, I thought that nest-site scouts begin searching for their new home only after the swarm has left its old home. I know now, however, that the nest-site scouts often begin searching for their new living quarters even before the swarm has left its old home. Moreover, I know now that the nest-site scouts sometimes finish choosing their new dwelling place even before the swarm has left its old one. When this happens, the swarming bees can do something that still amazes me: they can skip the process of forming a temporary bivouac on a tree branch near their original home and instead can fly directly to their new residence! In this article, I will describe the results of studies reported in 2010 that revealed that honey bees are far more adept at choosing and moving to a new homesite than I realised when I wrote Honeybee Democracy.
Piping and buzz-runs
This story starts in the summer of 2007, when Juliana Rangel (then a PhD student at Cornell University) and I identified the two mechanical-acoustical signals that trigger the explosive departure of a swarm from its home: worker-piping signals and buzz runs (Rangel and Seeley 2008). We did so by working with colonies living in observation hives with walls covered with fine, black netting (tulle) instead of sheets of glass. This way, we could both observe and listen to the bees in our hives. We discovered that starting about 60 minutes before a swarm bursts from its home, several dozen bees (‘activator bees’) begin to walk slowly across the combs while producing worker-piping signals (Diagram 1). To produce these signals, an activator bee grabs other bees one at a time, presses her thorax onto each bee and then vibrates her wings for about one second. This produces a vibration of about 250Hz that passes directly to the other bee’s body and, to the human ear, sounds like a short, shrill whine.
Diagram 1
A worker bee producing the piping signal. She stands still, presses her thorax to the surface she is on, pulls her wings together tightly over her abdomen and activates her wing muscles to produce a vibration in the substrate. Although the substrate shown here is a wooden surface, usually the substrate is another bee’s body.
We knew already, from a study conducted in 2000 (Seeley and Tautz 2001) that when a bee in a swarm cluster gets piped by a scout bee, she warms up her flight muscles and so prepares to launch into flight to travel to the swarm’s new homesite (Seeley and Tautz 2001). Thus, in our 2007 study, Juliana and I discovered that the bees also use the worker-piping signal to prepare themselves for the sudden exodus of a swarm from its home.
Juliana and I also discovered in our 2007 study that, starting about five minutes before the bees in a swarm begin to stream out of their home, many of the activator bees will scurry across the combs and produce buzz-run signals in addition to worker-piping signals. These buzz-runs are “Let’s go!” signals that a bee makes by running around while buzzing her wings and butting into other bees (Diagram 2). We knew already, from a study conducted in 2005 (Rittschof and Seeley 2008), that nest-site scouts produce buzz-runs in swarm clusters to stimulate everybody to launch into flight for the journey to their new homesite. It was now clear from what Juliana and I had seen that honey bees also use buzz-run signals to trigger the massive outpouring of bees from a hive when a colony casts a swarm.
Having learned in 2007 that it is the same signals – worker-pipings and buzz-runs – that trigger both a swarm’s departure from its nest and a swarm’s takeoff from its cluster site, Juliana and I wondered: is it be the same bees – the nest-site scouts – that trigger both a swarm’s departure from its nest and its takeoff from its cluster site?
To answer this question, Juliana and I, assisted by a Cornell undergraduate student, Sean R Griffin, conducted in July 2008 the study that I will now describe (Rangel et al 2010). When we began this study, our goal was to answer just one specific question: is it nest-site scouts that trigger the mass exodus of a swarm from its home? But by the end of our study we had learned much more than just the answer to this one question.
Diagram 2
A worker bee performing the buzz-run through a group of lethargic bees
Panel 1: The buzz-runner runs toward a knot of bees
Panel 2: One second later, the buzz-runner has spread her wings and is buzzing them as she makes contact with the cluster
Panel 3: One second after making contact, the buzz-runner is pushing through the cluster, still buzzing her wings
Panel 4: The buzz-runner has broken contact with the bees but continues buzzing her wings as she runs on. Sequence based on frames from a video-recording.
The set up
Our summer 2008 study went as follows:
- We took three colonies that were preparing to swarm (ie, swarm cells were present in each colony’s hive) to a special location with few natural nest sites (Appledore Island, Maine)
- We installed these colonies and some of their combs with queen cells in observation hives set up inside a barn on the island
- We set out one attractive nest box about 225 metres from the barn (Photo 2)
- Sean labelled (with a dot of yellow paint on the thorax) most of the scout bees that visited the nest box before each colony swarmed (Photo 3)
- Juliana and I watched the behaviours of the labelled bees when they returned to their hive (Photo 4). By labelling the scouts bees at the nest-box, and then observing the activities of the labelled bees back at their hive before their colony’s swarm flew out, we were able to address our main question: is it nest-site scouts that trigger a swarm’s exodus from its home (by producing piping signals and buzz runs)?
Photo 1: The corner in a barn where we set up three observation hives. Plastic tubes between the hive entrances and two holes drilled in the barn door enabled the bees to fly outside. This photo shows one observation hive already installed and the two stands for the other two hives. The two window-walls of each hive were usually covered with insulation boards, one of which is shown leaning against the nearest hive stand.
Photo 2: Nestbox (green) mounted in a small lean-to shelter for protection from the wind, sun, and rain. It was positioned 225 metres from the three observation hives. The Atlantic Ocean extends to the east in the background. Sean Griffin sits in front of the nest box and is holding a paint set and a small insect net. When an unmarked scout bee was seen entering the nest box, Sean captured her when she came out by holding the net over the entrance hole. He then pinned her gently between folds of the netting and applied a dot of paint to her thorax. Then he released her and recorded the marking so that we could tally the number of scout bees that had visited the box. Scouts showed no sign of being disturbed; nearly all went back into the nest box to continue their inspections.
The result
We were lucky, our study unfolded smoothly.
Colony 1
On 1 July, scouts from Colony 1 began to visit our nest box and, over the period
1–3 July, Sean labelled 462 scout bees at this box. This colony swarmed at 12:52pm on 3 July and the swarm settled in a sumac bush beside the barn. We quickly shook
the swarm into a hive, to clear the nest box of Colony 1’s scouts.
Colony 2
On 5 July, scouts from Colony 2 began to appear at our nest box and, over the period of 5–7 July, Sean labelled 86 scout bees at this box. This colony swarmed on 7 July at 10:58am and – amazingly – the swarm flew directly to our nest box! Later that day, we removed the swarm bees from the nest box by shaking them into a hive that we had positioned 20 metres away from the nest box.
Colony 3
On 8 July, scouts from Colony 3 discovered the nest box and, over the period of 8–9 July, Sean labelled 572 scout bees there. This colony swarmed at 1:45pm on 9 July and the swarm settled in another sumac bush beside the barn. We shook the swarm into a hive, and so concluded our data collection.
Diagram 3
Signal intensities in Colony 1 as its bees prepared for the departure of its swarm. Left: waggle dancers. Middle: buzz-runners. Right: pipers. Each bar shows the number of labelled bees (known nest-site scouts) and unlabelled bees. The swarm departed from the hive at 12:52pm. The diagram shows that in the hour before the swarm emerged from Colony 1, nearly every worker bee that was a buzz-runner or a piper was a nest-site scout.
Which are the activator bees?
What did we learn about the identity of the bees that produced the piping signals and buzz runs that triggered each swarm’s departure from its hive? Did the nest-site scouts produce both of these signals? Indeed, they did!
Diagram 3 shows a sample of the evidence (from Swarm 1) that we gathered. The three panels show for each of the three types of signal produced at the hive – waggle dances, piping signals, and buzz-runs – the proportion of the original scout-labelled bees producing each signal. Most of the bees that produced the piping signals and the buzz-runs were labelled bees, which showed us that the mysterious ‘activator bees’ were indeed nest-site scouts.
Because it was impossible for Sean to capture and apply a dot of paint to every scout bee visiting the nest box, it is not surprising that there were some pipers and some buzz-runners without paint marks.
If it had been possible to put a dot of paint on every scout bee at the nest box, then I believe that we would have found that every bee that produced a piping signal or a buzz-run would have had a paint mark.
Photo 3: One colony’s ‘beard’ of bees on the outside of the barn door around the entrance holes for the colony‘s hive. Because the weather was warm and sunny, it became hot inside the uninsulated barn, so many of each colony‘s bees moved out of their hive. This helped us, by making it easy to observe the behaviour of the scout bees (bearing yellow paint marks) and to track them with a microphone to know which ones were producing piping signals.
Photo 4: Juliana Rangel observing scout bees in the ‘beard’ of bees of a colony whose scouts were busily visiting our green nest box. A small microphone (linked to the video camera) at the bottom of the beard records the worker-piping signals in the beard. She is also making scan samples of the number of scout bees producing buzz runs on the surface of the beard.
Nest-scout bee versatility
What has this study revealed about the behavioural sophistication of the worker bees that function as nest-site scouts?
First, some of the nest-site scouts spring into action several days before their colony has cast a swarm.
Second, sometimes the nest-site scouts complete the nest-site selection process before their colony casts its swarm.
Third, the bees functioning as nest-site scouts trigger the emergence of a swarm, by producing worker-piping signals and buzz-run signals inside (or just outside) their current home.
And fourth, if the nest-site selection process has been completed before a swarm has been cast, then the swarm can fly directly to its new home, guided by the numerous nest-site scouts that know its location.
Besides answering the original question – is it nest-site scouts that trigger the mass exodus of a swarm from its home? – this study deepened my respect for the behavioural versatility of worker honey bees. I had long known that worker bees are extremely sophisticated creatures; their ability to share information about the locations of rich food sources (and good nest sites) by means of waggle dances is, by itself, almost fantastic. But it is seeing the behavioural complexity and flexibility of worker bees when they are functioning as nest-site scouts that evokes my deepest feelings of admiration for the behavioural talents of these bees.
I hope this report on the ways of nest-site scouts will fill you, too, with feelings of profound admiration for these small, but immensely marvellous, creatures.
Photos: courtesy Tom Seeley
References
Rangel J and Seeley TD, 2008. The signals initiating the mass exodus of a honey bee swarm from its nest. Animal Behaviour 76:1943-1952.
Rangel J, Griffin SR and Seeley TD, 2010. An oligarchy of nest-site scouts triggers a honeybee swarm’s departure from the hive. Behavioral Ecology and Sociobiology 66: 979-987.
Rittschof CC and Seeley TD, 2008. The buzz-run: how honey bees signal ‘Time to go!” Animal Behaviour 75: 189-197.
Seeley TD and Tautz J, 2001. Worker piping in honey bee swarms and its role in preparing for liftoff. Journal of Comparative Physiology A 187:667-6
Professor Thomas D Seeley
Professor Thomas D Seeley is the Horace White Professor Emeritus in Biology at Cornell University. His books include ‘The Lives of Bees’, ‘Following the Wild Bees’, and ‘Honeybee Democracy’ (all published by Princeton).
To read about how these signals were discovered and find out more, see Tom Seeley’s new book to be published in April 2024: Piping Hot Bees and Boisterous Buzz-Runners, Princeton Press.
