Great tits are known predators of bumblebees

It is known that great tits (Parus major) will predate bumblebees, as filmed by S O’Connor as part of her research about the nesting ecology of bumblebees and Forster-Johnson, A.P. 2002. She mentions that they even displayed -‘ stalking behaviour’ remaining outside the nest entrance hole, looking inside and removing leaf litter from the entrance. Presumably for them to get a better view of the bumblebees entering and exiting.

Indeed I have observed them myself outside one of my own nest boxes and also a robin catching a bumblebee as it entered the nest box. All of them watched as bumblebees approached the nest entrance which was set up in a tree. They would wait on a branch nearby and as the bumblebee landed on the platform to enter through the entrance hole, they would pounce on it and carry it a short distance to another branch, to kill it gripping it with one foot and continually pecking at its body wiping it along a branch with its bill. The robin continually banded the bee against the tree branches. Then flew away to devour it I suspect.

Opportunistic bumblebee hunters

This and my own behavioural observation suggests to me that once a bumblebee nest site is found, some great tits will hunt them outside a nest or hunt them when an opportunity arises. For example, when a bumblebee is drowsy.

How do birds predate bumblebees?

When they are vulnerable to being attacked is probably the answer. How do they become vulnerable to attack you may ask? When bumblebee queens display nest site seeking behaviour by flying above the ground, dropping to the ground and walking around to investigate potential nest sites. I believe this happened when I found 29 dead Bombus terrestris queens in a small area in woodland. Hence a predatory bird could more easily attack them whilst they were on the ground and literally pounced on them. Secondly, any bumblebee queen infected with the nematodes Sphaerularia bombi has to dig a hole in the earth to defecate and thereby spread the nematodes. I actually was fortunate enough to make a film of this process, some years ago. Being on or very near the ground would leave very little room for the bees to take any evasive action from an unseen predator attack from above. Another opportunity for birds to attack bumblebees was very eloquently put in this article undertaken by researchers at Kew Gardens, whilst investigating numbers of dead, dying and drowsy bumblebees underneath lime trees, Do linden trees kill bees? It’s not beyond improbability that insectivorous predatory birds, if they see a mass of dead and dying bees underneath a tree would nip down and snack on one of two then fly off.

What about Blue tits as a predator?

I have searched high and low regarding blue tits being predators of bumblebees. The nearest I could find was Benton 2006, which states that “probably blue or great tits – have mastered the art of capturing bumblebees without getting stung”. We know observers have seen great tits capture and kill bumblebees. I have not seen a blue tit doing this, but it would not surprise me if they did, occasionally kill bumblebees if an easy opportunity arose, especially if the bee is foraging on a flower, drowsy, basking in the sun or otherwise on the ground. O’Connor found no interactions with the blue tit, Parus caeruleus.

Shutt et al, state that tits will prey upon various insects including Lepidoptera, Hemiptera, Diptera, Coleoptera, Hymenoptera and spiders. Shutt et al (Appendix 1 Table S3 page 12) found evidence of white-tailed and buff-tailed bumblebees in the diet of blue tits. However based on metabarcoding of the poos of adult blue tits, they only found one case of each clearly showing that bumblebees are not a major component in their diet. Of course, the blue tits may have scavenged on a dead bumblebee and not caught and killed one.

Bombus hypnorum- in the UK

The tree bumblebee, B. hypnorum was first reported in the summer of 2001, near the village of Landford, on the northern fringes of the New Forest, most fittingly found by Goulson and Williams, both of whom are bumblebee researchers. As the specimen was brightly coloured, with little or no obvious hair loss and little damage to its wings, researchers concluded that there was little evidence that it had flown across the Channel. In other words, it was simply too healthy-looking to have made such an arduous and dangerous journey. That being the case it was presumed that it was a resident bumblebee. At that time researchers could not decide whether the bee came from an established colony or had been imported deliberately from Europe for use in a greenhouse. They could not confirm how it arrived.

At that time researchers concluded that this was one of the species of bumblebee that was capable and most likely to become established in the UK. They were right! It had previously been described as showing a stronger preference for areas inhabited by man than the remaining Scandinavian Bombus sp. Right again!  Another researcher considered that it had become more abundant in Belgium and Germany during the twentieth century, possibly as a result of increasing urbanisation. Goulson’s studies found that in England, gardens were shown to be a stronghold for other bumblebee species, e.g. B. terrestris. How right they all were!  They also conclude however that there was no reason to believe that it will do any harm, for example by competing with native British species, since it was already an established European species.

Competition for nesting sites?

That is true for British bumblebee species, however, it may now not be true for other species of wildlife, such as blue and great tits who may now have to compete with them for urban nest boxes or other suitable nesting sites. Something that back then would simply have not been considered. However, Broughton et al, some 14 years later state that there is no evidence of widespread significant nest site competition between cavity-nesting birds and bumblebees or social wasps. The paper recognises that a small number of bumblebees will usurp small passerines from nests. This is something I have witnessed several times. now over the years involving bird nest boxes, this particular bumblebee, blue and great tits.

That conclusion may be the case although tell that to the great and blue tits which have been ousted by the tree bumblebees which successfully nest in ‘their’ nest boxes! So how do tree bumblebee queens oust these birds? Carry on reading!

According to Stenning, blue tits (Cyanistes caeruleus) respond to bumblebees by performing the ‘snake alarm’, consisting of a hiss and a flick of the wings resembling a striking snake such aggressive action to a potential threat is known as Aposematism. He continues to say they may well kill bumblebees that enter their nests. However, he continues that the tree bumblebee is another matter. It often takes over a blue tit nest at any stage of the breeding attempt. This is irreversible, as once in residence it is probably expedient to let nature take its course. I would like to tease these comments open. The blue tit would perform the snake alarm when she sees the bumblebee enter the nest or inside her nest, regarding it as an intruder. The sighting of the bumblebee initiates the alarm. What would be the point if the bird did not see the threat entering its nest and the threat did not see the ‘snake alarm’?

Broughton et al, found that tree bumblebees invaded 3 partially built or lined blue tit nests and one nest containing nine incubated eggs and described how the queen bumblebee restructures the nesting material into a distinctive dome.

Here in the photograph, you can clearly see how the nest entrance hole is covered over, probably to keep more heat inside and a small entrance hole made for the bees.

In the case which I will show in the film, it shows how a possible prey item (bumblebee)) evicts the resident nesting bird which may or may not be a predator.

How does it work?

When the nesting bird enters the nest it is deep inside the nest box with the nesting material beneath it possibly with eggs as well. Neither species could see the other so visual warnings and displays would be futile. I have seen queen bumblebees enter bird nest boxes when birds are in the process of making a nest to probably check on the quantity and quality of the nesting material. If there is not enough material for her to utilise, she will leave the nest and revisit another time. Obviously, without marking the queen I could not say the same queen returns, but I think they do. This is an active and potential nest site for the queen to utilise for herself when there is enough material. It makes sense she will return to check on its progress. Wenough material is available, the queen tree bumblebee buries herself at the bottom of the nest and stays still and silent. When the bird returns to her nest and settles down inside the cup, in the dark, this movement tells the tree bumblebee that the bird is present. She will then start her threatening high pitch buzz. Her eviction buzz. The resident bird sitting in her nest suddenly hears but does not see the bumblebee, who is also vibrating and buzzing. Both the buzz and the vibrations are enough to ‘spook’ the bird so that she leaves the nest. When I opened the nest box and removed the front, I did carefully so as not to disturb the actual nest materials. It was only when I touched the nest itself and moved my finger that the queen initiated her eviction buzz.

Jablonski et al, found that bumblebees buzz at a frequency that causes incubating birds to leave their nests. They found that in their playback experiments, the warning buzz by bumblebees contributed to the successful eviction of birds in their nests. Enter the cavity-nesting tree bumblebee Bombus hypnorum, first recorded in 2001 by Goulson and Williams. What better cavity to nest in than the ample numbers of bird nest boxes provided by the British public in our gardens? Especially when the resident bird has collected nesting material for its own brood, or the nest box has now been used and abandoned by the owner, thereby leaving its nesting material in situ. Bumblebee queens are not physically equipped or capable of carrying nesting material themselves.

In my own garden over the years, I have had a nest box, which some years the tree bumblebees use, sometimes blue tits and sometimes great tits. I have observed a queen tree bumblebee using it even though great tits had already started to gather nesting material. She never entered when the birds were inside only once the female bird had left. It makes me wonder whether or not they take a chance of the resident bird not being present or even if she observed the female bird leaving. However, they were having none of it and I saw the female bird empty the nest box contents, which contained pollen and a nectar pot of the now evicted tree bumblebee.

Bumblebees displacing birds is not something new

Bumblebees displacing birds from nest boxes is not something new, nor is it just tree bumblebees involved. Broughton et al state “Further records in the literature suggesting usurpation of birds by bumblebees included Buff-tailed Bumblebees taking over single Blue Tit and Coal Tit nests in England”

Abandoned nest

I already had great tits nesting in the garden so was very pleased to have blue tits as well. But only ever a pair of each. The great tits always battle with the blue tits chasing them away. Through sheer persistence though, the blue tits nest as well. When I saw the female blue tit dragging nest material into the red nest box, as Stenning stated that nest boxes should be a minimum of 30 metres apart and as this one was only about 20 metres away it was unlikely to be another female entering another blue tit and great tit territory. There simply would not be enough food available to sustain 3 pairs of birds in such a small area. I concluded that this was the same female using the other nest box.

Angry Bombus hypnorum queen

I am aware that tree bumblebees can aggressively defend their nest especially when they feel vibrations or movement of the nest. How would the queen without workers react? To find out if a bumblebee box is being used, a knock or two on the sidewall triggers the bumblebees to give a warning buzz. This is what I did after I removed the nest front entrance panel. You can hear the response. It was the longest buzz I have ever heard which lasted 52 seconds! Then as she moved outwards inside the nest bedding, I was uncertain if she would aggressively defend her new nest site. So I moved a safe distance away from the nest and saw her fly away. Neither she nor the blue tit returned to that nest box.

If you found this interesting a more in-depth article I wrote some time ago can be found here Tree bumblebee queen steals nest from blue bit