It was a Saturday afternoon in September 1989, I had just finished working and I was walking back to our bungalow when I saw about four Thick-billed ravens (Corvus crassirostris) flying about around an acacia tree that stood alone in the park of the laboratory.
These birds, usually noisy, were rather common but their behaviour was rather odd as it was obvious that they were more agitated than usual. So, I rushed home to tell Mabel and bring the binoculars and the camera, just in case.
As we returned, a Harrier Hawk (Polyboroides radiatus) flew off with two of the ravens pursuing it. They all disappeared from sight, but the ravens returned shortly after. To our increasing amazement, what appeared to be a Wahlberg’s Eagle (Aquila wahlbergi) also flew away from the tree with another couple of ravens in pursuit. It landed in a nearby dead tree, but the ravens soon forgot about it. This situation was clearly odd, and we could not believe that we had already seen two uncommon raptors!
But there was more to come.
Although two possible candidates for the commotion had been chased away, the ravens were still very agitated. The racket attracted a pair of rather vociferous Cape Rooks (Corvus capensis) that joined in. It was now obvious that there was some other cause for the mobbing behaviour of the ravens that was now focused deep inside the acacia.
It required a thorough search to detect the presence of a fully grown Verreaux’s Eagle Owl (Bubo lacteus) surrounded by the ravens while the Cape Rooks provided a more distant but very noisy support. The owl had chosen or it was driven by the ravens to a rather thick tangle of thorny twigs. These shielded the owl from the raven’s beaks which were only able to attempt to reach it one at a time through one opening among the branches.
At this point one of the ravens (frustrated in its attempt to reach the owl?) began to deliberately break off the twigs with its beak in order to increase the size of the hole.
Corvids are regarded among the most intelligent of birds and they have brains with a similar number of neurons as some monkey species! So their “twig-removing” behaviour is not surprising .
After a few minutes, all four ravens adapted this activity, and soon made an opening large enough for them to enter and reach the owl from several directions. Despite their numbers, large size and powerful bills, the ravens were very wary of the owl, never facing it and always striking at it by jumping up and pecking at its rear.
After a few minutes of being forced to fend off the four attackers simultaneously the owl took off still pursued by the ravens. It alighted again in a nearby tree but was almost immediately forced to fly again into another one where it remained until dark, continually mobbed by the ravens, clearly unhappy to see it in their neighbourhood. The following morning, there were no signs of the dispute of the day before and the ravens were back to their normal noisy life!
Of particular interest to us was to see the congregation of several birds of prey on the tree as well as the persistence and ferocity of the attacking ravens.
The late Leslie Brown in his book African Birds of Prey  has reported Verreaux Eagle Owls preying on the young of pied crows and buzzards, so that probably all the corvids and raptors involved in this incident were reacting to the owl as a potential predator of their young.
Note: during the subsequent years spent watching birds in Africa, we have observed the great respect aht most birds of prey have for large owls, clearly seeing them probably as dangerous to their young.
 This observation was published much later as: de Castro, J. & de Castro, M. (2013). Verreaux’s Eagle Owl Bubo lacteus attacked by Thick-billed Ravens Corvus crassirostris. Scopus 32: 51–52
Link to the published work: https://www.ajol.info/index.php/scopus/article/view/108343
 For a recent study on crowns see: https://www.statnews.com/2020/09/24/crows-possess-higher-intelligence-long-thought-primarily-human/
 Brown, L. (1970). African birds of prey. Collins.