The Black Ring Explained

A video of a mysterious black ring hovering in the sky over Leamington Spa has caused a bit of a commotion, especially as an army of experts cannot explain it.
The ring was captured on a phone video camera and appears to hang around Warwick Castle for around three minutes before vanishing.
Obviously a fake everyone said but Iain McArthur, an expert at Audio Video Forensics said: "It doesn't look like it's faked. It's not like other video of unexplained sightings in particular the hazy edges to the black circle'.
Not a fake so obviously some sort of weather phenomenon everyone then said. 'There's no meteorological reason why it's happened' said the MET Office, 'and there was nothing unusual happening in Leamington Spa that day, we've checked'.
Some sort of insect swarm everyone then suggested hopefully but Frederic Tripet, an entomologist at Keele University explained that: 'Although insects do swarm when mating, mating is not done as high or in ring formation'.
Birds then, it has to be birds everyone then said looking towards Richard James, wildlife adviser at the Royal Society for the Protection of Birds who shook his head and explained 'Birds won't form a shape like this. It's certainly not birds'.
So it isn't faked and isn't insects or birds or some sort of bizarre weather event and because it hung around for three minutes it certainly isn't someone chucking a hula hoop in the air and videoing it which was my first thought.  
Of course we shouldn't jump to any crazy conclusions and we keep our senses about us and not read more into what probably has a sensible explanation which is this is a reconnaissance mission sent ahead before the main alien invasion party. Seems obvious to me.