When I first read the article Perris linked to I assumed there were other pages which would bear out Perris's view that the 80% sightings of single birds in some way suggests that flocks merge.
The article is pretty sketchy at best. One only needs to read the how the data was collected to appreciate this; 2.5 hours spent counting birds twice a month. Another point one should beat in mind is the article isn't about the jungle fowls social behaviour, it's about what ground conditions (vegetation, trees grasses etc) exist where the jungle fowl were counted.
The table layout is just plain wrong. The is no such thing as a flock containing a single bird. Flock is always more then one. So the sightings of single birds cannot be labeled or considered as a flock.
Nor can one assume that the sightings of single birds meant that bird was not attached to a flock.
I could stand at point on the land in Catalonia and see many sightings of single birds but the majority of such sightings were of birds traveling to and from the flock they lived with, roosters responding to hen calls, hens looking for nest sights, a bird going to a water site; there are many possible explanations for such sightings.
The study doesn't identify different individuals. This is quite understandable given one jungle fowl looks much like another especially from a distance and even more so when one gets a fleeting glimpse. The sightings recorded of single birds could be a few birds seen often, they move about a lot and they're fast. It could at the most unlikely be a different bird each time. Somewhere in between would seem most likely.
The observers had no idea, or at least didn't make note of any where these single birds came from, or where they went to.
Absolutely no inferences about how these birds are arranged in a flock or not can be made from the information in the study.
All the sightings of birds in the first column under the heading flock size can be dismissed when considering flock size, firstly because as mentioned a single bird is never a flock and secondly because there is no data on that bird bar the sighting.
Only 3 sightings of flocks above ten members were recorded they could be eleven birds or twenty five. The most common recorded flock size values are 35 for a pair and 34 for up to five members. The next range could be 6 birds or ten birds.
So, back to the original statement jungle fowl live in small groups in a defined territory. There is no evidence that flocks merge in this study, or any other I've read. There is plenty of evidence that individuals from a flock form new flocks but that is not the same as flocks merging.
I usually look at the cited papers and noticed this study sited a paper I have on file which is quite informative about the difficulties of observing jungle fowl as well as the jungle fowls social behaviour. Interesting that they tracked flocks by the senior males crowing.
Have a read. It's not a bad article. Notice the authors use the word group rather than flock.
https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=9843&context=condor