Previously, I was fond of nature scientific documentaries like BBC life series, actually I'd rather to say that those documentaries with magnificent views are the reason why I chose my major. However, during my graduate school period, I happened to see a documentary that called Earth Flight, it said that the thermal updraft from urban agglomeration could help migratory birds to fly, at that time, I was thinking about the disturbing factors of migration, and I was confused that if urbanization is a kind of negative effect in migratory routine. Then I asked some ornithologists, one of them told me that he did not find a reference about it, it's better not to draw a conclusion in it easily.
So I wonder that if BBC documentaries refer to references of just refer to their video sights
No, typically wildlife documentaries are not referenced. In some cases there maybe a book accompanying the programme/series which will have some references in, but this isn't usually/common. One issue is that in the case of wildlife programmes the film makers are often reporting/documenting brand new information/insights and are often ahead of the science. So it's not always the case that a programme will digest all the science and then set out to film a particular behaviour or phenomena, rather they will set out to film a species or habitat which is poorly known (or nearly unknown) and then what they find in the filming is shown in the programme. Whereas we, as scientists, would want to put things down in a paper before showing the film!
This is especially true of the BBC Life documentary series with David Attenborough. Often these find new behaviours (or even new species) before the science literature. And in some cases there are subsequent scientific papers published formally presenting the finding (particularly where new species are found), but these would probably be later than the filming (given how slow science publishing works)
Don't forgot that depending on what you are writing and the context, referencing things doesn't have to be restricted to journal articles or books. It is perfectly fine to reference other media (see online for reference styles for television programmes). Just remember that as a TV programme is not peer reviewed you just need to be more cautious in how much weight is put into it. For example it would be fine to say "it has been suggested.." or "it has been observed.." .. "that migratory birds can make use of thermal updrafts from urban agglomeration to assist in fight [Television Documentary, 2017]". Obviously exercise caution and take advice from supervisor/mentor, but if a good quality documentary has really good evidence I wouldn't automatically exclude it from being referenced in your work.
A very important question! The " answers" and situations are variable. It depends on the subject but generally these programmes are far from being rigorously referenced. Let alone the hidden messages these complex media organizations are intended or directed to vehicle.