Thanks for your visualization of the BS family member sites and paths. It is really nice to see in which kind of landscape they live now and guess how they feed. Just we don't know whether they are alone or have company.
Does the quoted info above mean that the BS let thermals rise them also during long travel flights? To which altitude they rise and what is their gliding ratio? E.g. Average altitude 250m, lower limit 100m, -> upper limit 400m. If gliding ratio is 1:10 they could continuously glide forward 3km plus wind.
Let say that the speeds are 10m/s for gliding and 20m/s for blowing. So they would come down from thermal heights to lower limit in five minutes and simultaneously get forward nine kilometres.
Or could they take more altitude by thermals, e.g. to 1300m and glide down to 100m from there, having an average travel altitude of 700m and a gliding altitude difference of 1200m. In that case they would glide for 20minutes continuously and get forward 12km without wind or 36km assisted by a tail wind of 20m/s.
If they spend 10 minutes to get up again by a thermal of 3m/s their average flight speed would be 24km/h (no wind) or 72+24=96km/h (tail wind 20m/s). The latter is maybe too high estimate, provided that daily legs can be up to 500km/day at a maximum. Maybe there are not so well developed thermals during high wind. I am sure that this has been well studied and calculated in science and aviation techniques already long ago.