I came across another contender this week for best thermalling bird,
although not really a bird... more an extinct flying reptile.
Apparently the Quetzalcoatlus (largest of the pterosaurs) might have
been quite the efficient soarer, with a 10m+ wingspan. Here's the
article: http://www.plosone.org/article/info%...l.pone.0013982.
Figure 2 is the most relevant for this discussion, with a nice two-by-
two diagram mapping bird and pterosaur species based on their aspect
ratios and wing loadings.

The theorized launch method was potentially quite unique as well
(quadrapedal launch), although not likely to replace the aerotow or
winch launch. There's a video recreation with this story:
http://news.sciencemag.org/scienceno...rs.html?ref=hp

Coincidentally, there even happens to be a photographic depiction of
an attempt to thermal with Quetzalcoatlus (http://tv.sky.com/3d-
attenborough-doc-to-debut-this-xmas). I think by the look on Sir
David Attenborough's face he must have been wishing the giant reptile
were FLARM equipped.