Interesting study of migratory raptors using soar / glide techniques to cover distance…
Species reacted differently to different climbing rates: Steppe Buzzards increased circling duration with increasing climbing rate (Spaar 1995), whereas Steppe Eagles only expanded circling duration up to a climbing rate of 2.5 m s-¹; if they reached higher climbing rates, they reduced soaring time and had a constant height gain of about 200-250 m in completed circling phases (Spaar & Bruderer 1996).
Very interesting to know that not only do birds vary their inter-thermal speed according to conditions (as in classic McCready theory), but also seem to adopt different strategies between species. Could this be in reaction to the performance differences (polar curves)? Some species adapt their airspeed according to prevailing wind direction while others ignore this.
All raptors seem to be able to climb at the same rates, despite their very different bank angles for given turn radius. Some species work thermals right to the top of lift while other have the habit to operate below a certain ceiling.
Worth reading the whole article, 20 pages.