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Re: Gliding/soaring requirements

Erik Boehm wrote:

Ahh... the danger of assumptions. A loon will not be thermal soaring anytime soon, because of behavioral reasons.

Yes, that's the precisely the point. I never said it couldn't ride a thermal - loons are not well adapted to thermal soaring, compared to other birds, and so we predict that they won't use them (more on this below). For a bird, the min sink of a loon is quite high (but that's not the primary constraint here, necessarily).

The performance specs you just described are very close to a "high performace" hang glider, which get sink rates of 1.0-0.9 m/s and max glide ratio's of 14-17:1.

Sounds about what I'd expect - good to have hard numbers from someone in the know, though.

Such gliders have flown 438 miles on thermals alone(at the time, the highest manufacturer claims were 15:1), and more than a few 400+ mile flights.
"recreational" hang gliders may only get 1.2 m/s sink rates, and glides of 12:1, they still regularly thermal for hours, and such gliders were setting the records back in the 90's and 80's with 200-300 mile flights.

Cool stuff.

I don't know the circle diameter of a loon, but I'm certain its much smaller than that of a hangglider.

At a 20 degree bank, it'd be about 35-45 meters. They have a tremendously high wing loading for a bird.

Loons seek out water, not soaring over just any open space that happens to be a thermal generator.

But they do fly over land between water sources, are occur over coastal ocean environments that produce marine thermals. Lack of access is probably not the constraint. Of greater importance is their aerobic capacity and flight speed - loons are highly adapted for rapid, straight flight over long distances. They can sustain constant flapping for very long periods, cruising at speeds comparable to some freeways. Given their very wide circle radius, and high min sink (relative to other birds), loons are looking at rather poor cross- country travel rates using thermal soaring as compared to continuous flapping. Note that many other birds with similar planforms and muscle compositions also rarely (if ever) utilize thermal climbing, including many that travel overland and certainly come in contact with thermals. Such taxa don't completely avoid external sources of lift, of course - cloud streets and ridge lifts may be used, for example, but thermal circling is rarely utilized by animals like ducks, geese, loons, or grebes.

So their behavior is why - they don't go to geographies likely to generate thermals, and probably would be very rarely interested in thermalling if they happen to come across one.

The second part is correct - a loon will generally be disinterested in loitering, and thus will not use thermals if encountered. The data I supplied predicts this trend, when compared to the same parameters for other birds.

Seaguls on the other hand, thermal quite often, and apparently so do some types flying foxes.

Yes, seagulls are well adapted to thermal soaring, gain significant benefits from doing so, and loiter often. The literature on soaring flying foxes indicates that only a few taxa have been seen to soar on thermal columns. I don't recall the frequency off-hand, but I have a vague recollection that it was not especially common.

Some birds extensively make use of thermals, and are optimised to do so.
You could compare a loon to them, and conclude they *can't* thermal, but you are not comparing them to the minimum threshold.

True - and I never said they can't thermal - I said that they aren't expected to do so.

I am quite sure a loon could thermal based on your numbers.

Me too. I'm also quite sure that they will almost never bother.

Out in the some of the hotter areas of the west US coast, come midday, thermals giving 8+ m/s climb rates are not particularly rare - practically anything can thermal inside one of those. A guy on this list sent me a message saying there were some days where he was, they could soar a J-3 cub single engine aircraft.

Yup - it's pretty intense. It also makes the point that many birds commonly cited as being specifically optimized for thermal soaring (like vultures) actually have planforms far broader than needed (or optimal) for such flight, if one assumes purely travel-based selection. Such taxa must have additional requirements - these appear to be related to a loiter-based feeding ecology and launch in cluttered environments. The need to carry very heavy gut loads after feeding may also factor.

One thing I would be very interested in, is the relationship between the much warmer climate, and thermal frequency and intensity.

It could be very relevant to the large pterosaur discussions, if not early birds.

Definitely worth considering - any thoughts out there?


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280-0181 habib@jhmi.edu