Re: Gliding/soaring requirements

[Mike Habib <habib@jhmi.edu>]

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?

--Mike


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