Pedantically, the wing plan-form is not the arbiter of soaring
capability. All birds can soar if you allow the Pizzey definition, ie
rise relative to the ground without wing beat. It just depends on the
birds wing loading (ie the birds weight divided by the wing area) &
the vertical velocity of the rising air. An example is slope soaring
where air (wind) rising over an obstruction can provide high vertical
velocities & swallows will & do soar in these conditions.
Aspect ratio plays a minor role as higher aspect ratios (ie span
divided by chord (effective width)) are more efficient, but at the
cost of manoeuverability. Other efficiencies such as low parasitic
drag (eg legs tucked out of the airstream) & effective wing sections
also play a part.
Chris Charles
Licole Pty Ltd
0412 911 184
On 20/11/2006, at 2:54 PM, brian fleming wrote:
I recently said on 'Birding-aus' that 'Wood-swallows are the
only soaring passerines". While I am quite certain from
observation that they do soar, I have been wondering where I first
saw this in print. After an extensive search of the thousand or so
bird-books in the house (no exaggeration), I can say that I first
found it in a work by E. Thomas Gilliard, Associate Curator of
Birds, American Museum of Natural History - "Living Birds of the
World" (Hamish Hamilton, London, 1958). Just because it's an
older book does not mean that it doesn't contain useful
information. On p. 303, he writes:
"The finest of passerine fliers, and excepting the ravens the
only songbirds known to soar, are the wood-swallows.... When
hunting, wood-swallows often soar high into the air on rigid wings.
At such times they resemble miniature vultures. Their food consists
entirely of insects captured in the air." [We now know that they
also take nectar with brushlike tongues, and very surprised I was
when I first saw them doing so in a flowering Ironbark at Cocoparra.]
The early 1980 edition of Graham Pizzey's Collins FG (the one with
Ray Doyle's illustrations) gave more room for description:
"On the wing their silhouette is rather like the Common Starling;
the flight is graceful and absolutely diagnostic: quick shallow
wingbeats, circling glides and soaring on shapely pointed wings and
spread tails; but pursuing flight can be dashing." (p.392).
Current edition, (1997) ill. Frank Knight, P., in the Family
Introductions section, p 542:
"Unrelated to true swallows, woodswallows are graceful songbirds
with broad, pointed wings; they are among the few songbirds that
soar."
And what is soaring flight anyway? Pizzey's definition (p. 551)
is "rising flight on still, extended wings, using thermals or
updraughts to gain height without flapping".
It is typically seen in broad-winged birds such as large
raptors, storks, pelicans etc - I recall resting during a mountain
hike, flat on my back, while we watched a Brown Goshawk circling up
in a thermal, never moving its wings until lost to human sight. I
understand that in Africa small vultures such as the Egyptian can
get airborne much earlier in the day than the very large heavy
species, enabling them to get a start on small and part-eaten
carcasses before the big ones arrive and bully them off. The
'dynamic soaring' utilized by albatrosses over waves, is a
different action and requires a long, narrow wing (high aspect-ratio).
Swallows have very narrow wings and cannot soar, though of
course they can and do glide between burst of flapping. I don't
really know whether Needletail Swifts soar, though their powers of
flight are prodigious. Half their weight is breast muscles,
according to the books. Can they gain height without too much
effort by using the updraughts in the thunderstorms which often
seem to bring them?
Anthea Fleming
in Melbourne
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