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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