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An attempt to classify the
arthropod capture techniques of hummingbirds employs the term
"hawking" for open-mouth capture in which the prey comes into
contact with the rear of the tongue and then is swallowed, and
the term "gleaning" for the method in which the bird uses the
bill as a tweezer to catch and fasten the prey, which is then
moved to the mouth cavity (Note 1).
Despite this, the assertion
that a hummingbird uses the tongue to "glue" small insects prior
to swallowing them has continued to be repeated since Gould's
times to the era of modern physiology; but experienced observers
such as Scheitauer (Note 2) and Mobbs (Note 3), who base their
conclusions on their first-hand experience with captive birds
seen at close range, clearly say the opposite.
Mobbs refines what is known
about insectivory saying that in captivity, the hummingbird
Chlorestes notatus inspects leaf and other surfaces hovering in
the air until a suitable insect is found, and then opens the
bill and with the tips "taps" the surface once or twice "CAUSING
THE INSECT TO FLY DIRECTLY INTO THE OPEN BILL".
When a foraging hummingbird is
hovering or in slow displacement flight, the funnel of air
pulled back by the wings might serve to close the prey's escape
routes, and the bird might even have the ability to "focus" the
vortex of air created by the wings so that the prey is pulled
just into the narrow mouth opening where both mandibles meet; in
other words, the hummingbird might be using a technique which is
not rare in animals living in a watery medium.
Thus the aerial insectivory of
these birds may thus be better understood, considering that
their long and narrow bill, as Mobbs says, "as an instrument to
catch insects on the wing ... seems clearly inferior to the
wide, flat bill of a flycatcher or swallow or to the wide mouth
of a swift or goatsucker .... Nevertheless, the hummingbird
compensates for the narrowness of its bill with its superb
flight control."
In fact, hummingbirds are not
endowed with the large buccal surfaces or with the peribuccal
bristles of insectivorous birds that catch airborne prey such as
swallows, swifts, whip-poor-wills, frogmouths, kingbirds, etc.
And this possibility would also
explain why hummingbirds prefer to feed on spiderlets and
slow-flying insects such as gnats, small wasps and leafhoppers,
rather buoyant in air and easy to catch.
If this hypothesis is
confirmed, hummingbirds would fully qualify as "filter-feeders",
to use a term popularly applied to many aquatic animals.
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REFERENCES:
1-Fritsch, E. and K.-L.
Schuchmann. 1988. Ibis 130: 124-132.
2-Scheitauer, W. 1967.
Hummingbirds. A. Blaker. London.
3-Mobbs, A. J. 1979. Avic. Mag.
85: 26-30.
*Translation of extract from
article in BirdLife International - Pan American Bulletin, Vol.
10, No. 4, Dec. 1995.
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