By Fernando Ortiz-Crespo
of Quito, Equador
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.
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.