Functional hypothesis for the role of the bill overhang in preening.  (a) the bill overhang generates a shearing force sufficient to damange lice as the lower mandible moves forward slightly during preening; (b) straight mandibles can only exert a relatively weak vertical force, by comparison.

From:  Clayton, D.H. and B.A. Walther, 2001.  Influence of host ecology and morphology on the diversity of Neotropical bird lice.  Oikos 1995.

The preening hypothesis was confirmed experimentally in another recent paper:
Clayton, D. H., B. R. Moyer, S. E. Bush, D. Gardiner, B. Rhodes, T. Jones and F. Goller. In press. Adaptive significance of avian beak morphology for ectoparasite control. Proceedings of the Royal Society of London Biological Sciences. 

Video clip of preening showing forward movement of lower mandible relative to upper mandible. This movement generates a shearing force sufficient to damage lice and other ectoparasites. Pigeons are capable of moving the lower mandible forward up to 30X per second! This clip was taken using a NEC high speed video camera at 250 frames per second. The preening bout shown represents a mere 320 milliseconds (1/3 of a second).

To View Preening clip effectively:
* Make sure your screen resolution is greater than 800 X 600 (e.g. 1024 X 768).
* Download QuickTime 4.0 or analogous video player.
* You can set your player to run the video clip in a loop, if desired.
* Clip is 4.1 MB, so be patient if you have a slow connection speed.
* Video clip at one-fourth actual speed.
* Video clip slowed to 1/24th actual speed.