Terence Ang of My Birding & Raptor Field Trip made an interesting observation when he came across a pair of Greater Flameback (Chrysocolaptes lucidus) in Penang, Malaysia. The male was actively brushing his bill against that of the female. This went on a number of times before the latter decided to move up the branch. The male did not give up as he followed the female who then flew off.
“Could this behaviour observed a part of the courtship?” asked Terence.
Ravens often indulge in mutual mouthing between pairs, less often seen in crows. This may develop into sharp jabs and brief fighting. Such behaviour is termed allobilling.
A pair of Collared Kingfisher (Todiramphus chloris) posted earlier may be indulging in allobilling but this would depend on what they were doing before and after the act. Unfortunately this was not observed. In some birds like Herring Gulls (Larus argentatus), bill-touching occurs when the female tries to coax the male to regurgitate food. But this is not allobilling, nor is mutual transfer of food.
Birds also indulge in allopreening as seen in a pair of White-crested Laughingthrush (Garrulax leucolophus).
More observations need to be done to note what the birds are doing before and after touching bills. This is a seldom reported behaviour outside of ravens and crows and we need more observations before we fully understand the phenomenon.
Marzluff, J. M. & Angell, T. (2005). In the company of crows and ravens. New Haven & London: Yale University Press. (p166)