Anting behaviour may be defined as the stereotyped movements with which birds, in order to get formic acid (or other organic liquids) on to the feathers for some purpose as yet not fully understood, actively apply ants (or objects in place of ants) to certain parts of the plumage, and/or more or less passively allow these insects to crawl on to the plumage.
Source A Review of the Anting-Behaviour of Passerine Birds 1957
Despite many decades of research into the prevalence of 'Anting' behaviour in birds, there is, to date, no generally agreed explanation for it.
There are several theories,which include :
Further info. : Wikipedia