I've wanted to film crows anting for quite a while. Anting is a behaviour carried out by a range of birds (I dealt with Blackbirds anting in a previous post), but it is rare to observe it. Anting can be active - the birds picks an ant (or ants) and rubs it on its feathers - or passive - the bird allows the ants to climb onto its plumage. The behaviour involves distinctive movements such as sitting close to the ground or partially opening the wings and tail dragging. The bird movements on top of the ant's nest or direct disturbing of the nest will also encouraging the ants to come out of the nest and climb over their feathers, as anyone who has had a picnic on an anthill will attest.
A Carrion Crow anting at my local park. 27/07/2016.
The first time I watched anting behaviour in crows I was driving, waiting on a red light. It is a very distinctive behaviour, but I had no chance for a video or a photo! I have seen it a few more times, but didn't get a good video of it. Today, as I got out of my car I noticed the local pair of crows on the verge opposite. I used the car as a hide and filmed them. First, one of them first seemed intent in getting some nest lining material, but the other went to the base of a tree trunk and pecked, and pulled some vegetation from around the tree. Then it pressed its body against the ground. The second crow joined the first and both enjoyed an 'anting' session.
Watch both clips and a clip of the ants nest:
It might be surprising, but is still unclear why birds carry out anting behaviour and there are a range of hypotheses. Some claim that it is some form of feather maintenance. Species of ants that eject formic acid (Formica, Lasius and Camponotus) as part of nest defence are preferred for anting. Formic acid has insecticidal and acaricidal properties. One of the first hypothesis is that the formic acid kills or repels bird ectoparasites such as feather lice or mites. This has had some support. In the summer of 1943, in Transbaikalia, Dubinin killed four Blyth's Pipits that had been anting, counted all the feather mites and checked the mites behaviour. Many mites were already dead and a total of a third of the mites died over the next 12 h, and the rest seemed very mobile. The birds feather smelled very strongly of formic acid even 12 h after the shooting. The control for this experiment were four pipits that hadn't been anting. Only 0.9% of the mites died after 24 h, and they stayed more or less immobile. However, other experiments tested this hypothesis were inconclusive.
Another hypothesis draws on the formic acid has anti-bacterial and anti-fungal properties, and could be beneficial to keep in check bacteria and fungi that feed on feathers. Although the experiment showed that pure formic acid inhibits feather bacteria and fungi's growth, the results were not conclusive when more natural doses were used that reflected ant's formic acid content. An ornithological enigma!
More information
Morozov, N. S. Why do birds practice anting? Biology Bulletin Reviews 5, 353–365 (2015).