Why Cardinals Cover Themselves in Ants

On a still afternoon in late August, a male Cardinalis cardinalis drops to the grass, picks up a live ant in his bill, and strokes it along the underside of his wing feathers in quick, deliberate passes. He is not distressed. He is not losing balance. He chose this ant, and he is using it on purpose.

The behaviour is called anting. More than 200 bird species practice it, and the Northern Cardinal appears consistently on that list alongside Blue Jays, American Robins, Common Grackles, and Baltimore Orioles. Ornithologist Eldon Greij, writing in BirdWatching magazine, lists the cardinal among two dozen common North American species observed anting in backyard settings. The behaviour is widespread enough to be unremarkable in ornithology and strange enough that the field has not agreed on why it happens.

Two kinds of anting

Cardinals practice active anting. The bird picks up an individual ant in its bill and strokes it through the wing and tail feathers, sometimes repeating the action with several ants in sequence. A session runs a few minutes. The bird works methodically, pressing the ant against the feather barbs rather than simply holding it.

Passive anting is different: the bird crouches directly on or near a colony with wings and tail spread, letting the ants climb freely through the plumage. American Robins and Common Ravens practice passive anting more than most species do. Cardinals generally prefer to be in control of the process.

The ants involved are not arbitrary. Across documented anting observations, birds strongly prefer species from the subfamily Formicinae - ants that produce and eject formic acid as a defence mechanism. Wikipedia’s detailed survey of anting behaviour records 24 ant species used across all documented cases, with Formicinae the most commonly chosen. When a threatened Formicine ant is pressed against a feather, it ejects acid from glands at the abdomen. The bird appears to want that response. Ants from subfamilies that do not produce formic acid are used far less often.

What formic acid may or may not do

Here is where the confident explanations tend to outrun the evidence.

The most widely cited hypothesis is parasite control. Feather mites and lice feed on the keratin structure of feathers, and formic acid is known in laboratory settings to inhibit the growth of bacteria and fungi that degrade plumage. The antiparasitic reading is plausible, and it is the hypothesis with the broadest popular coverage.

Whether formic acid will kill mites and lice on wild birds through anting has not been demonstrated. - Eldon Greij, BirdWatching

Greij’s phrasing matters. The gap between “formic acid can kill parasites in a lab” and “anting actually reduces parasite load on a wild bird” has not been closed by field evidence. The behaviour is real. The mechanism is proposed, not confirmed.

A second hypothesis connects to the timing. Anting peaks in late summer and early fall, precisely when Northern Cardinals are moving through their annual prebasic moult. New pin feathers are pushing through newly exposed skin. Some ornithologists suggest the acid may soothe that irritation. The timing overlap is consistent, but the explanation is, as Greij notes, undemonstrated.

The third hypothesis runs in a different direction entirely. Thomas Eisner and Daniel Aneshansley at Cornell University ran controlled experiments presenting hand-reared Blue Jays with two groups of ants: intact ants with functioning formic acid glands, and ants with the acid-producing apparatus removed. The jays ate 96% of the acid-depleted ants immediately. When given intact ants, 61% were subjected to anting behaviour rather than eaten. The Cornell researchers concluded that anting may be food preparation - the bird induces the ant to exhaust its defensive acid before swallowing it. Cardinals are not strict seed eaters, and insects make up a real share of their diet, so an ant scrubbed of its acid is a meal worth the effort. This is at minimum a hypothesis grounded in controlled observation, which puts it ahead of several of the others.

Four competing explanations. None of them settled. That is the position ornithology occupies on anting in 2026, roughly 200 years after John James Audubon provided the first scientific description of the behaviour in print.

Why the uncertainty has persisted

Anting is brief, unpredictable, and difficult to replicate in controlled conditions without altering the variables that produce it. The German ornithologist Erwin Stresemann described it formally in 1935, and Salim Ali translated the concept into English in 1936. Since then the literature has accumulated hypotheses faster than it has accumulated decisive tests of them.

The cardinal at your feeder carries no awareness of this debate. He picks up the ant - a Formicine ant, specifically, not the first one he encounters - and applies it with the same precision a preening bird applies to a damaged feather. The behaviour is older than our interest in explaining it. For a species whose range now stretches across most of eastern North America and into the Southwest, and whose plumage condition matters directly to breeding success, the investment in feather maintenance is not optional.

Whether anting is parasite treatment, skin relief during moult, or meal preparation, the bird has been refining it for longer than we have been watching. The formic acid was useful before anyone named it. The explanation may arrive eventually. The behaviour will not wait.