The viceroy resembles the monarch closely enough to fool a casual observer, differing chiefly in size and in a black line that crosses the hindwing veins. For most of the twentieth century this was presented as textbook Batesian mimicry: an edible species gaining protection by resembling a toxic model.
That account was tested directly. Researchers offered red-winged blackbirds viceroy abdomens, with the wings removed so the birds could not use appearance as a cue, and found them rejected at rates comparable to monarchs. Viceroy larvae feed on willows and poplars, which contain salicylates, and the adults appear to retain compounds that make them distasteful. The pair is therefore better described as Müllerian mimicry, in which two defended species converge on a shared warning pattern and each reinforces the lesson a predator learns.
The correction is not absolute. Palatability varies with host plant and locality, and in some regions viceroys more closely resemble the queen butterfly than the monarch. The relationship is probably a continuum rather than a clean category, which is itself the more interesting result.