Mourning Cloaks are almost impossible to differentiate by gender without looking at their tail ends with at least a strong pocket scope. The way I differentiate most individuals is like this: males tend to establish small territories, rest on trails and "stick around" an area. Males are often curious of humans or large animals passing through their territories and will "attack" by flying low over the trespassers, often buzzing peoples' heads so closely that one can hear their wings flapping. Females tend to wander more and won't stay in one area long. They will also fly over and around vegetation, inspecting potential hostplants. I also believe that females are more likely to nectar at flowers than males, though nectaring behavior is very uncommon in Mourning Cloaks.

Moved from Butterflies (excluding skippers).

Moved from ID Request.

however, Mourning Cloak gender is often difficult to tell from a single photo of a worn individual (males and females differ only a little bit in wing shape, and not at all in coloring).

I've been wondering the same thing. I don't have the accouterments necessary to photograph minuscule insects, but I can look at their features under a microscope, and hence meaningfully explain things to an expert. Therefore, it would be nice to be able to do what you are suggesting.

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