In Mute Swans:
Male are usually larger with thicker necks, while females have thinner necks and a smaller wingspan.

Another easy way to tell could be by the beak, as males tend to have a brighter orange beak with a black knob at the base of it that swells during mating season. The female beak can look slightly more grey.

These difference are not unusual in birds and may be caused by sexual selection…

I’m sure there are more ways to distinguish them!

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