Why do male butterflies chase other butterfly species?

BBC Wildlife contributer Richard Jones explains this aggressive butterfly behaviour.

Small tortoiseshell butterfly. © Paula French/Getty

Males of several butterfly species, notably peacocks, small tortoiseshells, red admirals and speckled woods in the UK, guard a patch of sunlight as their private territory. They fly up to see off other passing males, but hope to engage in courtship with a female should she chance by.

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Butterfly eyes (as with other insects) are well-adapted to detect motion, but their interpretation of shape and colour and the landscape around is probably highly pixellated at best.

Consequently, they don’t really know what they’re seeing off until they get quite close, and will regularly fly up to investigate other butterflies, bumblebees, dragonflies, birds and even the occasional aeroplane.

The behaviour does allow a neat trick to tell the sex of a butterfly without needing to peer closely: simply lob a small pebble about 1–2m over the top of a resting butterfly.

If it sits tight it is a bored female, if it dashes up it is a keen, if myopic, male.

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