Flight behaviour diverges more between seasonal forms than between species in Pieris butterflies

In flying animals, wing morphology is typically assumed to influence flight behaviours. Whether seasonal polymorphism behaviour might be associated with divergent fore wing shapes. Pieris individuals in the spring are small and have elongated fore wings, traits correlate with low fly speed and acceleration, and in butterfly morphology is linked to adaptive flight behaviour remains unresolved. Here we compare the flight behaviours and wing morphologies of the spring and summer forms of two closely related butterfly species, Pieris napi and P. rapae. We first quantify three-dimensional flight behaviour by reconstructing individual flight trajectories using stereoscopic high-speed videography in an experimental outdoor cage. We then measure wing size and shape, which are characteristics assumed to influence flight behaviours in butterflies. We show that seasonal, but not interspecific, differences in flight high flight curvature. Large summer individuals exhibit rounded fore wings, which correlate with high flight speed, acceleration, turning acceleration, and advance ratio. Our study provides quantitative evidence of different flight behaviours between seasonal forms of Pieris butterflies, and these differences were better predicted by body size and forewing shape, rather than other factors (e.g., species identity, ambient temperature). This points toward a possible adaptive role of seasonal morphology determining flight behaviours, although this needs to be thoroughly investigated to distinguish the effects of microhabitat (e.g., behavioural thermoregulation, resource distribution between seasons, such as nectar and larval food plants), predator and parasitoid pressure, and possible sexual behavioural differences.