Will I stay or will I go? Eye morphology predicts individual migratory propensity in a partial migrant

Billions of animals undertake migratory journeys every year, with powerful
consequences for ecosystem dynamics. Key behaviours that enable successful
migration are often guided by the visual system. The amount and quality of
information that animals can extract from visual scenes are directly
related to structural eye size — larger eyes can house larger pupils,
enhancing light-gathering capacity and vision by improving visual acuity
and contrast sensitivity. Migration should exert strong demands on
individual visual performance, for example via foraging, anti-predator
benefits or navigational requirements. Yet, it remains elusive whether
variations in eye morphology and corresponding visual capabilities are
associated with migratory propensity. Here, we capitalize upon
intra-population variation in migratory propensity (a.k.a. partial
migration) in roach, a common freshwater fish, to directly test for
migration-associated variation in image-forming eyes within a species. In
a multi-year field study tracking the migration decisions of over 2000
individuals in two different lake systems, we found that relative pupil
size was positively associated with individual migration propensity.
Computational simulations of the visual ecology associated with the
observed differences in pupil size show that migrants have an extended
visual detection range, and that the performance gain is most pronounced
for viewing small targets (e.g. planktonic prey) under low-light
conditions. These results suggest that the larger pupils of migrants
represent an adaptation for increased foraging efficiency to aid in the
accumulation of critical pre-migratory energy reserves. Together, our
anatomical and functional findings provide new perspectives on visual
system design in relation to individual-level migratory
decision-making.