Envision a pidgeon-sized scavenging bird that summers in Europe and spends winters in Africa. During migrations it travels in large flocks, and disperses at either end to look for gross dead things. During the migrations, other birds want to snack on them, and so when one jay gets dazzlingly shiny wings it survives the migrations slightly better just by being confusing to look at. As this mutations spreads in one flock, that flock becomes incredibly frustrating to hunt. Simultaneously the birds begin to consciously direct their dazzle during migrations and use it to drive off other scavengers when not migrating. It begins to be effective to stay together to scavenge, because being near these flocks is unpleasant at best and increasingly dangerous, at least to eyesight. Eventually the flocks become coordinated, at which point their victims are not merely blinded.

Envision a pigeon-sized scavenging bird that summers in Europe and spends winters in Africa. During migrations, it travels in large flocks, and disperses at either end to look for gross dead things. Also during the migrations, other birds want to snack on them, and so when one jay gets dazzlingly shiny wings it survives the migrations slightly better just by being confusing to look at. 

As this mutation spreads in one flock, that flock becomes incredibly frustrating to hunt. Simultaneously, the birds begin to consciously direct their dazzle during migrations and use it to drive off other scavengers when not migrating. It begins to be effective to stay together to scavenge, because being near these flocks is unpleasant at best and increasingly dangerous, at least to eyesight. Eventually the flocks become coordinated, at which point their victims are not merely blinded.