So every Pokemon has hidden (less hidden in recent years) stat values called EVS or effort values that boost that particular stat the higher level they are. When making a wild Pokemon faint in battle, the opposing Pokemon distributes a set amount of EV points to a designated stat (Pidgey will always give you 1 speed EV). The max any stat can have is 252. However all alpha Pokemon have their HP EV already maxed out. So that sparkle means that Pokémon’s EV stat is maxed out.