Depending on the pig's passive ability, it will either deal bonus damage to your opponent, heal your team or reduce your Super shot counters when knocked out. Every pig has a symbol above its head displaying the passive ability.
Extra Damage - These pigs will deal extra damage to your opponent's team when knocked out.
Super Shot Reduction - Reduces Super Shot Counters for all birds of the player who defeated the pig.
Health Boost - Heals the birds of the player who defeated the pig.
You can also check the pigs' abilities by tapping them.