Airgun hunting, compared to powder burners, is a relatively low power finesse sort of affair, so you're not going to blow chunks off of them with an air rifle. Head shots are the rule of the day. Fortunately, good quality airguns are very accurate, so if you know your trajectory plot for the gun and ammo in use, and can judge wind reasonably well, head shots aren't all that difficult to make. Looks like these guys did alright.