Hannibal was the name given to a male swan who attacked as many as 37 other swans, killing 15 of them. The swan had been living with his mate and their cygnet in Castle Pond near Pembroke Castle, located in Pembroke, Pembrokeshire, Wales. In February 2010, Hannibal began attacking any other swans who ventured into Castle Pond from the nearby Mill Pond. Hannibal would beat other swans with his wings, beak, and feet. If they remained in the pond, he would hold his victims' heads underwater until they drowned. If they attempted to leave the pond, Hannibal would "smash their feet to smithereens with his beak." After each attack, Hannibal would bring his son to view the aftermath while holding his wings up in celebration.
Male swans have been known to display aggressive behavior in order to protect their families, but such behavior generally decreases as the cygnet grows older. Wildlife worker Maria Evans suggested several factors which may have contributed to Hannibal's aggression, including poor water quality, limited food, pollution, lead poisoning, or the possibility that Hannibal was in pain after swallowing a fish hook. The aggressive behavior was only expressed towards other swans; humans were able to feed and later catch the bird without incident.
Hannibal's killings stopped only when he injured one of his wings during an attack. Evans, who had been waiting for permission to remove the swan for several months, brought Hannibal to the Tinker's Hill Bird of Prey and Swan Rescue Centre where she bandaged his wing and injected painkillers.