National Geographic reports that more than 500 instances have been observed over the past 15 years or so, before which time no one had ever seen the behavior.
The researchers believe that a boost in the population of Cape fur seals may be to blame. The babies — which are basically helpless when their mothers go off hunting — have long been seen as easy prey for big cats. It's likely that a surplus of helpless seal babies led some opportunistic gulls to hunt them in whatever way they could manage, and that other birds followed suit.
“It is not a pleasant behavior to observe, as the seals completely freak out and make a lot of noise,” lead researcher Austin Gallagher of Carleton University told National Geographic.
But as horrific as it seems, it's one smart survival strategy.