When Douglas W. Mock of the University of Oklahoma began studying egrets in Texas three decades ago, he knew that the bigger babies in a clutch would peck the smaller ones to death. Still, Dr. Mock was caught off guard by what he saw — or failed to see. He had assumed that the murderous attacks would surely take place while Mom and Dad egret were out fishing.

"I figured that, if the parents were around, they'd try to block these things," he said. "I have three older brothers, and I never would have made it if my parents hadn't interceded."

Instead, Dr. Mock witnessed utter parental indifference. The mother or father would stand by the side of the nest, doing nothing as one chick battered its sibling bloody. "The parent would yawn or groom itself and look completely blasé," said Dr. Mock, author of "More Than Kin and Less Than Kind: The Evolution of Family Conflict." "In the 3,000 attacks that I witnessed, I never saw a parent try to stop one. It's as though they expect it to happen."

Since then, siblicide under parental supervision has been observed in many bird species, including pelicans, cranes and blue-footed boobies.

One researcher watched a nest of African black eagles for three days as the larger eaglet alternated between tirelessly stabbing at its sibling and taking food from its solicitous mother's mouth. There was prey to spare, but the mother did not bother feeding the second, abused baby. When the eaglet's poor, tattered body was finally tossed to the ground, the researcher calculated that it had been pecked 1,569 times.

Pigs, too, have their own version of litter culling by sibling rivalry. Piglets are born with little eyeteeth that stick out sideways from their lower jaw, Dr. Mock said, and they use these teeth to slice at the faces of one another as they jockey for the best teats. The runt of the litter is so often sliced and bullied that it cannot get enough milk. It must spend every spare moment fighting to nurse and may get crushed by its mother.

Image ACCESSORY TO MURDER | PIG

Some pig. She looked the other way when some of her piglets turned on the littlest one, slicing its face and starving it half to death. Then she rolled over and crushed it to death. Credit... Lou Beach

In other cases, mothers turn infanticidal because they are born optimists, ever tuned to the sunny expectation that good times lie ahead. Each year they breed for a banquet, producing a maximum of begging bairns as the season starts; and when there is plenty of food, they will provision every young.