For the Birds Radio Program: Avian Siblicide

Original Air Date: Dec. 16, 1990

Many baby birds can get along well with their siblings, but in some species, siblicide is the order of the day.

Duration: 4′14″


Now that the Christmas gift-buying season is upon us, it’s a fitting time to consider the issue of avian siblicide. The first murder recorded in the Bible was a case of siblicide, as Cain killed his brother Abel with rather the same basic motivations we find in a nest of baby hawks, where the bigger, tougher siblings kill the smaller, gentler ones to garner every ounce of parental attention and presents for themselves. I went to a Catholic school where we learned that Jesus had no brother or sisters. That made a lot of sense to me, because I figured no one could possible lead a perfect life who had a brother or sister because they’d eventually end up fighting.

Of course some kinds of baby birds get along wonderfully with their brothers and sisters. I’ve watched baby Blue Jays in their nests and raised a whole family of them, and they all seemed very polite even when competing for food. When mom or dad flies in with a meal, they all shout “Me first! Me first!” but they don’t sabotage each other’s efforts beyond that, and never whine or fight about who got the best caterpillar the moment the parents leave. But birds in some families are horrible to each other even when the parents are watching. Anyone who has ever observed baby chickens knows how cruelly literal the expression “pecking order” is. The largest chick in a heron or egret nest attacks and usually kills the smaller, whether there is plenty of food or not. Most hawks squabble but don’t get murderous unless they’re hungry, but the most delicately beautiful hawk of all, the Swallow-tailed Kite, starts out life as an obligate siblicidal chick, meaning 90 percent of all Swallow-tailed Kites that we see dancing gracefully through a Florida sunrise once killed a younger brother or sister.

What’s the point behind baby birds killing each other, and why don’t their parents stop them or at least yell at them or take away their allowance now and then? Apparently some species of birds have more babies than they can raise on purpose, as insurance. Double-crested Cormorants usually lay four eggs, but virtually never do more than three survive. Whooping Cranes lay two eggs, but the moment the first hatchling dries off, they abandon the nest for good, leaving the second, usually viable egg unhatched and unloved. But if an accident or illness befell one of the older baby cormorants, or if the first crane wasn’t viable, there would be another baby to take its place, making the parents’ energy-expense well worth it.

In almost all bird species with siblicide, females start incubating as soon as the first egg is laid, so there is a size difference among the chicks. A mother Blue Jay never starts incubating until her full clutch of five eggs is laid, so the babies hatch out pretty much at the same time, and are on an even footing. Among hawks, sometimes the bigger sibling not only beats up its weak little brother or sister that couldn’t fight back during tough times—it then eats it. This makes complete sense from an energetics standpoint, but seems unfathomable to us humans. We may squabble with our brothers and sisters, filling Christmas with tension and unpleasant memories for some because of the difficulties present in a lot of sibling relationships, but that’s usualy as far as it goes. When a hawk gives its brother or sister a valentine saying “Roses are red, violets are blue, sugar is sweet, and so are you,” the little one knows it’s in big trouble. When a baby hawk says “you are what you eat,” it means it literally. This time of year, surviving brother and sister birds are safe from one another, living far apart and not even exchanging Christmas cards during the season most associated with brotherly love. Hawks have a whole different fear when they hear those ominous words, “Big Brother is watching you.”