The primary goals for birds are to survive, successfully breed, and dominate the gene pool. There are almost as many strategies to meet those goals as there are birds. Some lay small clutches of eggs, others large. Some begin incubation with the first egg, others after the clutch is complete. Some are monogamous, others are polygamous, and still more are promiscuous. The choices are endless and fascinating.
No strategy elicits emotions stronger than strategies involving siblicide, the death of a chick resulting from aggression by one or more of its siblings. The behavior requires asynchronous hatching, resulting from egg-laying intervals of preferably two or more days, and incubation beginning with the laying of the first egg. Siblicide usually occurs in clutches of two or three. Birds practicing the behavior include egrets, herons, pelicans, boobies, owls, and eagles.
Consider the Great Egret, which was studied intensely by Douglas W. Mock, the George Lynn Cross Research Professor of Biology at the University of Oklahoma. It lays a three-egg clutch that hatches asynchronously. The first chick gets all of the food brought by the parents and develops quickly. The second chick is not far behind. The third chick, however, is much smaller than the other two and weak because the parents rarely feed it. They make no attempt to parcel out the food equally; they’re content to feed the largest and most prominent chicks. And the chances are good that Chick No. 3 can be found spread-eagle on the nest floor with Chick No. 1 and Chick No. 2 standing on it.
Life looks pretty grim for Chick No. 3, and, unfortunately, it only gets worse. With Chick No. 1 taking the lead, the older siblings begin pummeling Chick No. 3. Relentless, No. 1 delivers vicious blows to the head, using its sharp bill. No. 2 pushes in and joins the assault. Both batter No. 3. Death results either from starvation or from wounds caused by the attacks. The youngster’s body is usually pushed from the nest.
Mock and his colleagues observed the behavior thousands of times. Sometimes, adults were in the nest when the carnage took place. How did they react? Most often, they preened calmly and deliberately, as if giving tacit approval to the attacks. But why?
For years, the behavior was thought to be a mechanism for brood reduction. Consider that egrets usually lay three eggs but most often raise one or two chicks. If food resources typically allow one or two chicks to survive, why do the birds lay a third egg?
Because, on occasion, food is very abundant and the adults can successfully fledge all three chicks. The observation that aggression among the chicks is greatest when food is limiting supports the idea. When Mock removed the youngest chick from the nest, the aggression stopped. When the chick was returned, so did the aggression.
If the parents tried to feed all three chicks when food was scarce, the young would likely leave the nest at a reduced weight and in poor condition, and they would be unable to compete successfully after fledging. It’s better to sacrifice one chick and have one or two healthy chicks. In a related study, Mock found that the amount of androgens (including testosterone) that Cattle Egrets add to the yolks of the first two eggs is almost double what they add to the yolk of the third. This presumably increases the aggression of Chicks Nos. 1 and 2, an advantage they appear not to need.
While siblicide definitely causes brood reduction, there is another explanation for the behavior. A third egg offers insurance against the loss of the first two eggs from infertility or predation, or the early predation of either chick. Studies of several species have shown that a third egg will hatch and survive under such conditions.
For years, it’s been known that associating with other species can be beneficial for one or more species within a group. Egrets in Florida, for example, enjoy a degree of protection from predators by nesting near alligators. Is it possible that siblicide, by providing chicks and carcasses, might play a role in attracting the reptiles?
Lucas A. Nell wanted to find out. In a recent study, he and colleagues from in the Department of Wildlife Ecology and Conservation at the University of Florida compared the body condition of alligators trapped in areas with and without nesting colonies of wading birds. Body-condition indices were significantly higher in alligators associated with the colonies. While it’s obvious that eating a young egret will benefit an alligator, this is the first study to document the effects of an egret colony on the body condition of a population of alligators. It’s as if both parties negotiated a deal. We’ll keep predators away, say the reptiles, so long as you provide an occasional morsel of egret. Everybody wins.
Siblicide is an unusual reproductive strategy that helps maximize surviving offspring. It again demonstrates the cleverness of those amazing birds. — Eldon Greij
Read the paper
Lucas A. Nell, Peter C. Frederick, Frank J. Mazzotti, Kent A. Vliet, and Laura A. Brandt (2016) Presence of Breeding Birds Improves Body Condition for a Crocodilian Nest Protector. PLoS ONE 11(3): e0149572.
This article from Eldon Greij’s column “Amazing Birds” appeared in the July/August 2016 issue of BirdWatching.