Wild Neighbors: Eagle Family Values: The Cain and Able Syndrome

By Joe Eaton
Thursday May 07, 2009 - 06:05:00 PM
Golden eagle egg and chick: beware Big Brother (or Sister.)
Johann Jantz
Golden eagle egg and chick: beware Big Brother (or Sister.)

We’ve been spending a lot of time with the peregrines lately, thanks to the folks at the Santa Cruz Predatory Research Group who operate the nest camera. This year the downtown San Francisco peregrines have chosen to nest on the PG&E building instead of the underpinnings of the Bay Bridge. Three of the four eggs hatched a couple of weeks ago, and Dapper Dan and Diamond Lil are busy feeding a trio of increasingly large and voracious chicks. 

(Dan is the former mate of Corona, the Queen of the East Span, with whom he displaced the former resident pair, George and Gracie, a few years back. Corona disappeared last year, and then Lil showed up. It’s a real soap opera. Dan was known as Tecate for a while, but the name didn’t stick.) 

A number of peregrine eyries worldwide, including one on the Rachel Carson Building in Harrisburg, Pennsylvania, Ron’s home town, have their own webcams. You can also watch bald eagle nests online: two in Washington State, probably others elsewhere. 

A friend who has been monitoring one eagle family informed me last week that the second egg had hatched. “Great,” I told her. “Stay tuned for the siblicide.” 

Yes, that’s a real word, and a widespread phenomenon among birds. In a few species, including some eagles, American white pelicans, and blue-footed boobies, the first-hatched chick almost always does in its younger brother or sister. That’s called “obligate siblicide,” or sometimes “Cainism.” In others, like the great egret and black-billed magpie, life in the nest is harmonious unless the food supply gets short, and then things turn nasty: “facultative siblicide.” 

Douglas Mock, a biologist at the University of Oklahoma, has written a book called More Than Kin and Less Than Kind, which discusses siblicide—and the broader category of brood reduction—in fascinating detail, and with a minimum of academic jargon. As he explains, siblicide has perplexed scientists for decades. It flies in the face of the idea that siblings should be predisposed to cooperate because of their shared genes (“I would risk my life for two brothers or eight cousins,” J. B. S. Haldane is supposed to have said.)  

The textbook case of obligate siblicide is the black eagle, an African species that preys on hyraxes. Calculating that only 2.5 percent of second-hatched chicks survived to fledge, ornithologist Leslie Brown called the female black eagle’s persistence in laying a two-egg clutch “an inexplicable example of apparent biological waste.” Food supply seems irrelevant; observers have watched alpha eaglets attack betas in nests piled with hyrax carcasses. Mock offers the caveat that the process has only been observed twice, since black eagles, like their relative the golden eagle, are wary birds that nest in inaccessible places. 

Mock also points to flaws in Brown’s logic, including the assumption that all the casualties in eagle nests that lost one of their two chicks were betas. He sees 20 percent beta survival as a more defensible figure. That would match the global average for the golden eagle, which conceals considerable geographic variation. Golden eagles in Scotland fledged an average eight-tenths of a chick per year; in Montana, just under a chick-and-a-half. 

In both black and golden eagles, the female begins incubating the first egg as soon as it’s laid. The second egg arrives two or three days later, by which time its older sib has had a considerable head start. The longer the lag time between eggs, the greater the tendency to obligate siblicide. Since females are the larger and stronger sex among eagles, a first-hatched female eaglet would have a near-insuperable advantage over a second-hatched male. 

Rather than inexplicable, Mock sees the production of the second egg as insurance. Birds that lay a single egg—kiwis and albatrosses, among others—are taking a considerable risk; if anything happens to it, there’s no time to replace it during the nesting season. A first-laid eagle egg is subject to the same hazards—infertility, mechanical cracking, predation. A second egg hedges the parents’ bets. If the second egg is smaller than the first, that lightens the parental investment. Although in theory the parents might be able to provision two nestlings to fledging, odds are that the prey base will fluctuate. That would make the first chick’s murderous behavior a preemptive strike against a future competitor. 

Facultative siblicide is not unknown among falcons. But Dan and Lil have an inexhaustible supply of city pigeons at their disposal, and the kids aren’t likely to get lethally hungry. (Not to equate siblicide with cannibalism; often the victim goes uneaten.)  

Siblinghood is deeply fraught even for those of us who didn’t have to fight our brothers or sisters to the death. I have to wonder if eagles are more neurotic than albatrosses.