20 Apr
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Sibling Day Siblicide

Brown-headed Cowbird chick dwarfs its nestmates. Its enormous size guarantees things will not go well for its roommates.

Brown-headed Cowbird chick dwarfs its nestmates. Its enormous size guarantees things will not go well for its roommates.

Note: By now, you’ve heard that there’s a Science and Nature Facebook page, right? Of course you have. Find photos, factoids, and more when you visit www.facebook.com/scienceandnatureforapie and hit “Follow”. You’ll feel better for it.

Apparently Siblings Day happened. I only know this because there was an eruption of comments related to the issue on Facebook recently: “To the best sister!” “So lucky to have siblings like you guys!” “Love ya!” Lots of happy emoticons, hearts, blurry pictures. Bloody awful. Likely a secret plot hatched by greeting card companies to combat the slump in sales due to the popularity of e-cards.

Birds have no Siblings Day. And it isn’t just because they have the good sense not to be duped so easily by corporate interests. It’s because bird siblings, at their best, tend to compete with each other for food. At their worst, they tend to kill each other for the same reason.

Dr. Charles Clarkson is an adjunct Professor of Ecology at Salve Regina College and Roger Williams University. He points out that many species of birds will not actually begin sitting on their eggs until the complete clutch is laid. Peregrine Falcons, for example, normally lay four eggs in a clutch. It takes the female four days to complete the process, and during that time the eggs will remain sitting in the nest, without her attending to them. This is because Peregrines want synchronous hatching, or having the chicks hatch at roughly the same time. Thus, for a brief period of time, the egg temperature will be too low to prompt development. Once the complete clutch has been laid, the female will begin attending to them so that the entire clutch develops at the same rate. In a good year, this means that the chicks will hatch at roughly the same time and Mom will provide them with sufficient food to increase the likelihood of survival. (Audubon Society of Rhode Island currently has a web cam trained on a Peregrine nest. Watch the developments live by going to www.asri.org.) That’s the way it is with most species. But not all of them.

Some species will lay what Clarkson refers to as an “insurance egg”. This is a kind of survival plan for the species of bird: if the eggs are damaged or eaten by a predator, having an extra egg comes in handy to increase the likely propagation of the species. If all the eggs hatch, though, the insurance egg becomes… well, unnecessary. “It’s very important in terms of your survival where in that laying order you lie.” Water birds will begin incubation on the first day an egg is laid. “By the time the last egg begins to hatch, chick one is four days older than chick four, and four days in terms of development is a tremendously long period of time in terms of size.” If there is ample food for all, it’s possible that the entire clutch will fledge. If not, things will get ugly.

“As the older birds get bigger and more competitive, they may begin to fight among themselves,” Clarkson says. Unlike in the human world, parents will step away and let the battle take place. “If the chicks sense that their opponent is the same size, they’ll back off from each other.” If not, one is likely to be killed. This is natural selection at its most raw. Birds reproduce with one aim in mind: to propagate the species, and 80% of the chicks will not survive. There is no sentimentality, no moral or ethical choice, only seeing to it that one of their offspring survive. Natural selection dictates that that one will be the strongest.

Some birds have even evolved weaponry to deal with the problem of food competition. Australia’s Kookaburra, for example, are born with a small, sharp hook at the tips of their beaks. In the world of ornithology this is called a siblicide hook, and it serves just one purpose: eliminating its nest mates. Even before the chick has its eyes open, it will seek out others in the nest that may compete for food and engage in mortal combat. “Monopolization of their food source (i.e., the food that the parents bring to the nestlings) makes the likelihood of survival much higher,” Clarkson says.

Perhaps the darkest practitioners of brutal nesting habits are ones found in back yards around Rhode Island at this time of year: Brown-headed Cowbirds. Cowbirds are parasitic nesters, which means that even before the eggs are laid, the mother is engaging in deeds worthy of a Shakespearean tragedy. To begin with, Cowbirds lay their eggs in the nest of other, often smaller birds. Clarkson provided the example of a tiny bird called a Marsh Wren. The Cowbird chases the female wren off her nest. Once the nest is unguarded, she proceeds to destroy and eat any eggs before laying one of her own in the nest. She then flies off, never to see her offspring hatch. The wren, genetically programmed to tend to her nest, eventually returns to the nest and tends to the egg, despite the fact that it is nearly as large as she is. The final blow comes when the Cowbird chick hatches: if there are any Marsh Wren nestlings left, the Cowbird will push them out of the nest, monopolizing the food and vastly increasing the odds of its survival.

Thus, I humbly submit that buying cards or posting Facebook messages on Siblings Day is a silly way of saying we are grateful for our siblings. Perhaps we’d be better off being grateful we’re not birds. I don’t think there’s an emoticon for that, though.

Newly hatched great egret chicks, also known to be practitioners of siblicide.

Newly hatched great egret chicks, also known to be practitioners of siblicide.

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