by Greg Butcher
May 2010
A few years ago I was walking in the woods when an Ovenbird shot out almost from underfoot. By stopping dead in my tracks and looking carefully at the spot where I first saw the bird, I was able to find the nest—my first Ovenbird nest ever. It wasn’t as well covered as I had expected, so I got a good look at the four eggs.
I was elated until I returned the next day to find the site all messed up and the eggs missing. What had taken them? The list of potential villains is large. It could have been any number of snake, bird, or mammal species.
Some bird species are predators on eggs and nestlings of other species. Grackles, crows, and jays are notorious, but many other birds also cause problems. A lot of eggs and nestlings are lost during competition for nest cavities. House Wrens, House Sparrows, starlings, and woodpeckers will all destroy (and sometimes eat) eggs and nestlings, primarily because they want to raise their own brood in that cavity.
To reduce bird predation on eggs and young birds, it is probably important to take care in bird feeding so as not to attract cowbirds, grackles, jays, and crows. If these species predominate at your feeders, it may be best to take a hiatus from feeding.
Although birds eat eggs and young birds, it is almost certainly mammals that do the most damage. A surprisingly large number of mammals eat bird eggs—from the tiny deer mouse, to rats, chipmunks, squirrels, weasels, and minks, to skunks, opossums, raccoons, foxes, and bears.
One of the greatest threats to baby birds and their parents—especially in urban, suburban, and farmland areas—is the domestic cat. Many individual cats specialize in birds; others take them as part of a more varied diet. Cats will catch birds even when they are not hungry. Because of the loss to cats, the American Bird Conservancy recommends that cat owners keep their cats indoors—for the health of the cat as well as for the birds.
The North American Bluebird Society lists eleven species of snakes as possible predators at bluebird boxes, with black rat snakes leading the list. Alexander Skutch, the legendary Central American ornithologist, hated snakes with a passion because of the eggs and baby birds they ate. However, there is no evidence that snake populations are increasing, so there is little sense that the snake problem is out of control.
In many situations, providing good habitat is enough to ensure healthy bird populations. But it is always good to keep an eye on predators. Small to medium-sized mammalian predators are quite common these days for two reasons: large “top-down” predators such as mountain lions and wolves are no longer common enough to keep them in check, and the medium-sized predators take advantage of garbage and other foods that humans supply, especially at feeders.
If predators in your neighborhood are out of control, a trapping program may help restore the balance. Importing wolves or mountain lions is not yet an option, although many bird-lovers are seeing improvements as coyote populations expand back into parts of the country where they have been absent for decades. Like wolves and lions, coyotes prefer to eat medium-sized predators over birds.