SCOTT SHALAWAY Predators: the other (unwanted) nest guests

The first of what will surely become many pleas for help arrived this week. A bluebird nest had been plundered -- the eggs were gone, and the nest itself torn apart. A raccoon was probably responsible.
It always seems cruel and unfair when helpless and fragile eggs and chicks are eaten by predators that include snakes, squirrels, chipmunks, weasels, cats and blue jays. But believe it or not, what appears to be a staggering rate of loss rarely influences the overall number of birds.
In nature, more often than not, life is brief and death comes violently. Fewer than half of most bird nests actually produce any chicks that leave the nest. Among open-nesting birds that build their nests on the ground or in trees and shrubs, as few as 20 percent of the nests fledge any young. (Biologists define a successful nest as any nest that fledges at least one bird.) Backyard favorites such as robins, cardinals, and mourning doves fall into this group.
Cavity-nesting birds, on the other hand, are usually more successful. Bluebirds, wrens, and chickadees succeed in fledging young 60 percent to 80 percent of the time.
Dangers they face
But even cavity-nesters suffer predation, particularly by black rat snakes. Many predators are excluded from cavities or nest boxes by hole size, but snakes slide right in. After eating the contents of the nest, the snake may be too large to leave the cavity. So it remains there for days until the meal digests, and it returns to its normal size. At least once each year while checking nest boxes, I reach in and grab a handful of well fed snake.
Despite the toll that snakes take on cavity-nesters, cavities are still a much safer place to nest than open nests in trees and shrubs. So why don't more birds, or all birds for that matter, nest in cavities?
It's a matter of supply and demand. Cavities are in short supply, so relatively few species use them. In North America north of Mexico only 85 species of breeding birds nest in cavities. Fewer than 40 local species nest in cavities. Competition for old woodpecker holes among bluebirds, wrens, chickadees and titmice, to name just a few examples, is fierce. Add house sparrows, starlings, mice, squirrels, snakes, bees, wasps and spiders to the list, and you begin to appreciate the price birds pay for the security of a cavity.
Adaptations
Birds cope with heavy nest predation in one of two ways. Open-nesters and some cavity-nesters (bluebirds and wrens, for example) offset frequent nest failure by nesting two or even three times each year. This increases the chances that at least one nest will succeed.
Other cavity-nesters such as woodpeckers, chickadees, titmice and nuthatches raise only one brood each year. But they improve their chances for success by nesting early in the breeding season. This puts eggs and chicks in the nest before snake activity peaks and before other predators have babies of their own to feed. And most renest if their first effort is destroyed by predators.
What does all this mean for long-term population trends? To keep a population at a stable level, each individual need only replace itself in the course of its lifetime. That means each pair of birds need only produce two offspring. And since most birds that reach breeding age live at least a few years, chances are good they will eventually replace themselves, and the population will remain stable.
Despite nest losses that may appear catastrophic to us, many populations remain relatively constant. We shouldn't begrudge predators their fair share. Their habits are not wrong or bad; it's just what they do.
On the other hand, if you maintain a few nest boxes for cavity-nesters, don't let them become predator feeders. Protect each with a predator baffle (an eight-inch metal or PVC pipe) beneath the box to discourage climbing snakes and raccoons. For a copy of plans for a simple baffle, send me $1 and a self-addressed, stamped envelope.
sshalaway@aol.com