Inside the Outdoors: High mortality is an inconvenient truth of nature
With some exceptions, the probability of a newborn wild creature being an early casualty—from predation, disease, starvation or another mortality factor—is high.
Humans, the dominant creatures on our planet, have come to expect virtually all of our offspring to survive. Modern medicine, and the post-natal care and attention we give them, have made it so. There are unfortunate exceptions—found chiefly in impoverished, under-resourced parts of the world—but these basically prove the rule that most newborns will survive their first year of life and well beyond.
But that’s not the case with most wild creatures. With some exceptions, the probability of a newborn being an early casualty—from predation, disease, starvation or another mortality factor—is high. Birds are among the most visible wild creatures whose lives intersect with ours, and they recently provided me with starkly contrasting examples of the uncertainties of offspring survival. Evidence, too, that the good fortune of survival is not evenly distributed.
I was returning home from an errand, slowing down as I often do when rounding a broad curve that follows the edge of “Twin Ponds,” a wetland that—from spring through fall—is home to or visited by many kinds of ducks, Canada geese, tundra swans, herons, sandhill cranes, songbirds and more. On the seat beside me was a camera with a telephoto lens. I’ve been known to decelerate to a slow crawl along the road’s shoulder—my vehicle’s hazard flashers on—scanning for anything photogenic within telephoto lens range.
Some creatures are extremely visible and stand out; a snow white tundra swan, for instance. Others are masters at being inconspicuous. But even so, there are tipoffs that can be picked up by an experienced eye. Like the unmistakable, statue-still silhouette of a great blue heron standing in the shallows, where it watches for any movement that spells opportunity for a lightning-quick stab with its dagger-like bill at a frog, a fish or maybe even an immature muskrat.
Herons seem to be among the creatures least tolerant of human intruders, and this one took to the air as I brought my vehicle to a stop and attempted to aim the camera in its direction: too late. But with the ruckus of its departure and my sudden arrival, three tiny wakes appeared on the surface of the pond. I had also startled a hen hooded merganser and a pair of very small ducklings.
As I watched, mama and one of her young dove beneath the surface. This is the way that mergansers—the hooded species being one of three common to North America—feed, mostly on small fish, aquatic insects, mollusks and the like. The two bobbed back up and began paddling away, heads turned to keep an eye on me, while the second youngster hustled in their direction to catch up.
I was struck by the fact that there were only two ducklings in the hen’s company. I knew that a typical clutch of eggs for most ducks we see here in Minnesota is much larger. My bird reference books (I’m old-fashioned enough to still use them) say the typical clutch of eggs for a hooded merganser is 8-12. Most likely they were laid in a down-lined cavity in a tree somewhere in the vicinity of the pond, or perhaps the hen occupied a nesting box that was intended for a wood duck.
What surprised me was the scarcity of young, as I’ve more commonly seen hooded merganser hens with a half dozen young in tow, give or take. Some of her young could have been lost in the egg stage to a nest-raiding weasel, mink, crow or even a snake, and young ducklings on the water fall victim to snapping turtles, herons and bass or northern pike if they’re present.
At the edge of this particular pond are several tall trees where I’ve often seen a bald eagle perched. Eagles are used to finding meals near water, where they commonly scavenge fish. I’ve also seen them take a harvested, motionless duck that was floating at the edge of a hunter’s decoys, and on another occasion exhaust a wounded duck by forcing it to dive, and eventually capturing it. Eagles are even believed to be a factor limiting the reproductive success of some ducks in the East, like the seagoing eider. Whatever the causes, this hooded merganser had little to show for her efforts.
That same evening, descending the steps to the dock at the family cabin, a hen mallard surprised me with her squawks of alarm, as she and a gaggle of young erupted from where they were feeling in the shallows. I carefully counted 11 youngsters, several of them taking to the air and proving that they were fledged, while the rest created little wakes as they practically walked on water to put distance between them and me.
Mallards commonly lay clutches of 8-10 eggs, so this hen was clearly an over-achiever, or had somehow acquired one or more ducklings in addition to her own. More to the point, this demonstrated that the math that makes up the law of survival averages includes exceptions and outliers, just like this one. There’s no guarantee that most of these young mallards will live to full adulthood, much less make the journeys south in fall and back north in spring, mate and successfully produce offspring. But they have a good start.
More sobering are the survival numbers for some other wild creatures. It’s believed that up to 90% of snapping turtle nests are found and destroyed by raccoons, mink, foxes or skunks. Fish, and even other snapping turtles, will eat many of those that do hatch and find their way to water. Only a quarter of the young robins that fledge will survive to November, when the time comes to migrate. And only about half the robins alive in one year will survive to the next.
According to data published by Pheasants Forever, only about 50% of pheasant nests produce any chicks, the rest raided and destroyed by raccoons, skunks, foxes and feral cats; some are destroyed by agricultural practices, too. Of those nests of 12 or so eggs that do produce young, chick survival is about 50%, the losses commonly owing to predation by foxes, hawks, owls, as well as human activity, such as mowing of grassland cover before the young can mature.
And so it is with most wild creatures, their abundance dictated by an unsparing law of survival of the fittest, intertwined with the law of averages, and just enough chance and randomness thrown in to keep us guessing.