By BILL CHAISSON
By Bill Chaisson
It is tough to be a bird. We all know what it’s like to be a little disoriented when we are in an unfamiliar place. This feeling does not work to the advantage of a migrating animal. Millions of adult birds die during migration due to exhaustion, predation, and collision with man-made objects.
In April 2019, the Chicago Tribune ran a story with the headline “As many as a billion birds are killed crashing into buildings each year — and Chicago’s skyline is the most dangerous area in the country.” (As a former newspaper editor, I have to say this is a very wordy, overlong headline.)
A billion is a lot of casualties. To drive the point home, the story featured a photo of a brown creeper — definitely not a downtown Chicago denizen — recovering on the sidewalk after a collision on Wacker Drive. The story was about spring migration, but the numbers unfortunately apply to fall migration as well.
Chicago Bird Collision Monitors estimates that 5 million birds representing 250 different species pass through the city en route north or south. The migrants are confused by reflections and by decorative trees inside the buildings and fly straight into the glass walls. The International style in architecture has not been a good development for birds.
Annette Prince of Collision Monitors expressed her admiration for these animals who weigh a few ounces and have made it this far under their own steam: “So, it’s unfortunate that the birds we find here in the spring have made it all the way from South America, almost to their nesting grounds in Wisconsin, and they hit a window.”
But as it was with human populations in the developed world until the 20th century and is still the case in much of the rest of the world, most deaths occur while birds are very young, sometimes while they are still in the egg. Among birds themselves, the corvids — crows, magpies, and jays — are the most notorious egg stealers. Certainly, a reason that birds build their nests in trees and shrubs is to cut down on predation, but in addition to other birds, snakes, rats, squirrels, raccoons, and various forms of weasel are all adept climbers with a taste for eggs. Ground-nesting birds — shorebirds, fowl, most waterfowl, a few songbirds — have a whole host of additional predators, including foxes, skunks, bears, and even your dog.
Nestlings have almost all the same predators. I have a vivid memory of walking to school one morning and seeing a crow dragging a struggling and vocal robin nestling across a roof. Upon reaching the peak, the crow paused and dispatched it with a few sharp pecks and then flew off with its inert prey in its claws. I didn’t look at crows (or robins) the same way after that.
Robert MacArthur, who was instrumental in bringing quantitative concepts to ecology, introduced the idea of r- versus K-selection. These are now thought to represent opposite ends of a continuous spectrum. R-selective organisms produce an enormous number of progeny, do so often, provide no protection for them, have high attrition rates, short lives, and disperse widely. These species are considered opportunistic and are adapted to exploiting unstable environments where niche spaces are not well differentiated.
K-selected species are essentially the opposite: they produce fewer young, less often, provide some care, have lower attrition, and are born into environments that are closer to carrying capacity and where niches are well defined.
Upon fledging, birds spend the summer and fall learning to forage and avoid predation. For most birds, there is a period during which they are fed and perhaps instructed by their parents. In the broad view, birds are generally K-strategists, but among birds there is a spectrum. Robins, for example, may have three or four broods of four to five eggs in a breeding season. In contrast, Baltimore orioles will generally have one. Most bird species are altricial; the nestlings are entirely helpless at birth. But many ground-nesting birds, like shorebirds (locally, killdeer and spotted sandpipers), have precocial young that are fully feathered at birth and, after emerging from the egg, can walk and forage as soon as they dry off.
Generally, the less abundant, less widespread birds have narrower niches and are closer to their carrying capacity, so they will be more K-selected. In theory, they should suffer less fledgling predation because they are investing more in each individual offspring. But, as they are adapted to more stable environments, natural disasters like violent storms, floods, and fires hit them harder.
Richard Adams mythologized the plight of the prey animal in “Watership Down.” Frith, the god of rabbits, told the first of their kind: “El-ahrairah, your people cannot rule the world, for I will not have it so. All the world will be your enemy, Prince with a Thousand Enemies, and whenever they catch you, they will kill you. But first, they must catch you …” Except for the largest among them, most birds are in this position. Even as adults, there are so many ways for them to die.
Although birds have many enemies, the single worst threat is the outdoor domestic or feral cat. While a billion birds die during migration through collisions, every year an estimated 2.4 billion birds are killed by cats in the United States. This is due to the combined impact of tens of millions of outdoor cats, with most mortality due to farm/barn cats, strays that are fed by humans but not granted access to habitations, cats in subsidized colonies and cats that are completely feral. In 2018, scientists estimated there were 60 to 160 million feral cats in the U.S. There is no native predator in temperate North America equivalent to these descendants of a wild Asian species. Without adaptations to cats’ hunting strategy, our wild birds are … sitting ducks.
Bill Chaisson, who has been a birdwatcher since age 11, is a former editor of the Eagle Times. He now works for the Town of Wilmot and lives in Sutton.
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