How cowbirds take advantage of other birds
The Brown-headed Cowbird (Molothrus ater or BHCO, 3 subspecies) They perch atop shrubs or trees to watch for nest building activities, or try to Dr. Lyle Friesen, a songbird specialist with the Canadian Wildlife Service. Many migrant songbird species specific to riparian communities during the breeding season are experiencing population declines. Conversely, the Brown -headed Cowbird (AAolothrus ater) benefits from fragmentation of, and directly and indirectly by human activities. . tained a symbiotic relationship with large grazing. Cowbirds on riparian-nesting migratory songbirds. Sara H. directly and indirectly by human activities. Conversely, the Brown-headed Cowbird ( AAolothrus ater) benefits from . tained a symbiotic relationship with large grazing animals of.
Veronica pulls over beside something we need—large boxes of snack bars—and begins inspecting our options: What was wholesale is now household. As we move steadily along, our eyes pass over the canyon walls, scanning for the familiar labels of items we might need.
Certain products are sold in such quantity as to seem like a visual joke—a gag on the expected proportions— and just for fun I pull one such prop off the shelf. In the middle of our next passageway sits a waist-high cube composed of orange blocks, each of which is, in turn, composed of 72 individually wrapped packages of cheddar-cracker-and-peanut-butter sandwiches. She takes the orange block from her cart and presses it back on me.
And so, starting with one edge of the warehouse, I march, my gaze fastened straight ahead. To my left, the glass cases of frozen meats pass as a beige-pink blur, and on my right, the openings to aisles are flashes in the corner of my eye—bright malls I manage to refuse.
But as I walk by the bakery, with its distinctive scent of glazed donuts, a sudden grey flicker pierces the sidewall of my private tunnel. It is a house finch: On the smooth cement floor, he hops, pecks once, pauses to tilt his head and observe the ground, and pecks again. The movements are quick and exact, at once mechanical and vibrantly alive. He flits up to a table, where he perches on the edge of a plastic bin and fixes me with a tiny eye—a pinhead of glossy obsidian.
He stays right where he is, and keeps his eye trained on me. He hops a quick, irritatedand fixes me with his other eye: Why the sudden interest? We convert almost every landscape—redwood forest or salt flat; peat swamp or sandy desert—into a patchwork of the weeds and grasses that provide house finches with their favorite seeds.
And when it comes to nesting, the little birds seem to adore stubby ornamental trees, the overhangs of parking structures, and the ivy that grows on building walls. Until the late s, house finches were hemmed into the southwestern third of the United States, mainly by the vast swath of grasslands that covered the center of the country. While these regions offered plenty of seed, they were severely short on nesting sites. A parsimonious guess at ecological history might suggest that the finches hopscotched gradually eastward, alighting on islands of human settlement until they reached the promised land of Midwestern suburbs.
But that is not, in fact, how it happened. Instead, when the invasion finally transpired, it came from the rear: But why Long Island? How did a small but momentous flock find itself next door to New York City? The seeds of biological invasions, like the sparks that start wildfires, are often obscured by the very process they initiate: But birds are uniquely beloved animals, monitored by a diffuse but nationwide federation of obsessively vigilant observers.
Like no other organism, birds get noticed. In this particular case, a red-headed finch was noticed in April,in Nassau County, New York, and the sighting was duly reported in the weekly ornithology column of the Nassau Daily Review-Star. The enthusiasts filing the report were well aware that their sighting represented an exception to the known range of the species, but they confidently identified the bird as Carpodacus mexicanus.
Elliott, followed up on the intriguing report, and over the next five years, he discovered several small colonies of house finches nesting in Long Island tree nurseries. He published his findings in the May issue of the Linnaean News-Letter. As to the origin of the birds, he was dumbfounded, though he did note that the Long Island birds appeared to be especially dusky, much like the subspecies of Carpodacus mexicanus found in the vicinity of Denver, Colorado.
In January ofhe wrote, he had entered a Brooklyn pet store, where his attention was drawn to a cage labeled Hollywood Finches. To his dismay, the 20 birds in the cage were in fact specimens of Carpodacus mexicanus.
Cowbird eggs and young in nestboxes
Fleischer had previously seen Bohemian Waxwings for sale in this same store, and decided to put an end to this traffic in protected American passerines. The Survey also dispatched Orin D. Steele, Game Management Agent, to pay Fleischer a personal visit and discuss the matter more fully.
The birdcages, they discovered, were empty of Carpodacus mexicanus. It seems quite likely, then, that the worthy efforts of Dr.
Edward Fleisher, of Brooklyn, not only put a deep dent in the transcontinental shipment of protected American passerines, but also prompted a number of worried pet-store owners to release their flocks of Carpodacus mexicanus in the vicinity of Long Island, N.
As for that little inconsistency in the story—if the birds had come from California, why were they dusky, like the subspecies in Denver? Life in New York had sooted the house finches. Eventually, Veronica found me by the muffin bin, watching a small bird forage for crumbs. But later, staring blankly at propagating waves of brakelights—5 p. Why treat Costco as a swirl of unwanted distractions, whereas a little grey bird instantly receives rapt attention?
Or to phrase it, frankly, as more of an indictment: Was there, I wondered, a real difference? Some critical distinction that made a warehouse of stuff less rewarding to investigate than a forest of birds? What occurred to me first was that the distinction might lie in ecology, for if Costco is viewed as a kind of foodweb, it exhibits drastic simplification, with all those thousands of products going straight to a single top consumer.
TPWD: Brown-headed Cowbirds
And yet, if one thinks just a bit less literally, ecological links do begin to appear, tracing their way throughout the warehouse. Certainly there is something like ecological competition Powerbar versus Lunaas well as symbiosis DVDs and red licorice.
And finally, to suggest that Costco lacks real ecology is to ignore a point of history: If Darwin saw ecosystems as economies, it ought to be possible to invert the metaphor. Ecology, then, was not going to offer the meaningful difference I was after. But every other distinction I tried—evolved versus designed, natural versus manufactured—failed just as soundly to vindicate my disposition toward the warehouse.
At that point, I began to feel a bit like a detective whose investigation is steadily gathering clues that point unmistakably to himself: But what was my problem? If I knew a bit more—some engineering, maybe, or economics—then surely I could approach Costco with real focus and interest. I remembered, for instance, what Primo Levi, the Italian chemist and writer, could find inside an ordinary can of housepaint: Just imagine, then, what he might have done with our bucket of synthetic licorice twists.
For five miles of traffic, I felt quite satisfied with my mea culpa: Costco could be truly engaging, if only I would follow the example of writers like Levi, who could read the manufactured world in such fascinating ways. But before I could make good my new resolution, it soured. I remember exactly where it happened, because the street signs had something to do with it.
We had just exited the highway and were now entering a vast residential development, where some friends had generously offered their home as base camp for gearing day. I was beginning to feel uneasy, wishing Veronica would get off the phone and remind me where to turn.
That very morning, in fact, returning from a short errand, I had unwittingly parked in the driveway of a neighbor, and blithely walked into his house.
Veronica was immersed in her conversation, so I was craning forward over the wheel, searching helplessly for any geographic cue. Irrationally, I sped up, then swung the truck around the next corner, looking up at the green street signs. Rock Ridge, said one. Tamarack, said the other. And at that moment, I disavowed my recent resolution. Each of us knows, from firsthand experience, that Costco is a small part of an increasingly widespread environment.
A plexus of shopping malls, parking lots, and residential sprawl now extends for hundreds of miles around every American city.
In many places, it actually is the city. Its consistency is both extensive and exact: When I pulled into the wrong driveway, I could have been in Albuquerque or Schenectady.
What was my problem? The metaphor of cancerous tissue is loaded, but it is also precise in certain ways: In cancer, cells of a single variety replicate rapidly; they form a strangely homogenous tissue, because differentiation—the development of the distinctive properties that characterize mature, functional cells—is a relatively slow process, and cancer cells divide before they manage to differentiate.
In fact, the less they differentiate, the more rapidly they tend to replicate, and soon they begin to invade and outcompete the various specific types of well-differentiated tissue. In ecosystems and economies—just as in the body—differentiation takes time: Whether it is the biological evolution that makes the sage scrub of southern California different from that of western Australia, or the cultural development that makes San Diego taco shops different from Maine chowder shacks, the accumulation of local differences happens at a stately pace.
But when that stereotyped plexus of malls and sprawl moves in, all the singular places give way to one single place. At the corner of Rock Ridge and Tamarack, I saw a signpost of the problem. Developers often name their new neighborhoods and streets after what was there before the development.
Near my hometown of Boulder, Colorado, for instance, there is a mile of replicated townhouses called Rock Creek, and I can testify, from childhood experience, that once upon a time there was indeed a lovely creek in exactly that location. I suspect that, similarly, on a certain hillside north of San Diego, there was once an actual and particular rock ridge.
Tamarack, on the other hand, does not memorialize a tree that once stood on that same hillside.
Of this we can be sure, because the tamarack is a cold-weather tree, which grows around the Great Lakes and northward into Canada. It is possible, however, that the street sign in San Diego does in fact hearken back to a particular tree somewhere—in Minnesota, say—because developers have, on occasion, constructed virtually identical subdivisions in multiple locations. Perhaps, then, somewhere in the native range of the tamarack, there is a corner, marked by a pair of street signs, identical to the one in San Diego, only there, it is the word Tamarack that marks the site of a towering tree now gone, while Rock Ridge refers to a place thousands of miles away.
Whether or not such a duplicate really exists, the juxtaposition of those disparate landmarks struck me as a concise testament to the erosion of geography, the end of distinctions between one place and another in the new American landscape. So maybe Costco is just as fascinating, or wondrous, or even, in its own way, just as beautiful as any other setting in which we might immerse ourselves.
At least, we could find a perspective that would make it so. But what I decided, as I turned the corner at Tamarack and Rock Ridge and found myself lost in a familiar neighborhood, was that I would not try to find that perspective. In fact, I would resist it, because we are now in the process of trading our entire library of unique volumes for so many copies of the selfsame text.
And the exchange seems not just foolish, but tragic. Twelve years after his column in the Daily Review-Star announced house finches in Nassau County, Elliott published this diagnosis: Although this small sample may shortly come to feel like a litany of modern blights and scourges, my intention is not to flagellate us until we achieve Job-like understanding of the bad things nature has to offer.
The female brown-headed cowbird is a bit smaller than a robin, rather drab, and quiet. Whenever I see her hopping around a lawn with a flock of other birds, the image called to mind is that of a shadowy spy, her brown overcoat wrapped tight, her gaze askance and ever-watchful.
And yet, strangely enough, the ruse usually works: A few bird species are canny enough to foil the con. Robins, for instance, will chuck out the cowbird egg nine times out of The perspicacity to spot an anomalous egg, therefore, appears to be a mental trait that has evolved in response to the danger of cowbirds. And this helps explain why cowbirds are such effective invaders: Consequently, quite a few little birds that nest in the U. There is something particularly grotesque about the way cowbirds invade—the deceit of it; the perversity of tiny yellow warblers struggling to feed their gargantuan brown chick—but other, less graphic stories of invasion will also prove instructive.
Across the desert Southwest, the Asian salt cedar, a scraggly tree with glaucous needles, was originally planted to provide a bit of shade. But salt cedars, it turns out, have an irrepressible tendency to take over: The invasive trees have now conquered every waterway in the desert Southwest, and what were once diverse forests of cottonwood and willow are now dense thickets of salt cedar. Another invader, a little weed now spreading across the American west, deploys a scorched-earth strategy of its own.
Eurasian cheatgrass looks innocuous—a thin stalk bends elegantly under the weight of a loose cluster of slender, tufted seeds—but come summertime, the pretty little plume becomes highly combustible, inviting wildfire. And when the blaze rolls through, it destroys most plant life, but the cheatgrass seeds are fire-resistant: In the century since cheatgrass arrived in North America, it has blanketed million acres. Just a couple more examples before we reckon with the problem in more general terms.
Ina colonial Ugandan administrator surreptitiously slipped a bucket of Nile perch into the waters of Lake Victoria, the largest tropical lake in the world. At first, it appeared the scientists had been wrong: The exotic predator established a local population, but nothing catastrophic came to pass. Finally, our litany should not omit the invaders that are less exotic to us but no less successful in expanding their range: Rats and cats eat the island-nesting birds; rabbits and mice eat indigenous plants that would otherwise feed native herbivores; and goats and swine denude whole islands.
This process began in the 14th century, when Europeans first reached the Azores, the Madeiras, and the Canaries, and it has continued ever since. All in all, the proliferation of European pests and domestics on previously isolated oceanic isles has annihilated dozens of species of reptiles and mammals, and at least species of birds.
Now, with our mixed bag of invaders in hand, we can ask the more general question such variety seems to beg: If all these species are invasive, what really is an invasive species?
What unites these powerful few and equips them to drive native inhabitants over the cliff of extinction?
To answer this question would be to solve the problem of invasive species—not just intellectually, but practically, as well, because if we knew in advance which species might turn malignant, we could nip their incipient invasions in the bud.
In certain respects, however, this general question—what makes a species invasive?
Songbirds in the Suburbs
In different groups of organisms, different attributes are associated with a penchant for invasion. Among insects, for example, invasive species tend to be small, while among vertebrates, they tend to be big. Among pine trees, invasives tend to have small seeds, while in another group of plants, the invaders have big seeds.
Such trends, confined to certain groups of organisms, are not without meaning, and in some cases they make a lot of sense: The fundamental difficulty is that invasions depend on the details of natural history. Cowbird eggs hatch sooner and the young grow faster, so the cowbird chicks get most of the food and have been known to push the host's chicks completely out of the nest.
The majority of the time the adult host birds end up feeding and caring for only the cowbird chick, rather than raising the next generation of their own kind.
While there is no single reason for this decline, one major contributing factor is the spread of the cowbirds due to landscape changes throughout North America.
Cowbirds once occurred only in short-grass prairies, where they followed buffalo herds, feeding on insects stirred up by the grazing animals. Human-caused changes to the environment, including the introductions of domesticated grazing animals and cultivated fields and lawns, has greatly benefited the cowbird, helping it to spread to new territories and prey on more songbird species.
Today the cowbird parasitizes more than species of North American birds.
The cowbird may pose a particular danger to already-threatened species like the Black-capped Vireo. Studies have shown that the removal of one female cowbird enhances the survival of 35 songbirds per year. In an effort to manage the brown-headed cowbird populations, Texas Parks and Wildlife has implemented a Cowbird Trapping Program. Anyone interested in participating is welcome to go through our training and certification process.
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