Who killed cock robin? Clues are transparent.
CORNELL, N.Y.––The Southern Tier city of Corning, New York, noted for glassmaking since 1868, may hold one of the key clues to the loss of 2.9 billion birds from U.S. and Canadian habitat in just under 50 years.
Corning lies at one end of the Finger Lakes, about 40 miles southwest of the Cornell University Bird Laboratory, located in Ithaca, New York.
The Finger Lakes are a favored haunt of birdwatchers, among other distinctions.
But a birder standing with binoculars looking out over the water would be facing in the wrong direction to see perhaps the biggest reason why bird numbers are down––and would be looking at the wrong things if he or she turned to train the binoculars on either a falcon swooping on a smaller bird, an egg-stealing crow, or a feral cat skulking past the dumpsters at the Corning Museum of Glass.
But the transparent can hide the obvious
Looking up at high cumulus clouds symptomatic of global warming or noticing an odd absence of bees and mosquitoes might also be misleading.
The major clue would be the Corning Museum of Glass itself.
Founded in 1951 by the Corning Glass Works, the museum holds more than 50,000 glass objects, including 3,500-year-old depictions of birds.
The windows of the 1978 and 1996 additions to the museum, though, might tell the most about where the birds have gone, if a visitor paid careful attention to the details of presentations about how glass windows are made and used, and how much this has changed since circa 1970, a landmark year for the U.S. and Canadian “flat glass” manufacturing industries.
“Float glass” & why birds are sinking because of it
Since the “float glass” production method came to the U.S. on a commercial scale in 1970, led by Corning Glass Works and the Pittsburgh Plate Glass Company, glass has become cheaper and more ubiquitous relative to other building materials than ever before.
Glass window production has approximately doubled and doubled again every 10 to 15 years since. There are now from 20 to 30 times more glass windows per year installed in the U.S. and Canada than 50 years ago––and the windows of today tend to be bigger and stronger, though on average only half as thick and heavy.
No other factor pertaining to bird abundance has changed as markedly or as steadily for as long: not the also fast-paced acceleration of global warming, not the soaring use of neonicotinoid pesticides, not land use patterns, and not anything associated with predation or roadkills, the largely irrelevant grand bugaboos of birders for more than a century.
To kill a mockingbird: through the looking glass
To be sure, though, all of these factors are related. The warming North American climate, for instance, is among the many reasons why sheathing buildings in glass has become ever more attractive to builders. And how glass contributes to bird losses is integrally related to both why pesticide use is a major factor, while predation and roadkills are non-factors in most habitats.
The loss of birds, amounting to 29% of the estimated U.S. and Canadian wild bird populations as of 1970, was reported in the journal Science on September 19, 2019 by Cornell University Bird Laboratory scientist Kenneth Rosenberg and colleagues with multiple institutions.
Organizations participating in the Rosenberg study included the American Bird Conservancy, the Smithsonian Conservation Biology Institute, the U.S. Geological Survey, and the Canadian Wildlife Service.
Birds clocked by radar
The study by Rosenberg et al, explained Science writer Jillian Mock, “marks the first time experts have tried to estimate sheer numbers of avian losses in the Western Hemisphere. Typically, conservation studies focus on a specific species, habitat, region, or type of threat. By taking a higher-level view, the study highlights that many birds we still consider common are posting heavy population losses over time.”
The research team, wrote Mock, “analyzed the breeding population of 529 species by pooling data from the North American Breeding Bird Survey, National Audubon Society Christmas Bird Count, U.S. Fish & Wildlife Service waterfowl surveys, and 10 other data sets. They also analyzed more recent data collected by weather radar technology that can track large groups of birds as they migrate to estimate their numbers.
“The weather radars indicated a 14% decrease in nocturnal spring-migrating birds in the last decade alone,” Mock summarized, “helping the authors to verify the longer-term survey trends—especially for those breeding in remote northern habitats that aren’t as well monitored.”
14% loss of nocturnal spring migrators
The 14% loss of nocturnal spring-migrating birds over the last decade coincides with the most recent doubling of U.S. and Canadian window glass use, and with the discovery, reported by some of the same researchers in February 2014, that as Susan Milius summarized for Science News, “Between 365 and 988 million birds die from crashing into windows in the U.S. each year––as much as 10% of the estimated total bird population of the country.”
Scott Loss of Oklahoma State University at Stillwater, lead author of the study of window collisions, reported that the 15 million buildings in the U.S. standing from four to 11 stories in height accounted for about 56% of the bird deaths.
The 123 million buildings of one to three stories in height accounted for 44%.
The 21,000 buildings in the U.S. standing taller than 11 stories had negligible overall effect, Loss projected, simply because there are so few buildings that big. Skyscrapers do on average kill about 24 birds per year, Loss calculated.
Night collisions more harmful than collisions by day
Some birds kill themselves by flying into windows by day. Sometimes daylight bird collisions with windows because the birds are intoxicated from pesticides or from eating fermented berries; sometimes because the birds swoop on reflections, believing they are attacking other birds; sometimes because the birds have been blinded by contagious diseases, such as the “feeder disease” mycoplasma gallisepticum.
But daytime collisions are relatively unlikely to affect bird species at the population level, mainly because many and perhaps most of the birds involved are already impaired in some manner, with reduced chances of contributing to successful reproduction. The same is also true of birds who are killed by cars.
Collisions with lighted windows are another matter. Most migratory birds, from the smallest to the largest, do most of their long-distance flying at night. Migrating birds tend to be the strongest, healthiest, who have survived either the breeding season or the winter, before heading toward the opposite end of their range. These are the birds most likely to breed successfully in the future––if they avoid collisions.
As birds usually migrate in fast-flying dense flocks, a single lead bird becoming disoriented by lights can lead to the deaths of many.
Common small birds are hardest hit
“About 90 percent of the missing birds came from 12 distinct and widespread bird families,” Mock wrote of the Rosenberg study findings, “including warblers, sparrows, blackbirds, and finches. Common birds found in many different habitats—even introduced, ubiquitous species like European starlings—experienced some of the steepest drops.”
The European starling population fell 63%, the study found, including an annual toll of just under a million starlings killed deliberately by USDA Wildlife Services, the U.S. government extermination agency, which kills birds mainly on behalf of farmers, secondarily to protect airports from the risk of bird/plane collisions.
Feds kill 2.5 million birds per year
USDA Wildlife Services in 2018 also killed––as it does every year––half a million redwinged blackbirds, 400,000-plus cowbirds, 200,000 grackles, 66,000 pigeons, 34,000 doves, and enough birds of other species to raise the total number of birds killed in the name of pest control to about 2.5 million.
“Feeder birds like the dark-eyed junco declined by nearly 170 million individuals, the [Rosenbeg] study models estimated, while white-throated sparrows dropped by more than 90 million,” observed Mock.
Altogether, the sparrow population is down by 750 million since 1970, the Rosenberg team found, including losses of 300 million non-native house sparrows and 150 million savannah sparrows.
Birds who catch insects in flight, such as purple martins, swallows and swifts, are also in trouble, Canadian Wildlife Service senior biostatistician Adam Smith told Windsor Star reporter Sharon Hill.
Altogether, grassland habitat lost nearly 720 million breeding individuals across 31 species since 1970, a 53% decline.
“There are likely many causes,” offered New York Times writer Carl Zimmer, “the most important of which include habitat loss and wider use of pesticides. “Silent Spring, Rachel Carson’s prophetic book in 1962 about the harms caused by pesticides, takes its title from the unnatural quiet settling on a world that has lost its birds,” Zimmer recalled.
Wrote Carson, “On the mornings that had once throbbed with the dawn chorus of robins, catbirds, doves, jays, wrens, and scores of other bird voices, there was now no sound.”
Silent Spring helped to bring about a 1972 ban on most uses of the insecticide DDT, and led to many other pesticide bans or restrictions.
Introduced in 1947, DDT had already supplanted more than 50 years of intensive use of an even deadlier insecticide, lead arsenate.
15 million more bald eagles
Over the next several decades, DDT notoriously accumulated in the food chains of predatory birds, causing the top predator birds––mostly hawks, owls, and eagles––to lay more fragile eggs.
As DDT use declined, formerly endangered raptor species including bald and golden eagles, peregrine falcons, and many others achieved relatively rapid recoveries. The U.S. and Canada now have about 15 million more bald eagles than in 1970.
DDT was supplanted by insecticides of the organophosphate and carbamate chemical families, also dangerous to birds and other wildlife, but less so.
Use of organophosphate and carbamate pesticides peaked circa 1981, as did pesticide use generally.
But organophosphate and carbamate use declined chiefly because of the emergence of neonicotinoid insecticides, which are chemically similar to nicotine. Among the neonicotinoids are Imidacloprid, now the most widely used of all insecticides.
Neonicotinoids are much less toxic to birds and mammals than the organophosphates and carbamates that the neonicotinoids replaced. But neonicotinoids may be too pervasively and enduringly lethal to insects, contributing to honey bee colony collapse disorder and bird losses due to loss of the insects that the birds rely on for food.
Former U.S. President Barack Obama restricted the use of neonicotinoids on land leased by farmers within National Wildlife Refuges, but current U.S. President Donald Trump rescinded the restriction in August 2018.
Why predation is mostly a non-factor
Many of the bird species showing the largest losses are also among those most often hunted by cats––and by many other bird predators, including bigger birds, who also steal eggs and take over nests. But––except on isolated islands, with a limited array of predator species––predation is rarely if ever a significant cause of lasting losses of prey species.
This is for two reasons.
First, almost all animal predation is “compensatory,” meaning that the predators kill and eat mainly the sick and injured, the aged and infirm, and unattended young, none of whom are likely to contribute any more to the reproductive success of their species than they already have.
Most animals killed by predators would not survive for much longer anyhow. Especially if the victim animal is suffering from a contagious disease, predation may help the species more than would the temporary survival of an individual who might infect many others.
Only “additive” predation actually reduces the long term abundance of the prey species. “Additive” predation by predatory animals, including parasites, cuts into the successful breeding population of the prey species, but rarely occurs for long, because if the prey population declines, the predators starve out long before the prey species disappears entirely.
By contrast, there is no natural brake on how many birds––or animals of any sort––can be removed from a breeding population by collisions with windows or pesticide intoxication.
“Subsidized” predators, such as cats who are fed by humans, may commit “additive predation” against birds by hunting them for sport, rather than sustenance.
Outdoor cat population is relatively steady
But the U.S. outdoor cat population, including free-roaming pets, has remained within a relatively narrow range more than a century, with no cat population increase that could account for the bird population decrease. On the contrary, the best available data suggests the outdoor cat population has steadily declined since peaking circa 1990.
Back when practically all cats roamed at large, Frank M. Chapman of the American Museum of Natural History in 1908 estimated that the U.S. had about 25 million cats.
National Family Opinion Survey founders Howard and Clara Trumbull conducted three exhaustive studies, published under the pseudonym “John Marbanks,” to put the U.S. population of cats at large at circa 10 million in 1927, just past the peak of a USDA-led national cat purge; 20 million in 1937; and 30 million in 1950.
Feral cats mostly hunt rodents
The U.S. pet cat population since then has gradually increased to as many as 90 million, but a variety of ownership surveys have established since circa 1990 that cat owners who keep their cats indoors have cumulatively more than twice as many cats than those who let their cats roam.
Cat colony surveys, meanwhile, suggest that the U.S. feral cat population has dropped to fewer than nine million.
In short, the number of cats who are at large to hunt birds is still about what it was in 1908 and 1950––and feral cats who genuinely hunt for a living are overwhelmingly nocturnal, preying on relatively easily caught small rodents, rather than birds, who are overwhelmingly diurnal and hard enough to catch that most genuinely feral cats do not expend the energy to try.
(See $1.5 million DC Cat Count: useful, make-work, or compiling a hit list?, TNR achieves 72% drop in kitten birth rate, finds Alley Cat Rescue, and Are Southern California coyotes eating 68% fewer cats than 20 years ago?)
Many of the bird population decline findings by Rosenberg et al point clearly toward effects of global warming.
“Birds who breed in at-risk habitats such as grasslands and the Arctic tundra are declining drastically,” Mock pointed out.
“Climate change looms large over the tundra and is the primary threat to this nesting habitat for many birds,” Mock elaborated. “Warming temperatures melt permafrost and threaten to put migrating birds out of sync with the food they depend on during the brief northern summer.”
But the net decline of about 80 million birds in tundra habitat since 1970 is partially contradicted by a net gain of about 34 million individuals among ducks, geese, and swans, many of whom summer in the Arctic Circle.
Forest birds are about half the losses
“Clearing for oil and gas development, logging, widespread fires, and climate change all threaten boreal forest habitat,” Mock continued. “Some 500 million birds have been lost in this habitat since 1970—a more than 30% decline.”
Among “forest generalist” species, “about 482 million individuals have been lost since 1970, a nearly 20 percent loss, according to the study,” Mock said, while the bird population has also lost about 417 million “habitat generalists.”
Neotropical migratory songbirds, who mostly breed in the hardwood forest understory of the eastern U.S. and Canada, wintering in Central and South America, are down by about 167 million, consistent with the overall rate of bird loss.
Previous studies have indicated that neotropical migratory songbirds are particularly affected by rainforest logging at the southern end of their range, and record deer numbers at the northern end. Browsing down the hardwood forest understory, hungry deer tend to leave songbird nesting habitat scarcer and more exposed to predators.
Losses are less in the west––& wetlands birds are up!
The net loss of western forest birds is comparable to the loss of neotropical migratory songbirds, at about 140 million, approximately 30% of the 1970 population.
Wildfire destruction of western forest bird breeding habitat has accelerated over the past several decades. While western forests typically recover over several decades, species dependent on particular areas or tree species suffer meanwhile.
Despite the explosive growth of cities such as Phoenix, Tucson, Santa Fe, Albuquerque, and their suburbs, bird losses in the arid U.S. Southwest have been somewhat less: about 35 million, or 15% since 1970.
Bird losses from coastal habitat have also been relatively small, at about 6% overall, with some species, notably oystercatchers, showing steady gains.
Finally, increased legal protection of wetland habitat since 1970 has produced a 10% net increase of wetland bird species other than waterfowl––about 20 million individuals altogether.