A 2008 report from Canada shows an alarming decrease in insect-eating species of birds.
The decline is most notable in Canada, but also affects the entire Northern Hemisphere – particularly the Northeast – with some species suffering a 70 percent decline in the last 40 years.
The report’s author, Jon McCracken, cites a number of possible causes, including loss of habitat and even natural variability. The most likely cause, according to McCracken, is the change in density of the insects the birds feed on. If this is the case, the decline has significant ecological and socio-economic implications, since not only do these bird species control crop-devouring insects, they also pollinate crops. Other causes likely include pesticides and global warming.
The decline, video-broadcast by CBC’s The National, on December 28, 2007, indicates the urgency. The Northeast is, however, not the only area experiencing the decline. The Western Black-Throated Sparrow, native to the Western U.S., has also declined from 55 million to 20 million in the last 40 years. This sparrow also feeds on insects. The Swainson’s hawk, which summers in Boulder, Colorado and winters in Argentina, is also disappearing, likely from the pesticides sprayed on Argentinian sunflower crops. Bicknell’s Thrush is on Audubon’s Watch List, as are the Golden-Winged, Golden-Cheeked and Kirtland’s Warbler. In all, the WatchList reports 187 species of birds at risk of extinction.
As early as 1996, The Bay Journal began reporting on songbird decline, showing that warbler populations alone were decreasing by 7 percent a year. In 1994, the New York Times, citing a study from the journal Ecology, noted that insect eating birds protect America’s forests by eating the bugs that feed on tree bark and leaves. A Missouri study, that netted some white oak trees to exclude birds, showed that protected trees lost twice as many leaves to insect as unprotected trees.
This result, which ecologists call the trophic cascade, shows that birds are top-level predators and can significantly impact the health of forests. Forests, as we all know, protect the environment by absorbing carbon dioxide and providing oxygen. Forest fragmentation, or urban sprawl, is likely one cause of bird decline. Landscape ecologists say only 3-5% of all US land is undisturbed.
There is now growing concern that pollinating insects such as wasps, butterflies and moths are also declining, again due to both global warming and pesticides. Parasitic wasps, which kill caterpillars attacking crops, trees and foliage, have been declining for years as a result of wholesale pesticide use by farmers, agriculturalists and homeowners.
The alpine butterfly, native to mountainous regions in the U.S. and Canada, is being impacted by the rise in timberline (the altitude beyond which trees are not commonly found). Timberline advance is solely due to global warming. Farming and land settlement, including such simple tasks as mowing tall grass, further reduce butterfly populations with surprising rapidity, since many common butterflies lay their eggs in native grasslands. In 2005, Monarch populations were reported down 75 percent, with the U.S. blaming Mexico’s logging industry, and vice versa. A year later, Wildlife Extra warned that Monarch populations would likely be reduced even farther by drought conditions in Texas and S. Oklahoma as a result of warmer-than-usual weather. Ninety-six percent of North American birds rely on insects to feed their young.
The decline in insect-eating birds also leads to the importation of “alien” insects, or bugs that didn’t evolve in the native food chain alongside the birds that feed on them. For example, in 1970, when aphids began decimating pecan orchards in the southeastern US, imported the Asian Lady Beetle to eradicate them. This beetle has since spread across the Northern Hemisphere, randomly devouring so many aphids (and native ladybugs) that the nine-spotted ladybug is now extinct, and other ladybug species are on starvation diets. This decline in native insects then further impacts bird populations, which have been fine-tuned by generations of adaptation to eat only insects they recognize. The U.S. has stopped importing these aliens, but the damage is already done, and the Catch-22 – declining bird populations leading to alien insect importation, leading to declining native species of insects, leading to more declines in bird population – now appears to be a treadmill with no “off” switch. This treadmill also incorporates global warming, which has apparently reached disequilibrium, in part due to natural forces like sun activity, but also to human occupation of the planet. The precise portions of blame have yet to be scientifically assigned.
The same situation is occurring all over Europe, with insect-eating bird populations (and the insects they feed on) declining precipitously. Locally, I haven’t seen a thrush, warbler or nuthatch since last year, and even then they were scarce. My over-wintering friend, the cardinal, has been absent since September. Chickadees (which eat both insects and sunflower seeds) haven’t touched the bird feeder, which remains at the same level month after month, and I fear Rachel Carson’s predicted “Silent Spring” has arrived.
You can view it at a forest or wildlife refuge near you, but don’t take binoculars – there’s nothing left to see.
Disclosure: I don’t own stock in any company associated with pesticides.