Bird species revert to the mean after European boost


commercialappeal.com (subscription), TN – Jun 28, 2014

By Van Harris
Special to The Commercial Appeal
June 29, 2014
I have noticed a lot of discussion on the Internet and with people I meet regarding recent news from the National Audubon Society about changes in North American bird populations during the past 40 years.

The organization noted drastic declines in populations of the northern bobwhite (82 percent, evening grosbeak (78 percent), northern pintail (77 percent) and greater scaup (75 percen
The usual culprits are blamed: Illegal mining and logging in the boreal forests of northern Canada, urban sprawl, pollution and global warming. It is of note that six of the top 10 decliners (northern bobwhite, northern pintail, eastern meadowlark, loggerhead shrike, field sparrow and grasshopper sparrow) are primarily birds of grasslands. The push for biofuels, with $4 per bushel corn and $8.50 per bushel soybeans, is causing farmers to convert grassy fields from cattle grazing to row crops.

Especially damaging is the destruction of overgrown fence rows, which provide the cover and nest sites for shrikes and bobwhite. (The only argument my father and I ever had about the management of our farm was when he had a fence row bulldozed.)

While these declines are deeply concerning, it is notable that none of the species is in any way in danger of extinction. The evening grosbeak, for instance, was quite a rare bird in the 19th century. According to the Audubon Society’s Christmas Bird Count, it experienced a population increase beginning in the 1950s, which became an explosion in the 1970s and ’80s and has now reverted to historical levels.

Grassland species were probably never numerous before European settlement converted much of eastern North America from hardwood forest to grass pasture and cropland.

An article titled “Accidental Good Deeds” by Kenn Kaufman in the July/August issue of Bird Watcher’s Digest puts much of this in perspective. Kaufman notes that species of the swallow family especially have done quite nicely since Europeans began altering their habitats.

The process began with Native Americans, who provided purple martins with gourds in which to nest in hopes the birds would control mosquitoes near their lodges. It is now unlikely that purple martins in eastern North America nest anywhere except in manmade structures.

Barn and cliff swallows have also benefited. Barn swallows once had to search hard for spots to build nests; ledges under sheltering rock overhangs.

Now they use every kind of structure imaginable. Kaufman notes that in four decades of bird watching all over the world he has never seen a barn swallow nest not on a manmade structure.

I have seen cliff swallow nests under rock overhangs on cliffs along the Snake River in Idaho. I have seen hundreds more under a highway bridge over the Tenn-Tom Waterway in North Mississippi. Cliff swallows attach their mud nests to vertical rough walls with surfaces sufficiently rough to hold them. Concrete bridges provide such surfaces and sheltering overhangs that prevent rain from washing them away. Bridges cross water, which provides the mud the swallows need to build the nests.

Even more dramatic has been the expansion in range and population of the cave swallow. Forty years ago, this species nested only in the shadow zone of Carlsbad Caverns, N.M., and a few caves in Texas. In the 1970s, it began showing up near bridges, particularly the box culvert type, in eastern New Mexico and Texas. By 1990, it had followed Interstate 10 almost to Louisiana.

Errant individuals have shown up as far east and north as New England.

Eventually, it will almost certainly use box culverts and bridges to extend their range into the Mid-South.

There is a lot that can be done to slow or reverse the decline noted in the State of the Birds report.

It is also important to remember that bird populations are naturally dynamic, not static.

Also, not all manmade changes in the environment are necessarily bad for birds.

Meanwhile, I need to go plant a fence row.

Van Harris is past president of the Memphis chapter of the Tennessee Ornithological Society. E-mail him: shelbyforester1223@ earthlink.net.

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