Laura Erickson's For the Birds

Thursday, June 6, 2013

Declining Numbers of Birds

Late spring in Port Wing, Wisconsin--taken on June 3!

 This spring has been exceptionally cold in my neck of the woods. As of June 3, a great many trees and shrubs hadn’t even started to bud out in Port Wing, Wisconsin, and there are far fewer Neotropical migrants on territory than I can ever remember. We lost a lot of birds during migration this year due to exceptional cold, both up here and down in Texas and the central states during migration, and I’m concerned about how successful the survivors will be at nesting. Studies have shown that for many species, nesting success is much lower when they arrive on their nesting grounds later than average.   

Whooping Crane carrying blue crab to edge of water

 There have always been bird losses on migration and bad years for songbird breeding, but these harsh cycles are growing increasingly dire, because for many birds, I suspect that there simply is no longer a “surplus population” that can take up the slack. What troubles me is that we have no tools for getting exact population numbers for bird species. The one exception used to be Whooping Cranes. Tom Stehn, a biologist with the US Fish and Wildlife Service, conducted aerial surveys over the Aransas National Wildlife Refuge and surrounding areas a few times each winter to get an exact count of wintering Whooping Cranes, but since his retirement last year, the Fish and Wildlife Service decided to stop conducting those precise censuses and use a simpler but far less accurate survey method.

  Breeding Bird Survey

 Most species aren’t as big and conspicuous as Whooping Cranes, and for most songbirds, the only consistent method for assessing numbers is the Breeding Bird Survey. This is an exceptionally invaluable tool, in use since 1966, so providing almost half a century of data now. But the Breeding Bird Survey has two serious limitations. First, it’s a roadside survey, and as traffic increases, a great many surveys have been re-routed away from dangerous traffic areas. This makes sense because it’s harder to hear bird songs as cars pass by, and traffic puts the people conducting the surveys in jeopardy, but little by little, what were originally randomly-situated routes are being moved away from developing areas, skewing results to make it seem as if birds requiring wilder habitat are more abundant than they are. I conducted one of these Surveys for 20 years, and my own survey route was re-routed twice during that time due to development.

  Indigo Bunting

The second limitation for the Breeding Bird Survey is that it counts only males successfully defending a territory, so doesn’t include any “floaters”—that is, birds quietly waiting in the wings for a suitable territory to open up. We have no idea how many of these are out there for any species, but there is strong reason to believe that there used to be far more than there are today.

In a 1945 experiment, a scientist studied what seemed to be an isolated pair of Indigo Buntings. To learn if the female could finish incubating and raise chicks on her own, the scientist shot the male. When he returned the next day, she had a new mate, which he also shot. Day after day, the scientist shot a new male until he’d killed nine male Indigo Buntings on that territory, finally leaving the tenth in peace to help raise the young. The Breeding Bird Survey would have shown exactly one Indigo Bunting on that territory throughout, until every male was gone.

We don’t have any way of counting these “extra” birds, but there’s powerful evidence that their overall numbers have dwindled. Sydney Gauthreaux, the first scientist to look at bird migration via NEXRAD radar data, found that movements of migrating birds over the Gulf of Mexico had dropped by almost 50 percent between the 1960s and 1980s, and by another 50 percent in the following two decades. Many conservationists disputed his findings because their Breeding Bird Survey data didn’t show a corresponding decline, but the simple truth is that you need to lose an awful lot of individuals of a healthy population of birds before you can detect any loss at all on a Breeding Bird Survey, if the health of the habitat remains the same. Those of us who have been birding many decades have seen fall-outs before, and watched species recover after setbacks. But we’ve also seen a definite decline in birds, during migration and on territory, over these decades.

  Canada Goose

 Unless someone knows and values individual species of birds, it’s hard to wrap our heads around bird population losses. Here in the North Woods, there are more Canada Geese, Wild Turkeys, and American Robins than ever, and naturally people with little knowledge about birds conclude from how conspicuous these birds are that birds in general are doing fine. But a great many species are not, and unless we’re willing to face the problem head-on, we’re not going to be able to find solutions in time.