In the last half-century, North America has lost over a quarter of its entire bird population nearly equal to 3 billion birds. This shocking declaration is a new estimate published in the Science journal by researchers who compiled a variety of data that has been collected on 529 bird species since 1970.
Ken Rosenberg, an applied conservation scientist at the Cornell Lab of Ornithology in Ithaca, N.Y, says, “We saw this tremendous net loss across the entire bird community. By our estimates, it’s a 30% loss in the total number of breeding birds.”
Rosenberg and his colleagues were aware even previously that the number of bird populations had been decreasing.
He adds, “But we also knew that other bird populations were increasing. And what we didn’t know is whether there was a net change.”
Scientists believed that this was due to a possible shift in the total bird population towards more generalist birds adapted to existing around humans.
To explore this possibility, the researchers collected information from long-running surveys conducted with the support of volunteer bird spotters like the North American Breeding Bird Survey and the Audubon Christmas Bird Count. They combined this data with a decade’s worth of figures on migrating bird flocks recorded by 143 weather radar installations.
Their results appallingly show that over 90% of the loss can be attributed to just a dozen bird families encompassing sparrows, warblers, blackbirds and finches.
Rosenberg says common birds with declining populations include meadowlarks, dark-eyed juncos, horned larks and red-winged blackbirds. Grassland birds have also suffered a 53% decrease in their numbers, and more than a third of the shorebird population has already been lost.
In contrast, bird populations that have improved include raptors, like the bald eagle, and waterfowl.
Rosenberg says, “The numbers of ducks and geese are larger than they’ve ever been, and that’s not an accident. It’s because hunters who primarily want to see healthy waterfowl populations for recreational hunting have raised their voices.”
According to Applied ecologist Ted Simons of North Carolina State University, trying to tally bird populations and tracking them over time is a daunting task involving a lot of uncertainty.
Simons says, “People are doing a wonderful effort to try and understand our bird populations, but the actual systems that we have in place to try and answer really tough questions like this are really far short of what we need. We’re certainly far from having the tools and having the resources to have real high confidence in our estimates of these populations.”
Even then, he says, “I think it is very likely that we are seeing substantial declines in our bird populations, particularly migratory birds.”
Other researchers comment that this continentwide decrease in bird figures is similar to what they expected.
Kristen Ruegg, a biologist at Colorado State University in Fort Collins says, “I think that I buy the magnitude of the loss. Overall, the conclusions weren’t necessarily surprising. I mean, they were depressing but not surprising”.
Ruegg adds that there have been indications that the loss was of such a high magnitude from a variety of sources over the past few decades. However, in most cases, they were species-specific accounts of local extinctions or representations of projected losses resulting from factors like climate change.
This study, she acknowledges , “really sort of wakes people up to the idea that this is happening.”
Elise Zipkin, a quantitative ecologist at Michigan State University believes the loss of individuals can be a big problem and says, “Just because a species hasn’t gone extinct or isn’t even necessarily close to extinction, it might still be in trouble. We need to be thinking about conservation efforts for that.”
Zipkin notes that the research cites a variety of possible causes for the loss of birds such as habitat degradation, urbanization and the use of toxic pesticides,
She summarizes, “And so I think this kind of lays the gauntlet, for people to be thinking about ‘All right, how can we estimate maybe the relative contributions of these things to individual populations and their declines.’ “