Collisions vs. Impacts

In the first of a two-part series (the first part of which aired Wednesday) on NPR’s Morning Edition, American Bird Conservancy ornithologist Christine Sheppard described her research into how birds respond to different types of glass. And the motivation behind it: the numbers of birds killed each year in building collisions.

“Our best estimate is 100 million to a billion,” Sheppard said.

“It’s an incredible number,” observed NPR Science Desk Correspondent Christopher Joyce, “and she acknowledges it’s an estimate. But it’s based on reasonable assumptions.”

Reasonable assumptions? Like those used by ABC to “estimate” the number of birds killed each year by cats?

“Almost anybody you talk to has seen a bird hit a building or heard a bird hit,” Sheppard continued. “The fact that so many people are seeing this means it’s happening all the time.”

Let’s hope there’s more substance to their estimate than that. Methodologies aside, though, I found it curious that once again the focus was on mortalities and not the effect of those mortalities.

“Many people believe that birds have an intrinsic right not to be killed,” Sheppard explained. “Birds are seed dispersers; they eat tons of bugs. So every bird that’s killed on a building is an ecological service that we lose.”

(Well, many people believe that cats have an intrinsic right not to be killed, too—but let’s stick to building collisions here.)

Sheppard’s point is well-taken. The thing is, according to a 2011 study that examined collisions with both buildings and communication towers, all those mortalities may not be so costly in terms of lost “ecological service.”

“Although millions of North American birds are killed annually by collisions with manmade structures,” explain conservation biologists Todd Arnold and Robert Zink, both from the University of Minnesota, “this source of mortality has no discernible effect on populations.” [1]

I’m all for bird-friendly glass, whether or not there are population-level benefits. But it’s telling that ABC makes no mention of these findings—indeed, doesn’t acknowledge the research at all as far as I can tell (and I sent it to Darin Schroeder, ABC’s Vice President of Conservation Advocacy, and Steve Holmer, their Director of the Bird Conservation Alliance, months ago).

As we’ve seen with their position on free-roaming cats, though, once ABC settles on a story, they stick to it.

Literature Cited
1. Arnold, T.W. and Zink, R.M., “Collision Mortality Has No Discernible Effect on Population Trends of North American Birds.” PLoS ONE. 2011. 6(9): p. e24708.


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