This is Science Friday. I'm Ira Flato. We've heard about steep declines in the populations of certain species,

for instance, just this February, reports of a drop in insect populations worldwide.

Now another possible warning sign of global environment in crisis. A report in the journal Science this week says that North American

bird populations have declined by nearly 3 billion, yes, would it be 3 billion birds since

1970? That's a loss of nearly one in four birds. What is the cause? And how do you count that kind of population decline?

Ken Rosenberg is one of the authors of that report and an applied conservation scientist

at the famous Cornell Lab of Ornithology in Ithaca.

Welcome back to Science Friday.

Thanks.

Hi, I, Ira.

That's a lot of birds, isn't it?

It is a lot of birds. There are a lot of birds out there, but we've been seeing this

steady decline, and people who love birds and are out there every day have been noticing

fewer and fewer birds, and now we can put some hard numbers on those declines.

But there's still a lot of birds left, right?

This is not, we're not talking about an extinction here.

We're not, and this is different from other studies that are showing, and of course,

there are birds that are dropping towards extinction.

But what we're seeing here is like a whole another level of biodiversity

loss because we're seeing the loss of abundance among common species of birds. So it's not just the

rare and threatened species, but what we're finding is this pervasive loss among common, familiar

everyday birds. Giving me an idea of what we're talking about. What kind of birds?

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