(CBS) Healthy bats, when waking from hibernation, form small clusters after emerging from months of slumber.

But when CBS News correspondent Daniel Sieberg went with Biologist Al Hicks to an abandoned underground mine near Kingston, N.Y., it looked like a bat morgue, with many of the fragile mammals already dead, or dying, in our hands.

"This is the biggest threat to bat populations I've ever seen, no question about it," said Hicks, a state wildlife biologist.

Normally, the small brown bats should respond to lights and noise.

But their bodies are so weak, they are too lethargic to move. Their fat stores have been mysteriously depleted.

"If what we see here continues to play out over the winter, we'll lose most of them," Hicks said.

One of the most startling things that biologists are finding isn't just the condition of the bats themselves, but it's when they wake up and in a desperate bid for food, fly out into the dead of winter.

"These guys are leaving the mines and leaving the caves all across New York state," Hicks said.

And they're starving?

"They're basically starving to death," he said. "They can either die hanging upside down, or come outside here and hope they'll find flying insects flying around a snow bank."

But why is this happening?

There are three main theories, but no conclusive answer.

Some researchers believe it's because of a fungus Hicks discovered growing on their noses and wings. He calls it White Nose Syndrome.

"Their bodies are wasted," said New York State wildlife pathologist Ward Stone.

Stone has another idea. He believes the bats are the canaries in a climate change coal mine.

"These bats are warning us that there are some big changes," he said. "Ecological changes taking place that are impacting them."

Stone says warmer weather means bats hibernate much later, long after their insect food supply of insects is gone. So they enter winter with much lower fat stored.

But Texas bat conservationist Merlin Tuttle dismisses both those theories.

He suspects airborne toxins from pesticides; maybe even from spraying for the West Nile virus.

And when bats decline, he says, we all lose.

"We need bats, like them or not," Tuttle said. "Bats are primary predators of vast numbers of insects that fly at night and without them, whole ecosystems could be jeopardized."

Whatever the cause, the problem has exploded, from just four New York caves last year to caves and mines in three different northeast states.

This year's spring awakening for bat populations looks very dark indeed.

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