Then a wildlife biologist and a veterinarian get to work. They kneel in the heaps of melting snow surrounding the moose, working quickly to collect vital statistics and collar it with a GPS tag. Their urgent goal: to better understand why moose are dying at alarming rates.

The tranquilizing dart contains a drug 10,000 times stronger than heroin, but even then a 1,000-pound moose will stagger for as long as seven minutes before finally falling to the ground with a thud that's nearly as loud as the helicopter's spinning rotors above.

But it can also be devastating. Of the 80 percent of collared moose that have died, 40 percent died from an insidious infection known as brain worm, 20 percent died from a heavy winter tick load, and the rest died from a combination of both scourges.

And it can be a lot of fun. "I always call it 'National Geographic' type of work: flying around in these helicopters and darting these animals that are just huge," said Moore.

That's why Seth Moore, chief biologist to the Grand Portage Band of Lake Superior Chippewa, a Native American tribe in the remote northeast corner of Minnesota, has been attaching GPS collars to moose every winter since 2009. His goal is to keep 30 collared moose on the 60,000-acre reservation at all times. Moore's primary intent was to determine what habitat types moose are using in the warming climate and then maintain those habitats or produce more.

It's a trend that can be seen across the United States. "Minnesota is having problems, quite a few Canadian provinces in the southern edge of their ranges are seeing issues, and one of the Maritime Provinces had almost a complete die-off," said Kristine Rines, a moose biologist for the state of New Hampshire, who's also concerned about declining moose populations in New Hampshire and Maine.

In Northeast Minnesota, moose numbered about 8,000 a decade ago. Today, that number is roughly 3,500. As new evidence unspools, one clear thread has emerged: in years of warmer, shorter winters , the moose are plagued by health problems.

Moore and his colleagues intended to collar this calf, but by the time they arrived the calf had been killed by wolves, creating a puncture wound in its head. The calf's legs, however, were accidentally broken by its mother while she was trying to defend it. Credit: Seth Moore

Winter ticks are also the result of short winters. Because moose didn't evolve with ticks, they don't know how to groom them off. Instead the ticks cause the animals to lose so much blood they become anemic, and to scratch so much that they tear off huge patches of hair. Without their warm coats, the moose can essentially freeze to death during cold rains.

Both brain worm and ticks appear to be worsening as the climate warms. The brain worm parasite thrives in white-tailed deer, which are resistant to it. But when snails and slugs that have eaten deer feces are, in turn, ingested by grass-eating moose, the worm can quickly become deadly. Not only do infected moose become disoriented and lethargic, they frequently cannot stand or even move their huge heads.

One of the newest concerns Moore's research has uncovered is that while adult moose are dying from warm-weather parasites and not predation, their young are. Wolves and bears feast on the weak calves, and Moore worries that those kills are increasing.

So in addition to their winter collaring work, in 2013 Moore and Wolf started to head back into the field to search for moose calves each spring. Later this month, they will once again be on the ground—without a tranquilizer—and likely within charging distance of the moose mother.

Luckily in the past, the weakened moose has charged only once or twice before running away, or circling the team from a distance. "The moose cows are just emaciated when we get there. You can see their scapular, clavicle and ribs," said Moore. "And so we speculate that some of the health issues that we're seeing in Minnesota may be affecting their parenting."

But Moore's research partner, veterinarian Tiffany Wolf, isn't convinced yet. She thinks the mother runs away as a defense mechanism; she's trying to lead the predator away from the calf, said Wolf.